A RECORDING INTERFEROMETER

by

Jacques R. Maroni

Submitted in Partial Fulfillment of

the Requirements for

the Degree of

BACHELOR OF SCIENCE IN MECHANICAL ENGINEERING

from the

Massachusetts Institute of Technology

1943

Signature of Author.

Department of Mechanical Engineering..............

Date.. . . .. . .. . ........ .. . ....ss* .. . .

Professor in Charge of Thesis.

A C K N O W L E D G E M E N T

The author gratefully acknowledges the

aid of Dr. W. M. Murray in suggesting the

subject, and in offering helpful comments

on the work.

TABLE OF CONTENTS

Section I

Introduction

Section II

The Light Source

Section III

The Interferometer

Section IV

The Light-Sensitive Device

Section V

Amplification and Simple Relay Circuits

Section VI

Automatic Recording of thePhotoelectric Current

Section VII

Conclusion

Section VIII

Appendix

Section IX

Bibliography

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S E C T IO N I

INTRODUCTION

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1.

INTRODUCTION

The accurate measurement of strains is an important

part of the tension test. In this respect, the Huggenber-

ger tensometer is a valuable strain measuring instrument

because it is sensitive, rugged, relatively inexpensive

and easy to handle. In 1934, Mr. Vose1 made an extensive

study of the characteristics of the Huggenberger tenso-

meter, and found that, due to wear of the instrument parts,

a calibration was desirable from time to time, for best

accuracy. In that connection, he devised a special inter-

ferometer which makes possible a direct comparison of the

tensometer reading against the actual displacement as

measured from the number of interference bands going by

a reference point.

The purpose of this thesis is to determine whether

it is possible to make the use of the interferometer more

reliable and less tedious by providing an automatic re-

cording in place of the eye-straining visual observation.

Such a study has been very much hampered by the fact that

most of the equipment needed for actual tests was unavai-

lable due to the present war emergency. It will therefore

have to be a preliminary survey of the problem, in which

we will only attempt to profit from previous investiga-

1: A.S.T.M. Proceedings. 1934

3.

tions made on the different pieces of equipment needed,

in such a way as to open the path for future studies.

A definite solution will no doubt have to wait until such

a time that a complete test may be set up.

The equipment needed may be listed under five hea-

dings, each of which will have to be considered.

(1) the light source.

(2) the interferometer.

(3) the light-sensitive device.

(4) the an. plifier.

(5) the recording device.

Numerous studies have been made on these different

items, and we propose in the following pages to indicate

what considerations must be taken into account to direct

and coordinate the necessary selections of equipment.

S E C T I 0 N II

THE LIGHT SOURCE

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5.

THE LIGHT SOURCE

In making the choice of a proper light source, the

requirements are that it be simply and conveniently ope-

rated while at the same time giving out a highly intense,

constant and monochromatic radiation. We can easily under-

stand the reason for these requirements: the radiation

must be monochromatic so that the interference pattern

will be black and white, and not colored as it would be

if white light were used. It must be intense , in order

to increase the energy which will be distributed along

the different interference fringes, thus making easier

the detection by the light-sensitive device. Finally,

it must be constant because the detection cannot be ex-

pected to be reliable if the light source intensity varies

from time to time.

At this point, it is important to point out that,

in most cases, the light-sensitive device does not res-

pond equally well to radiations of different wavelengths.

It is apparent then, that we will want to use a wave-

length of such a magnitude as to correspond to a range

of high sensitivity for the light detecting device. We

see therefore, that the selection of a light source is

rather closely connected with the choice of a light-sen-

6.

sitive device. This means that we will have to make a

first selection which we may have to revise later from

the light detection point of view.

There are several types of light sources which will

satisfy our general requirements:

One solution would be to use a high intensity incan-

descent lamp with the proper filter arrangement. This

provides obviously a simple and convenient answer but it

has the great disadvantage of requiring filtering to a

point where most of the energy emitted is absorbed.

Another solution is to use the flame of a burning

gas. Alcohol, sodium salts, and acetylene have been used

in the past. This method affords a remarkably constant

and reliable light source, but there again the intensity

is quite low.

A third and better method consists in using as a

light source, an electrically lighted gaseous substance.

This can be done by either one of two ways:

a) gaseous discharges in rarefied atmosphere.

b) electric arcs.

The gaseous discharge method is extremely valuable

in many applications. Lord Rayleigh, Michelson, Fabry,

used it in spectroscopic studies to determine the width

of spectral lines and the maximum order of interference

4f

that could be observed with different vapors. They found

that narrow lines could be obtained only with vapors of

high atomic weight under low pressures and temperatures.

Mercury was used successfully although the best line

(X = 5461 A ) is surrounded by a few " satellites ".

Cadmium gives a very fine red line (= 6438.4696 A )

which is used as a reference in spectroscopic work. The

use of cadmium tubes , however, is not very convenient,

as they have a short life and do not always radiate as

intensely as is desirable. Krypton tubes immersed in li-

quid air have produced the finest lines ever observed.

Neon tubes give very intense lines mostly in the orange

and red region. This method affords the best light sour-

ces for accurate experimental work. However, it requires

a rather high voltage and it is not especially desirable

when extremely fine lines are not necessary.

The electric arc method seems to be more desirable

in our case. Carbon or tungsten arcs are used in several

applications and they provide very intense sources of

light. However they usually require large control boxes.

Furthermore the intense heat generated may injure the

reflector and the required filters. Mercury arcs of the

Cooper-Hewitt type have many distinct advantages. They

provide very bright spectral lines that are so far apart

8.

that they can be easily isolated with filters. They are

produced commercially and can be operated conveniently.

They have proved to be an invaluable tool in innumerable

optical experiments. They send out a light which is cha-

racteristic of mercury vapor: in the visible spectrum we

find violet radiations convenient in photographic work,

green radiations desirable for visual observation, and

yellow radiations. There is a deficiency in red radia-

tions. Many styles of mercury vapor lamps have been de-

vised. The tube is usually made of fused quartz and we

may note in passing that all fingerprints on the tube

must be removed before the arc is started. If this is

not done, the salts contained in the grease on the fin-

gers will unite with the quartz when the latter is heated

and cracks in the tube may result. The brightness of the

are stream may be anything from 1 to as high as 1000

candles/cm 2. The lamp for best results is enclosed in a

lamp house in which is mounted a reflector, so that most

of the light is reflected back on the lamp house window.

The window may or may not be fitted with adjustable shut-

ters, the purpose of which is to regulate the size of the

light beam. In addition, it will be desirable to use a

collimating lens to obtain a parallel beam.

We have up to now put the emphasis on the fact that

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9.

the light source should be highly intense. There is, how-

ever, a definite limit as to the desirable light inten-

sity; obviously a highly intense light source means that

a great amount of heat is generated and must be radiated

away. This consideration involves a heat transfer problem

which determines a practical light intensity limit.

The recommendations which have been made thus far

concerning the selection of a light source, have been

motivated in part by our preoccupation to be on the safe

side. It may well be that tests will show the relative

importance of the several factors to be not what we ex-

pected. But we know that a perfect black and white inter-

ference pattern requires a perfectly monochromatic light

source, and that satisfactory light detection and recor-

ding demands a highly constant and intense light source.

These ideal qualities should guide the choice of the light

source. Their relative importance can be determined only

in specific cases through actual tests.

T

IrI

S E C T I 0 N III

THE INTERFEROMETER

10.

11.

THE INTERFEROMETER

We can divide the methods of obtaining interference

effects into two broad classes:

(1) Methods which require a point or line sour-

ce and which produce interference effects by a division

of wavefront.

(2) Methods in which the beam is divided by

partial reflection into two or more beams, and which pro-

duce interference effects by a division of amplitude.

Examples in the first class are the Fresnel biprism,

Lloyd's mirror, Billet split lens, and the Rayleigh in-

terferometer. Examples in the second class are the inter-

ferometer systems of Fizeau, Jamin, Mach, Michelson, Fa-

bry Perot and Lummer Gehrcke. The systems in the second

class offer the distinct advantage that extended light

sourcesmay be used, so that in general the effects are

much brighter. In the present discussion, only the sys-

teis of the second class are of interest, due to the fact

that the brightness of the fringe pattern is of paramount

importance.

On the original interferometer devised by Mr. Vose

to calibrate Huggenberger tensometers, the optical sys-

tem is similar to the one of the Fizeau interferometer.

12.

It consists essentially of two thick glass disks. The

front surface of the first disk is inclined at a slight

angle so that the reflected light does not enter the eye

of the observer and the back surface of the second disk

is either inclined or blackened. Under these conditions

the interference effects are produced by the inner surfaces

of the first and second disks. If these surfaces are exac-

tly parallel and are separated by an integral number of

wavelengths, the point at the foot of the perpendicular

dropped from the point of observation will be dark. This

point is surrounded by alternate bright and dark rings.

As the distance separating the two inner surfaces is in-

creased, the spot at the center becomes alternately bright

and dark. If the two surfaces are not accurately plane,

the rings will have irregular shapes similar to the con-

tour lines of a topographic map.

In practice, this system is not satisfactory for

our purpose. For one thing, the two plane surfaces must

be adjusted continuously for parallelism; for another,

the interference fringes are not sufficiently wide to

make it easy for the light-sensitive device to single

them out. It is important to keep in mind the fact that

the optical system of the interferometer must be such as

to satisfy the requirements of the light-sensitive device.

13.

These are, in general terms, that the interference bands

be of such a width and such intensity as to make possible

the detection of one band at a time. In addition, it would

be advantageous to use a system which does not need to be

adjusted continuously.

If, for example, we used a system consisting of an

optical flat and of a spherical lens, we would obtain a

pattern of Newton rings not very different from the pat-

tern obtained with the Fizeau method when the two inner

surfaces are exactly parallel. There would, however, be

no need for frequent adjustment. Furthermore, we could

make the diameter of the central spot whatever we want,

by choosing properly the radius of curvature of the sphe-

rical lens. The longer the radius of curvature of the lens,

the larger the diameter of the central spot. It seems the-

refore that such an arrangement would be quite satisfac-

tory.

Assuming now that we have decided to use such a sys-

tem, we can try to determine whether it is best for our

purpose to observe it by transmission or reflection, and

whether partial silvering of the surfaces can improve the

fringe pattern from the intensity point of view. In that

connection, the first property which must be mentionned

is that the patterns observed by reflected and transmit-

14.

ted light are always complementary, the maxima of one

system corresponding to the minima of the other. This

property can be easily verified by placing light sources

of equal brightness in front and behind the apparatus.

The pattern observed by transmission is superimposed

on the beam of light going through the optical system in

such a way, that the dark rings do not appear dark but

merely less bright than the bright rings. The reflected

pattern presents a stronger contrast between bright and

dark rings, but the light intensity is not as high as in

the case of transmission, largely due to the low coeffi-

cient of reflection of unsilvered glass. It seems that

it is preferable to observe the pattern by reflection but

it may be that no convenient light-sensitive device is

sensitive enough to detect such a low intensity. If that

is the case, we may have to observe the pattern by trans-

mission.

We may also silver partially the surfaces of the

optical system of the interferometer. The silvering of

the surfaces has, however, the disadvantage of making the

interference phenomenon more complicated and rather un-

predictable, due to a series of inter-reflections. As a

general rule, it seems that partial silvering of the sur-

faces causes the interference bands to appear brighter

15.

but narrower. Such a modification of the pattern could

hardly be considered as a great improvement from the point

of view of detection. It may be proper to note that in

the case of silvered surfaces, some light is absorbed in

the silver film. which means that the reflected and trans-

mitted patterns are no longer complementary.

One disadvantage of the simple system consisting of

a spherical lens and an optical flat is, that, when the

angle of incidence is increased, there is a considerable

loss of light by reflection at the first glass surface.

A large angle of incidence is decidedly detrimental to

the brightness of the rings. This difficulty may be avoi-

ded by using a system consisting of a prism and a lens.

One face of the prism is placed against the curved sur-

face of the lens, so that light falling nearly perpendi-

culary on one of the other faces will enter the prism in

large quantity. This arrangement has the additional ad-

vantage of making large the angle of refraction into the

air film between the prism and the lens, which increases

the radius of the rings as it depends not only on the ra-

dius of curvature of the lens, and on the wavelength used,

but also on the secant of the angle of refraction into

the air film. Viewed through the third face of the prism

the rings obtained in this manner are both bright and

large. The same result may be obtained with a glass plate

16.

and a prism having one face polished into a spherical

form of small curvature.

In any of these systems where prisms are used, we

have to take into account the effect of dispersion. It

is interesting to note that, especially in the neighbor-

hood of total reflection, dispersion has an achromatising

effect. In other words, the bright rings, obtained with

white light are nearly white instead of being highly co-

lored. We see then, that it may be possible to use a

prism-lens system with an intense source of white light,

and with no filters required. It may be that such a system

would actually be more satisfactory than a plate-lens sys-

tem.

Summing up this discussion, we can say that there

seems to be a definite advantage in the use of a plate-

lens system in place of a two plate system. The plate-lens

interference pattern will show a greater contrast between

dark and bright rings if viewed by reflection. If however,

the light intensity is too low to be easily detected by

a convenient light-sensitive device, that situation may

possibly be improved by proper partial silvering of the

surfaces. If silvering turns out to be impractical it is

possible to view the pattern by transmission, in which

case the actual recording of interference bands is likely

17.

to be satisfactory only if the light-sensitive device

has a linear characteristic over the range of illumina-

tion. Finally, it is possible that the best results will

be obtained with a prism-lens system which can give bright

and large achromatic rings even when the angle of inci-

dence is large, and when the light is not monochromatic.

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S E C T I 0 N IV

THE LIGHT-SENSITIVE

DEVICE

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18.

19.

THE LIGHT-SENSITIVE DEVICE

In the past, a number of more or less satisfactory

instruments have been used to measure radiant energy.

There are non-selective radiation meters; e.g., thermo-

pile, thermocouple, radiometer and bolometer, wherein

radiant energy is absorbed by a blackened receiver and

converted into heat. Also, there are selective light-sen-

sitive instruments.

The automatic recording of the number of times a

certain spot in the interferometer system changes from

dark to bright illumination requires the use of one such

device. It is apparent however, that selective light-sen-

sitive cells are better suited to our purpose, not only

because of their selective capacity, but also because of

the many qualities which have been incorporated into them

through many years of development.

A light-sensitive cell is essentially a device by

means of which light energy can be used to control elec-

trical energy. There are two broad classes of light-sen-

sitive cells: the first class is said to be photoconduc-

tive. It includes the cells which merely change their

electrical resistance when illuminated. The outstanding

example is the selenium cell. The second class is said

20.

to be photoemissive. It includes the cells which exhibit

the photoelectric effect. A photoelectric cell is essen-

tially a light-sensitive material which, placed inside

an evacuated or gas-filled bulb, emits electrons when ex-

posed to radiant energy of short wavelengths, thus cau-

sing a current to flow to a positively charged collecting

electrode in the bulb.

WIe will immediately eliminate the selenium cells.

They have a high sensitivity which in certain instances

may make current amplification unnecessary, but that qua-

lity is more than counter-balanced by several grave dis-

advantages. These are:

(1) They have an appreciable and sometimes un-

predictable lag in response.

(2) Their resistance varies greatly with tem-

perature.

(3) They are vulnerable to moisture and shock

and the constancy of their behavior is uncertain.

(4) They do not have a straight line charac-

teristic.

We may now confine our attention to cells of the

1: The increase in current due to the action of the lightis very nearly proportional to the square root of the il-lumination, in the case of selenium cells. This may notnecessarily be a disadvantage, however.

21.

photoemissive class. This class is essentially made up

of the so called photovoltaic and photoelectric cells.

A satisfactory cell in this class should have the follo-

wing desirable characteristics:

(1) Constancy and speed in response.

(2) A definite and constant relationship between

intensity of illumination and current given off.

(3) A high spectral sensitivity well fitted to

the light source used.

(4) Low " dark current ".

Photoelectric Cells.

The photoelectric cells have been used extensively

in many applications, and we will consider them first.

However, as there is a great number of photoelectric cells

available, it may be in order to see what the factors

which influence their characteristics, are.

The first factor we will mention, is the one which

determines the ratio of the primary photoelectric current

to the incident light: it is the material of the cathode.

When the cell is to be used with visible light, our choice

in this matter is closely limited; for there are only

seven metals known that are sensitive at all to such light.

They are sodium, potassium, rubidium, caesium, lithium,

strontium and barium. All these metals oxidize rapidly

22.

in air and must be prepared in a sealed vessel. Only the

first four can be distilled in glass; the difficulty of

preparing the three others is such that they have almost

never been used.

The second factor determines the proportion of the

incident light which falls on the cathode: it is the area

of the cathode and of the window. This factor is not too

important as long as the window and cathode are large

enough to admit the whole beam of light. It is known that

it makes practically no difference how the light is dis-

tributed over the cathode of a photoelectric cell. The

cathode material is usually deposited on a spherical or

on a plane surface. The shape of the cathode surface has

apparently a definite influence on the cell characteris-

tics.

The third factor has to do with the conditions in-

side the photoelectric cell. The cell may be either pum-

ped out and sealed; or it may be filled with some inert

gas. The presence of the inert gas increases appreciably

the sensitivity of the cellas the primary photoelectric

current is magnified by the process of ionization by col-

lision. The magnification can be increased or decreased

1:This is not true in the case of a selenium cell.

23.

within certain limits by varying the pressure inside the

bulb, and the applied electric field.

There are many other factors of lesser importance,

some of which are not yet completely understood and are

outside the scope of this discussion. The factors men-

tionned above will be essential in guiding the choice of

a cell.

Vacuum versus Gas-filled Photoelectric Cells.

The vacuum cell is theoretically the simplest form

of photocell. With a given amount of luminous energy in-

cident on the cathode, the vacuum cell will show a gradu-

ally increasing current with increasing applied voltage.

As the voltage is further increased however, the current

reaches soon a limiting value known as saturation current.

The gas-filled cell has a similar current-voltage

relationship for low voltage values. For larger values of

the electric field, the magnification due to ionization

by collision becomes increasingly large and a limiting

value is reached when the voltage is such that the ioni-

zation becomes cumulative and a continuous glow discharge

takes place. This effect makes the gas-filled cell more

sensitive; for stable operation however, the cell should

be operated well below the glow voltage, and always con-

nected in series with a high resistance. It has been found

Ir

III

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24.

that the glow discharge, if allowed to pass for a short

time, increases considerably the sensitivity of certain

cells. If maintained for a long time, the glow discharge

damages and eventually destroys the photoelectric proper-

ty of the cell. This makes the use of gas-filled cells

always a little uncertain, as an excessive illumination,

as well as an excessive electric field, can cause the

glow discharge.

If now, the voltage is kept constant and the amount

of light incident on the cathode of a vacuum cell is in-

creased, it is found that the current given off is a li-

near function of the illumination. This linear characte-

ristic, in addition to an almost instantaneous response

constant over several thousands of hours of operation,

is a distinct advantage of the vacuum cell.

The gas-filled cell may exhibit a similar linear

characteristic, although in most cases the increase in

sensitivity is obtained at some sacrifice of linearity

of response. The speed of response of the gas-filled cell

is limited by the rate of decay of the ionization of the

gas, but it is satisfactory in most applications. The

constancy of operation of a gas-filled cell is probably

not as satisfactory as that of a vacuum cell, as " fati-

gue " phenomena, not too clearly understood, have been

1'

II1III

25.

observed.

As a result of this brief discussion, it is apparent

that the increase in sensitivity is practically the only

point of superiority of the gas-filled type of cell, and

the vacuum type should always be used when possible.

Cathode Material

From the study of the photoelectric properties of

different materials, as well as from theoretical consi-

derations, it has been established that each material is

affected by what is called the photoelectric threshold,

which in effect defines a maximum wavelength above which

the photoelectric emission can no longer take place. For

most materials which might be used as a cathode, the

threshold value is shorter than the shorter wavelength

of the visible spectrum, and, as we, have pointed out be-

fore, only four metals have been used so far in the visi-

ble spectrum, although practically any metal will give

a photoelectric response to ultra violet radiation.

The earlier forms of photocells contained a potas-

sium cathode sensitized by a discharge in hydrogen by the

so called Elster and Gertel process. Subsequent develop-

ments however, have shown that the most satisfactory cells

are those which have cathodes consisting of very thin

alkali metal coatings on a suitably prepared base.

26

Various combinations have been investigated with the re-

sult that the caesium-on-silver-oxide cathode has proved

most efficient.

The "1 mass " layer potassium cathode gives a maximum

spectral sensitivity for a wavelength of about 4400 Angs-

trom units. The " thin " layer caesium-on-silver cathode

gives a maximum sensitivity for a wavelength of about

7600 Angstrom units. The caesium-on-silver maximum is ap-

proximately three times the potassium maximum, in compa-

rable units. In that respect, it appears that the use of

the mercury line of wavelength 5461 Angstroms, is unfor-

tunate as the relative emission of most cathodes at that

wavelength is comparatively low. However, that situation

can be remedied: it is possible to bake the caesium oxide

until the cathode becomes ivory-colored and the sensiti-

vity drops to a low value; if pure argon is then intro-

duced and a glow discharge is established, as during oxi-

dation, it will be found that the surface is re-activated

and the sensitivity returns to its initial value; the gas

may then be removed. A cell prepared in this fashion will

have an excellent sensitivity with its maximum in the

green region of the spectrum.

Dark Current

It has already been pointed out that the voltage

27.,

applied to gas-filled cells should be well below the glow

discharge voltage. In the case of vacuum cells, no gain

is made by increasing the voltage above a value which

gives approximate saturation. Actually, high voltages are

detrimental to satisfactory operation of the cell, as lea-

kage current under such conditions becomes of increasing

importance. In order to minimize the effect of leakage

current, a vacuum cell should be provided with an inter-

nal guard ring. Leakage current tending to pass between

the electrodes can then be shunted through the guard ring,

and so avoid passing through the measuring instrument in

the circuit. It is often necessary to reduce external

leakage by the use of an external guard ring.

In every photocell, a small current passes when the

cell is not illuminated. This current is due not only to

the leakage current but also to a cathode emission simi-

lar to that produced by a small illumination. This is

usually termed " dark current ". It is obvious that if

the illumination under consideration is small, the dark

current may be very troublesome, to the point of making

measurements meaningless. The cell construction most sui-

table to reduce the dark current to a minimum is that in

which the electrodes consist of a centrally placed anode

of small dimensions in the center of a spherical surface,

28.

the whole of which forms the cathode.

Unfortunately, the types of thin film cathodes which

are most efficient, have an abnormally large dark current

which is in the nature of a thermionic emission, appreci-

able even at atmospheric temperatures. In most industrial

applications, where bright lights are used, this dark cur-

rent is negligible, compared to the photoelectric current.

This is not the case when it is desired to measure faint

illuminations. These considerations emphasize the neces-

sity of using an intense light source and of producing

wide and bright interference fringes. They also compli-

cate further the intelligent choice of the proper photo-

electric cell.

Photovoltaic Cells.

Photovoltaic cells are those in which light produces

directly or indirectly an electromotive force.

A number of electrolytic photovoltaic cells have been

developed in the past but they have been overshadowed by

the recent development of " dry " photocells, also known

as " rectifier " cells or " barrier layer " cells. Ever

since the success of Lange in 1930 in obtaining relatively

large currents from the dry photovoltaic cuprous oxide

cell, the photovoltaic cells have come to a new level of

prominenc e.

29.

The cuprous oxide cell however, has one disadvan-

tage in common with the selenium cell: it suffers from a

slow I fatigue " when subjected to high intensity illumi-

nation. This is not too important as it " recovers " nor-

mality very rapidly, after a brief rest, and no permanent

harm results. On the other hand, a very interesting fea-

ture of the dry type of photocell is the relatively large

currents obtained with them. With an orthodox photoelec-

tric cell, 60 micro-amperes per lumen is a very good emi-

ssion; but for the same illumination, a dry cell will de-

liver 120 micro-amperes or over twice the current obtai-

nable from a gas-filled photocell.

The dry photocell is not used entirely in place of

the ordinary photoelectric cell, however, the reason being

that it unfortunately possesses a very low internal resis-

tance, which prohibits its use for television or sound

reproduction. It is used wherever practicable, for example

in small pocket photometers.

Another type of dry photocell is the Weston Photro-

nic cell. It consists of a special iron-selenium light-

sensitive disk which is both sensitive and permanent. It

uses the barrier-layer principle which does not require

an evacuated space as in the usual forms of photocell.

It has a color response quite similar to that of the hu-

30.

man eye which is a favorable characteristic if a mercury

arc is used as light source. It has, however, a large

electrical capacitance which limits its use at high fre-

quencies. This last point is not a disadvantage in the

present investigation. Since the cell uses no batteries

or any source of external current, there is practically

no dark current. The cell will deliver at least 150 mi-

cro-amperes per lumen. Such a high current output makes

amplifying unnecessary in many cases. The Photronic cell,

like most photovoltaic cells, has a straight line rela-

tionship between current and illumination if the exter-

nal resistance is small relative to the cell resistance.

As far as is known, it is not harmed by high intensity

illumination nor is it subject to physical or chemical

change.

Therefore we can say to conclude this discussion,

that the Weston Photronic cell seems to be preferable to

either a vacuum or gas-filled photoelectric cell. The

Photronic cell satisfies all of our requirements. The

photoelectric cells offer probably a more perfectly li-

near characteristic and a more perfectly instantaneous

response, but it is at the cost of higher dark currents

and a lower sensitivity. The linear characteristic and

31.

the speed of response of~ the Photronic cell will certain-

Ily be better than our purpose really warrants.

32.

S E C T IO N V

AMPLIFICATION

AND

SIMPLE RELAY CIRCUITS

33.

AMPLIFICATION

AND

SIMPLE RELAY CIRCUITS

Investigations involving the measurement of photo-

electric currents range from those in which the photoelec-

tric effect itself is the center of interest, to those

in which a photoelectric cell is merely a tool for indi-

cating light intensities. The currents may vary in magni-

tude from 10-15 amperes to 10-3 amperes. They may be cons-

tant or may fluctuate rapidly. Hence a wide variety of

methods of measurement has been developed.

The purpose of the use of a photocell is in general,

the measurement, the comparison or the detection of a

number of objects.

Measurements are usually the concern of the physicists.

They favor direct measurements, conveniently made with a

galvanometer if the current exceeds 10-9 amperes, or with

an electrometer of the Compton, Lindemann or Hoffmann

types if the current is smaller than 10~9 amperes. Cur-

rents as small as 10-18 amperes can be measured. It is

even possible to detect the emission of single photoelec-

trons by means of a modified Geiger-Mfiller tube.

In most photoelectric applications however, such as

photometry, colorimetry, telephotography, sound reproduc-

34.

tion, or television, the purpose of the cell is more to

detect and compare than actually measure changes of light

intensity. This can be done with a much greater measure

of success if the photoelectric current is amplified by

means of thermionic vacuum tubes, or gas-filled relays.

A great amount of work has been done in the field

of current amplification and a number of standard circuits

have been developed in addition to many arrangements used

in specific cases. Small and rapidly varying currents can

be reproduced with great fidelity and amplified by a tre-

mendous factor. In the present investigation, however,

it appears that the amplification of the photoelectric

current will be far from being a critical phase of the

completed apparatus. The neatness of the interference pat-

tern and the reliability of the cell are certainly of more

vital importance.

Just how much amplification is required depends en-

tirely on the use to which the current is going to be put.

In most industrial applications, the use of an ordinary

relay, which is either " on " or " off ", according to

whether the exciting current is above or below a critical

value, is perfectly satisfactory. However, mechanical

relays, even of the best type, have several disadvantages

which restrict their use. Usually they cannot be opera-

35r

ted by the photoelectric current directly, so that an

initial stage of amplification is required. The electri-

cal contacts after repeated operations deteriorate. They

may not operate instantaneously and hence introduce an

undesirable time lag. Finally the threshold value of the

current at which the relay is supposed to operate is not

sharply defined.

All these objections are eliminated if a Thyratron

is used in place of a mechanical relay. The Thyratron is

essentially a hot-cathode mercury-arc rectifier in which

a control grid has been placed. It is so designed that no

current can pass through until the grid potential is rai-

sed above a certain critical value. One simple Thyratron

relay circuit is connected in such a way that control is

obtained by applying an alternating current to the anode.

In that way, the tube acts as an on-and-off relay with a

very high power amplification and with practically no

hysteresis such as a mechanical relay would have. There

are several other Thyratron relay circuits which can give

a very nice control.

The Thyratron is admirably adapted for use as a re-

lay in conjunction with a photoelectric cell of either

the vacuum or gas-filled type. Since it passes a current

of several amperes, it can serve as an excellent relay

Now----

36.

which can be controlled directly by the photoelectric

current without preliminary amplification. In the case

of a photovoltaic cell however, no external battery is

used, and the voltage generated by the cell is so small

that the direct Thyratron control cannot be used.

The manufacturers of the Weston Photronic cell, in

view of that fact, have developed a sensitive relay, which

they call Sensitrol. They claim that it can operate on

values down to 2 micro-amperes or 1/2 millivolt. Such a

relay can usually be made to operate directly on the Pho-

tronic cell current or voltage. However, some amplifica-

tion may be necessary in some cases.

It appears therefore, that the Thyratron control is

extremely desirable if a photoelectric cell is used, whereas

the Sensitrol relay or a similar device is better adapted

to use with photovoltaic cells such as the Weston Photro-

nic cell.

V 37.

S E C T IO N VI

AUTOMATIC RECORDING

OF

THE PHOTOELECTRIC CURRENT

38.

AUTOMATIC RECORDING

OF

THE PHOTOELECTRIC CURRENT

The automatic recording of the photoelectric respon-

se can be obtained in several ways.

We can have a continuous recording if we use a self-

balancing potentiometer such as the Leeds and Northrup

Curve Drawing Potentiometer Recorder. This device will

automatically draw a continuous curve showing the varia-

tions of the photoelectric current. By calibrating pro-

perly the recording paper and by making approximately

constant the speed of motion of the interferometer, it

should be possible to determine with accuracy the dis-

placement to be measured, without actually having to count

the number of maximum of current as recorded on the pa-

per. This method however, requires more intricate and

fragile equipment and does not simplify very much the

human labor required.

Another possibility consists in the use of a Veeder

magnetic counter, suitable for applications involving

counting up to 3 or 4 a second. A few words on the ope-

ration of the counter may not be out of place:

The cycloneter portion of the counter is actuated

39.

through a link from a crank bolted to the armature spindle

which is supported in bearings attached to the field po-

les. A side knob is provided so that the counter can be

reset to zero by hand. When the field circuit is clcsed

through the photocell relay contacts, the armature rotates

into the most intense part of the field against the tor-

que of a spring which returns it to the zero position

when the impulse ceases. The rotation of the armature

actuates the connecting link and operates the counter.

Such a counter would clearly have the advantage of

giving a direct and almost immediate answer. The counter

could actually be geared in such a way as to read direct-

ly in any desired unit of displacement. In that respect,

it would be quite satisfactory. It is apparent, however,

that if a high accuracy is desired, it is necessary to

measure, or at least be able to estimate, fractions of

the number of light reversals on the interferometer, and

not only integral numbers as would be the case with a

simple magnetic counter.

To do that, it may be necessary to use auxiliary

counters. However, it should be reasonably easy to esti-

mate fractional numbers, if the displacement is made re-

latively uniform which is not too difficult.

In that connection, it seems that it should be re-

40.

membered that the error found in most testing machines

is from 1/7% to 1%. Therefore little is gained in the

final analysis by making the measurement of strains dis-

proportionately accurate.

The magnetic counter recording is attractive because

it makes for extreme simplicity, although it is probably

somewhat detrimental to accuracy. The continuous paper

recording offers a more tedious but more accurate method.

41.

S E C T I O N VII

CONCLUSION

CONCLUSION

In the preceding sections, the factors involved in

the selection of the different pieces of equipment have

been discussed.

The following conclusions have been reached:

(1) The mercury arc is probably one of the most

conveniently operated highly intense source of

monochromatic light.

(2) A bright pattern of concentric interference

rings with a large central spot, can be obtained

without the necessity of frequent adjustments,

if the optical system consists of an optical

flat and a lens with a large radius of curva-

ture. A prism may be substituted in place of

the optical flat to increase the brightness of

the pattern and facilitate the design of the

apparatus.

(3) A Weston Photronic cell is likely to be a

satisfactory light-sensitive device. It has a

maximum response at the wavelength of the mer-

cury line used. It is both sensitive and perma-

nent. It has practically no dark current and

can be made to give a linear response if the

42.

43.

external resistance is made small relative to

the cell resistance.

(4) A Thyratron relay circuit is to be recom-

mended if a photoelectric cell is used. If how-

ever, a Photronic cell, or any other barrier-

layer photocell, is selected, a relay such as

the Sensitrol should be preferred. Amplification

may or may not be necessary.

(5) The relay circuit can be made to operate

a Veeder magnetic counter which automatically

records the electrical impulses originating at

the photocell each time that the central spot

of the interference pattern is bright. If a

self-balancing potentiometer is used in place

of a magnetic counter, a continuous recording

of the current variations can be obtained.

The above results are intended to be of some use to

future investigators who may work on this problem. It is

clear that the survey that has been done so far is but

a small step toward the solution. It is now necessary to

go through the actual optical, electrical and mechanical

design. However, this will probably have to wait until

the equipment required is available. In this connection,

44.

it is likely that tests will be found to be an indispen-

sable check on the design.

The optical design will be concerned mainly with the

relative disposition of the light source, the interfero-

meter and the cell. It will also involve the determina-

tion of the dimensions of the lens and prism and of any

auxiliary lenses or prisms that may be required. The op-

tical design will be influenced by the size of the cell

window and will be continually guided by the conditions

of satisfactory operation of the cell.

The electrical design will deal principally with the

relay circuit and the determination of the critical va-

lue regulating the on-and-off action. In addition some

control may be necessary in the light source circuit.

The mechanical design will have to respect closely

the optical arrangement. It may be found necessary to pro-

vide a cooling system, if the light source is very hot

and the apparatus compact.

The completed apparatus, if successful, will provide

a means of calibrating strain measuring instruments, such

as Huggenberger Tensometers, with the utmost simplicity

and speed and an appreciable degree of accuracy. The ac-

curacy could be improved upon, if a continuous recording

of the photoelectric response were taken. This could be

45.

done in several ways, all of which would involve some

complication of the procedure. Whether such a complica-

tion is warranted can be determined only in specific cases.

The use of such a recording interferometer may not

be limited to the calibration of Huggenberger Tensometers.

It is likely that many other uses could be found for it

in science and industry.

46.

3 E C T I 0 N VIII

APPENDIX

47.

APPENDIX

In the following pages, technical information on

Weston Photronic cells and Sensitrol relays taken from

this Company's Bulletins B-19-B and B-20-A, will be found.

Also shown are two models of Veeder magnetic counters.

It is hoped that this information may prove to be

of some use to future investigators.

48.

T ECHNICAL DATA ON WESTON PHOTRONIC CELLS e 1

THE PHOTRONIC CELLThe Photronic cell is a self-generating "Solid" or so-called

"Barrier Layer" type photoelectric cell. This cell is distin-guished from other types in that the semi-conducting mediumbetween its electrodes consists of a dry solid, whereas the semi-conducting medium in phototubes is a partial vacuum or gas,and in photovoltaic cells is a liquid. The light-sensitive sele-nium material used in Photronic cells is mounted between twometallic electrodes, the positive electrode serving as a rigidbase and the semi-transparent negative electrode serving as anelectron-current collecting surface.

Photronic cells are entirely electronic in their action, theenergy of the impinging light radiation being converted directlyinto electrical energy without altering the structure of the cellitself. The magnitude of the current developed by the cells, forlevels of illumination normally encountered, is sufficient tooperate direct-current instruments and relays directly, withoutamplification or other auxiliary equipment. Details on cell char-acteristics, applications and use will be discussed later.

Photronic Cells have been made in three types, differingonly in details of construction and performance characteristics.The Type 1 Cell is of medium output and is the most rugged.It is capable of withstanding prolonged high intensity illumi-

nation but has fatigue characteristics which are sometimestroublesome.

The Type 2 Cell had high output per unit of light intensityand has been superseded by the Type 3.

The Type 3 Cell is the result of continued research anddevelopment. It not only has the same high current sensitivityas the Type 2 but also has less fatigue and greater permanence.It approaches the Type 1 Cell in its ability to withstand highintensity illumination and surpasses the Type 1 in sensitivityand freedom from fatigue.

The Type 1 cell is recommended for use where the illumi-nation exceeds 600 to 800 foot-candles over prolonged periodsbecause of its ruggedness and the fact that high sensitivity isnot required. The Type 3 cell is recommended for general use,such as for light, color or turbidity measurements and in par-ticular (because of its greater sensitivity) for measuring lowlight intensities. In order to provide cells having a high degreeof uniformity, the Type 3 cells are individually tested, grouped,and listed according to output and linearity. Cells speciallyselected for spectral uniformity, and paired cells matched foroutput and linearity, are also available as listed on page 12.

49.

TiCELL SPECIFICATIONS &

CELLS FOR GENERAL USE

TYPE 3 CELLS. The following table and curves in Figure 7

summarize data on five selected groups of Type 3 Photronic

cells recommended for general use. Average values of output

and of linearity, and limits within which they are selected, are

given along with their color code. Each cell is color coded by

means of colored dots on the front edge of the case. Threaded

type terminals are standard on Type 3 cells.

TABLE 3(a) (b)

Approx. Approx.Microamperes Linearity Color

Model per Foot-Candle Factor Code Prices

594RR 4.4 ± .3 .86 ± .04 Red-Red $14.00594YR 4.0 ±L .3 .86 ±: .04 Yellow-Red 12.00594GB 3.5 -- .5 .84 ±- .09 Green-Blue 10.00594BB 2.9 ± .5 .84 ± .09 Blue-Blue 8.00594YY (c) 2.6 ± .3 .92 ± .03 Yellow-Yellow 12.00

(a) The approximate current output and limits of selection are for alight intensity of 20 foot-candles and an external circuit resistanceof 200 ohms.

(b) The approximate linearity factors and limits of selection repre-sent the ratio of output per foot-candle at 200 foot-candles to thatat 20 foot-candles for an external circuit resistance of 200 ohms.

(c) Specially selected for use at high illumination and exposure toNorth sky.

SPECIAL CHARACTERISTICS. Cells can be furnished se-

lected for similarity in spectral sensitivity, matched for linearity,

or matched in pairs for output and linearity as listed below.

When ordering selected or matched cells, it is advisable to

mention the approximate range of light intensity and external

resistance that will be encountered.

Symbol Special Characteristics Surcharge

L Linearity matched within 2% as describedin Table 3 --------------------------------------------- $ 3.00

0 (e) Output and linearity matched in pairs within2% as described above-.................................... 5.00

S (f) Spectral selection ............................................... 7.50

V (g) Viscor Filter equipped....................................... 7.50

W Weather-proof housing ..........-................. ..... 15.00

(e) Limits of selection for a large number of cells will depend uponthe quantity involved and the requirements of the application.

(f) All photoelectric cells differ somewhat in spectral sensitivity.Cells spectrally selected will be within 1/5 of the normal spectralvariation.

(g) When Viscor Filter equipped, the cell output is reduced to approx-imately 40% of the normal cell output for 2700*K. light.

PHOTRONIC CELL MOUNTINGS

WithThreaded Terminals Pron

In Bakelite Case

UIWith

q Terminals Single Cell Three Cell

In Weatherproof Housing

ORDERING INFORMATIONTYPE 1 CELLS. Only one group of Type 1 cells is carried in

stock. These cells have a current sensitivity of 1.6 ± .5 micro-

amperes per foot-candle and a linearity of .75 or more for

standard test conditions defined above. These cells are not

color coded. They list at $10.00. Prong type terminals are

standard on Type 1 cells.

HOUSING. Cells are normally housed in a black bakelite

case 1" deep and 2 " in diameter. Type 3 cells are regularly

supplied with threaded binding post type terminals, and a

threaded back for mounting purposes whereas Type 1 cells are

regularly supplied with prong type terminals. Either type ter-

minal will be supplied on request at no extra cost. Cells can be

supplied in a 3 " weather-proof metal case provided with

standard " pipe connection for conduit wiring as listed above.

The weather-proof housing is recommended where cells will be

subjected to abnormal humidity, corrosive fumes, etc. Cells are

normally provided with a protecting window of glass, but when

required Viscor, quartz or other special filter glasses can be

supplied in place of the regular glass window at extra cost.

ORDERING. Example of the use of the above symbols:

Model 594GB-OS indicates that Type 3 cells having an output

of 3.5 microamperes, matched for output and linearity, and

selected for spectral uniformity at a list price of $22.50, are

desired.

CELLS FOR USE WITH RELAYS

Differently colored back plates identify Type 3 Photronic

cells for relay use. These cells are carefully selected for char-

acteristics most suitable for such application. To properly order

and identify relay cells which are also rated at 20 foot-candles

and 200 ohms external resistance, consult the following table.

Approx. MicroamperesModel per Foot-Candle Identification Prices

* 594R 4.5 Red Back Plate $15.00* 594B 3.5 Blue Back Plate 10.00

594Y 2.5 Yellow Back Plate 10.00

Selected for highest possible output. Minimum output is statedbut usually will be found higher.For relay applications not involving skylight and where highestsensitivity is not required.Processed for relay applications where the cell faces the sky orsun. This cell will operate under continuous exposure to Northsky but a water or glass filter is recommended to reduce theheating effect if continuously exposed to direct sunlight.

ho

50,

250

EXT.

225 #OH

4- 05

20G 2?

175 Y

150-

0 10 3 4 5

25 I

0/0 10 20 30 40 5

ILLUMINATION IN FOOT CANDLES

RES.-HMS100

300700

600

w

< 5000w

z 400zw

U 300

200

100

500

700

1000

1500

2000

3000

411111 1 1 1 1 1 1 1 1 1

.75

.8A.8A

III z

I I 1.W

ljYY

16RR6YR

4GB "

4BBI

50 100 150ILLUMINATION IN FOOT CANDLES

Fig. 4-Effect of Illumination and External Resistanceon Current Output of Type 3RR Cell.

(Tungsten lamp at 2700*K.)

REMS

300031020

2500 --- -- 40

/ -60

2000- 80

100

1500

__ 150

10ff :200Fy ru

u

500

fi

S.I IEXT.

500 .1000, 1500 2000

Lt4 /TI/V/,____ILLUMINATION IN FOOT CANLSL

Fig. 5-Effect of Illumination and of External Resistanceon Current Output of Type 1 Cell.

(Tungsten lamp at 2700"K.)

Lii

Liia-

0Ix

z-

IAA

U)

a-

7qI ZFVV I

Rflfl i i i i i i i i i i i

t

EXT. RES. OHMSCOLOR CODE

LINEARITY FACTOR

.90

}RR

.90

.821...------- .9 3

9 1

- 95KBBI

00 200

51.

ATTAGE OUTPUT. When cells are used with rhich require a relatively high torque for good contact,

inear output characteristic is less important than the maximumwattage output obtainable from the cell. Cells for relay use Fare, therefore, not normally selected for linearity, but areselected for high internal leakage resistance which, in additionto current sensitivity, determines the relative efficiency of thecell as an electrical converter of radiant energy. The cellsselected for relay use are listed on the basis of current sensi-tivity as shown on page 12.

The curves in Figure 14 show the microwatts output for atype RR cell for various values of external resistance and illu-mination. The values of maximum wattage output were takenfrom these curves and are plotted in Figure 15 against illumina-tion as a log-log function.

The result is nearly a straight line relationship. It is usefulin determining the external resistance for maximum poweroutput at any given illumination.

600

400

O RFOOT CANDLES

20 500

10080 1 0060

40 110

301 T- - 1

20 50-

210 28206

4 MAXIMUM WATTAGE3 OUTPUT CURVE.

0 1000 2000 3000 400EXTERNAL RESISTANCE IN OHMS-

-

zwU0LL..w

olx0

w-jw

rL

35010*Ct

300 300C-- - -40Co

250

150 - - - - - - _ _

100150- / I I

0---

50--------------__

LU 50 100 150

ILLUMINATION IN FOOT CANDLE

Fig. l1-Electromotive Force of Type(Tungsten lamp at 2700*K.)

200

3GB CeA

Fig. 14-Wattage Output of Type RR Cell at Various Values ofIllumination and External Resistance.

CL

0LID0-

4UUU

3000 -----

2000

1000

7 600 -0LLn 400 -

o 300 -Z

u3 200-

10020 30 40 60 80 100 200 300 400 600 800

ILLUMINATION IN FOOT CANDLES

ig 15-External Resistance for Maximum Wattage Output

L9.ternal Type RR Cell. 4

0

x ES

00

I I I I IILLUMINATION20 FOOT-CANDLES

27 oK0 j

N EXT.

ooz RES

100--0

0 40- --- 0

z

z4w0-n

-103 10

t 00

IE _.~ H

{ [ j tions. It also lowers the effective internal resistance and con-sequently, reduces the current delivered to the external circuit.

30 ** Where cells are mounted outdoors or subjected to moist condi-20 30 40 tions indoors, weather-proof housings are advisable.

TEMPERATURE IN DEGREES CENTIGRADE

Fig. 17-The Effect of Temperature on Current Output of Type 3Cells at 20 Foot Candles Illumination.

1In

0N~z0

'a

0.

zz

I-

ILLUMINATION2OOFOOT-CANDLES

2TOO* KEXT.RES.

300

200 EXT.------- RES.

0

sw

-10 -- 4--

7*WE

150 -o

U 10 20 30TEMPERATURE IN DEGREES CENTIGRADE

40

SAFE TEMPERATURE. Photronic cells should be protectedagainst being heated to a temperature beyond 60'C. (140*F.)since continued heating may result in permanent changes insensitivity. The cell surface absorbs infra-red or heat rays andunless care is taken, may become heated to a greater degreethan is indicated by the ambient temperature or that of theouter case.

Radiation from tungsten filament sources contains a largeproportion of infra-red radiation. When cells are used tomeasure high intensities, say 500 f.c., or more, from suchsources, or from sunlight, they should be exposed only longenough to obtain readings unless protected by a water or otherspecial infra-red filter.

INFRA-RED OR HEAT ABSORBING FILTER. In order to pro-tect Photronic cells against heating, an Aklo filter made of solidgreen tinted glass manufactured by the Corning Glass Com-pany, can be supplied instead of the regular clear glass win-dow. It removes a large portion of the infra-red radiation foundassociated with light. Since infra-red radiation produces novisibility and very little cell response, but is very effective inproducing heat, it is often desirable to remove it from the lightbeam before it reaches the cell disc. It is preferable to mount aheat absorbing filter outside the cell when possible because thefilter may become quite hot and transfer its heat by conductionand secondary radiation to the cell disc if mounted in the cell.

The Aklo filter alters the effective spectral sensitivity of thecall and, therefore, requires special correction factors whenused for measuring light from different sources. Its total trans-mission to 2700'K. light, measured as cell response, is about65%.

Fig. 18-The Effect of Temperature on Current Output of Type 3 SPEED OF RESPONSE. The cell response is so much moreCells at 200 Foot Candles Illumination. rapid than that of meters and relays used with them that it

may be considered instantaneous for such use. Actual tests

FATIGUE. Photronic cells have no dark current or drift and have demonstrated that the cell response is sufficiently rapid

are not subject to erratic or non-reversible changes. However, to record the passage of a rifle bullet through a beam of light

upon an initial exposure to a luminous source the current output incident upon the cell.tends to fall off slightly with exposure so that a short time inter-val may be required before the cell current reaches a constant LIGHTvalue. Therefore, when relatively high accuracy is desired, a Eminute or so wait is recommended before taking readings. rSEMI-TRANSPARENT- -X

Individual cells differ in the amount of fatigue they exhibit. CATHODE --The Type 3 cells have less fatigue than the Type 1 cells and t LCsome may have zero fatigue or even a positive fatigue, that is, SEMI-CONDUCTOR .x

an increase in current output. However, a decrease in output of I

1 or 2% between and 20 minutes of illumination at 100 foot METAL BASE ~candles and low external resistance is normally to be expected.I___-------------

igher values of illumination and external resistance tend toncrease the apparent fatigue primarily because of temperature1-Cross Sectional View of Photronic Cell Iut

es produced in the active surface of the cell. Cell Action. (Not to scale.)

ci L t 1

52.

-1.

r-PERMANENCE. Experience indicates that Photronic

have a long life if thoroughly protected against unfavorableexternal influences-particularly moisture, heat, corrosivefumes, and excessive applied voltages. When damage occurs,it is usually traceable to some outside influence other than illu-mination itself. Many cells that have been in use for nearly tenyears are still functioning satisfactorily.

As a rule, the Type 1 cell is recommended in preference tothe Type 3 where the illumination may exceed 600-800 f.c. overprolonged periods because it is more rugged and because thehigh sensitivity of Type 3 cells is not required.

APPLIED EXTERNAL VOLTAGE. The cell surface is not de-signed for carrying large current densities; hence, cells mustnot be connected in circuits in such a way that damage fromburning will result from external voltages. Here also the possi-bilities of high voltage arising by induction must not be over-looked.

MOISTURE EFFECTS. Experience has shown that moisturehas a harmful effect upon the cell. It may permanently damagea cell by promoting destructive electrolytic action between theconstituent parts or between the cell and its external connec-

'

'I+

-

I

if

LMODEL 721otoelectric Potentiometer

53.

MODEL 721 PHOTOELECTRIC POTENTIOMETERThe Model 721 photoelectric potentiometer is fundamentally a d-c amplifier fo

the actuating of a 10-milliampere instrument, recorder or relaying device, or agroup of such devices in series, provided their total resistance is not over 1,000ohms. The gain is such that 10 milliamperes output can be had with inputs as low

as 5 microamperes or 2 millivolts.

The instrument functions as a potentiometer in that it takes no energy from the

input circuit, since, in a manner similar to a potentiometer recorder, the input energy

is balanced out.

The device combines a sensitive mirror galvanometer, light source, photo tubes

and vacuum tubes for output, the output energy being fed back to oppose the

input. All power for operation is supplied from an a-c line, less than 40 watts

being required at 60 cycles 110 volts.

The amplified output of the Model 721 is truly linear in terms of the input and

instruments in the output circuit may be calibrated in terms of the primary quantity,which in turn may be temperature applied to a thermocouple, light on a photo cell,

and the like.

Measurement of extremely small currents or voltages are easily possible with

Model 721, and instruments or recorders can be operated at high speeds. Balanc-ing is continuous and response to change is virtually instantaneous. Descriptivebulletin available upon request. Correspondence invited.

Model 721 Photoelectric Potentiometer with two range standards and

Model 301 Indicator (portable or switchboard mounting)-- --------------------------Price $

S.PAT. T$.NT VO

RO3S A

29 MODEL 705M DEiMODEL 705 [Single Fixed Contact with

SigeFxe otctobeAdjustable Contact table Index andl Solenoid

SSENSITROL RELAYS-MODEL 705Weston Sensitrol Relays are the first commercial types combining high sensi-

tivity and high contact capacity. Positive operation is obtained on values as low as1 millivolt or 2 microamperes d-c; and 0.5 volt or 10 microamperes a-c with anexternal rectifier. Contact capacity is 5 watts at 110 volts.

Operation is as follows: The stationary contact is a small powerful permanentmagnet and the movable contact is an iron "rider" mounted on the pointer whichtravels over the relay scale. The operating torque moves the pointer into the fieldof the stationary contact. This contact then draws the movable contact in and holdsit firmly. Perfect contact is assured and chattering is eliminated. Contacts remainclosed until reset manually or with an electrical solenoid reset. The latter is mountedon the base of the relay and is arranged with 2 small arms which mechanicallypush the contact arm away from the contact when the solenoid is energized. Accu-racy in general may be considered as within 5% of the range in Types 1, 2 and 3.Types 4, 5 and 6 may be considered accurate within 10% of the range. Whenspecial characteristics are required these accuracies may not apply. See typicalscales and dimensional diagrams on pages 10 and 11.

Description of TypesType i--Single Fixed Contact. Stationary contact remains in a fixed position. Canbe supplied to make contact on increasing or decreasing values. Reset from frontor back.Type 2--Single Adjustable Contact. Stationary contact can be adjusted to anydesired operating value over either lower or upper half of scale. Provided with amovable index which indicates operating point. Adjusted by means of knob onfront of case. Reset from front only.Type 3--Double Adjustable Contact. Has two adjustable stationary contacts, onefor high and one for low values. Provided with two indexes for indicating high andlow operating points. Reset from front only. Closest that contacts can be settogether at center of scale is approximately 20 % of scale range.Type 4-Single Fixed Contact with Solenoid Reset. Supplied to make contact onincreasing or decreasing value. Solenoid is mounted permanently on back of relay,extending approximately 2 ". Designed for 6 volt d-c intermittent service. Can besupplied for any d-c or 60 cycle service.Type 5--Single Fixed Contact with Solenoid Reset and Adjustable Index. Similar toType 4 but equipped with adjustable index to indicate operating point. Turning theindex shifts the control spring similar to the action of a zero corrector. Has twoscales; one for setting index, the other for reading pointer position.IType 6-Double Fixed Contact with Solenoid Reset. Makes contact on bothincreasing and decreasing values.

When ordering Model 705 Relays, please use type numbers printed aboveand specify whether contacts are to close on increasing or decreasing values.

55.

MAGNETIC COUNTER, FORM UDDIRECT CURRENT ONLY

SPECIFICATIONSDIMENSIONS:

24" long, 3" wide, 5 7/32" high.

FIGdRES:

.166" high.;White on black back-ground.

SPEEDS:700 counts per minute.

RESET OR NON-RESETP Reset.

LuBRICATION:

Not required.

FINiSH:

Black shrivel.

WHEN ORDERING:

Specify Voltage.

THESE counters are practically the same, in action and operation,as Form US Counters, except that they will operate satisfactorilyonly on direct current. Thus they may be connected to dry cellsor storage batteries, as well as to generated current.

An eighty ampere-hour storage battery, with the time of contactof commutator or circuit-breaker adjusted to the minimum re-quired, should run the small Form UD Magnetic Counter to5,000,000 counts on one charge.

Form UD Counters can be furnished for the following voltages,6, 12, 24, 55, 110, 220.

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56o

MAGNETIC COUNTER, FORM USALTERNATING CURRENT ONLY

ACTUATED by electrical energycounters may be placed at any distancethe count is to be taken - a feature which per er ofcounters to be mounted together at a central point. Where articlesto be counted are very small, thin, or light in weight, these counters(if furnished with coils to suit) can be connected in shunt or in serieswith the device.

Form US Counters have stator cores of special dynamo electriciron, equipped with shading coils. Rotor shaft is fitted with aninertia governor. These features permit the use of alternating cur-rent of the following voltages: 55, 110, 220 (50 to 60 cycles). Atextra charge, counters can be furnished for other voltages from 12to 550.

Power consumption is approximately 15 watts irrespective ofvoltage. The voltage variation permissible is 10%.

Combined registers may be supplied on this magnetic base, seepage 20 for type referred to. Can furnish non-reset registers 6 to8 figures, see page 18.

For remote indicationof machine operations.

SPECI FICATIONSDIMENSIONS:

24' long, 3" wide, 5739 high.

No. FIGURE WHEELS:5 only.

SPEEDS:600 counts per minute.

RESET OR NON-RESET?Reset.

LUBRICATION:Not required.

FINISH:Black shrivel.

WHEN ORDERING:Specify Voltage.

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57.

S E C T IO N IX

BIBLIOGRAPHY

58.

BIBLIOGRAPHY

Physical Optics R.1.V7ood.

Principles of Optics A.C. Hardy and F.H. Perrin.

Light for the Student F. Edser.

The Theory of Light T. Preston.

Cours de Physique G6n6rale H. Olivier.

Interf6rences Lumineuses C. Fabry.

Applications of Interferometry W.E. Williams.

Photoelectric Cells N.R. Campbell andD. Ritchie.

Photoelectric Cell Applications R. C. Walker and T. Lance.

Photocells and their Application V.K. Zworykin andE.D. Wilson.

Photoelectric Phenomena A.L. Hughes andL.A. DuBridge.

La Cellule Photo6lectrique et ses ApplicationsConf6rences d'Actualit6s Scientifiques etIndustrielles. 1929 L. Dunoyer.

History of Photoelectric Cells E.W. Cowan.Electrical Enginnering Seminar,Mass. Inst. of Tech. 1942

Electrical Engineering Thesis T.B. Perkins.Massachusetts Institute of Technology 1929

Electrical Engineering Thesis F.B. Parks and B. PoehlerMassachusetts Institute of Technology 1934

Photocell Multiplier Tubes C.C. Larson and H. SalingerReview of Scientific Instruments 1940. Vol.11.p. 226

59.

Sensitive Light Meter with ElectronicAmplification Photovolt Corporation.

Review of Scientific Instruments 1941. Vol.12. p.39

Counting Automobiles on Ambassador BridgeElectronics June 1930.

New Fields for Magnetic Contact Relays A.H. Lamb (WestonElectronics Dec. 1940 Elect. Instr. Co.)

Applying Magnetic Relays for Sensitive Control A.H.Lamb (WestonElectronics Feb. 1941 Elect. Instr.Co.)

A Recording Photoelectric Color Analyzer A.C.HardyJournal Opt. Soc. of America, 1929. Vol.18. p.96

Relays and Photronic EquipmentWeston Elect. Instr. Co.

Counting DevicesVeeder-Root Inc.

Hot-cathode ThyratronGen. Elect. Review.

Characteristics of theHuggenberger TensometerA.S.T.M. Proceedings

Catalog 1941.

1940. Catalog No G 40.

A.W. Hull.1929. Vol.32. pp.213 and 390.

R.W. Vose.1934.Vol.34. Part II p. 862