216 PHILIPS TECHNICAL REVIEW Vol. 2, No. 7

A RECORDING FIELD-STRENGTH METER OF HIGH SENSITIVITY,

by M. ZIEGLER.

Summary. An' apparatus for measuring the signal strength of radio transmitters radiatingon wave lengths between 10 and 2000 m is described .. The sensitivity of the apparatus iscalibrated during measurement by means of an oscillator incorporated in it. Field strengthsfrom approximately 1 microvolt per m upwards can be measured with this apparatus.

, .Introduction

The measurement of the field or signal strength ofradio transmitters is now a matter of routine inwireless engineering and may be carried out fora variety of practical purposes. A few applicationsin which the field-strength meter has proved in-dispensable may' be quoted from the innumerableuses to which this instrument has already been put:The energy radiated from a transniitting aerialwith a known input is a criterion for determiningthe efficiency of the aerial, while the distrihutionof field strength may be explored over the areasurrounding a transmitter to ascertain the intensityof interference at, a particular point, due either to aninterfering transmitter or to an oscillating receiver,The directional characteristics of an array of trans-mitting aerials can' be investigated, as well a~ theabsolute minimum' and maximum signal strengthsdue to fading ascertained, The purpose of a meas-

'urement of the field strength may have purelya scientific interest, but in the majority of casesmeasurements have primarily a technical value,such as those enumerated above. Practical dataconcerning the power radiated from a specific,typeof aerial, as well as a close knowledge of the effects.of local geological and topographical conditions on''the propagation of waves, are esserrtial for planningand selecting the best site for a transmitting stationwhich shall operate efficiently and be economicalto construct and run.

Thè variou~ problems entailed in à measur~mentof the field 'strength a~e very diverse, and the prac-tical and theoretical requirements imposed on themeasuring apparatus 'used are widely divergent.Firstly, very weak' fields due to remotely-situatedtransmitters must be capable of measurement, andsecondly, 'measurements must be feasible in theimmediate vicinity of the transmitting aerial;in other cases again the apparatus must be used onsites to whlchit caûnot be conveniently transportedby vehicles. For still other purposefl, it may bedesirable to' instill, the. measuring arrangementspermanently jn ordef to carry out measurementsat a partienlar 'spot. of the variations in 'signalstrength with time over periods of several weeks.Furthermore, on certain occasions measurements

have to be carried out on long waves and onother occasions on ultra-short waves. '.Every type of measurement of the field strength

required in normal practice can be carried outdirectly with the apparatus described below. Beforepassing to i, description of the apparatus, theprinciple 'Oh ~hi~h the field-strength meter operatesmust be briefly outlined.

Principle of the Field-Strength Meter

A field-strength meter is composed of an aerial. in which a certain electromotive force is inducedby the field of 'the transmitter, and a voltmeter,for measuring this e.m.f.

The effective height of the aerial is of particularinterest in t~is connection, and is defined as follows:If the electric field strength for a partienlar waveis e = E'Siri.·w 't; then an e.m.f. of

v = he (1)

will be induced ip. an aerial of effective height h.If V is measured and the effective height is known,the signal .strength can be calculated. The effectiveheight of a vertical aerial with a high top capacity(fig. la) and whose length l is small as compared

FFvz~

Ia b .22~O!Sc

Fig.!. a) Vertical aerial with high top capacity (capacityaerlal), A vertical electric field.Induces a voltage Vwhich is equal to the product of the field strengthand the length of the aerial.

b) Equivalent circuit of the capacity aerial. Theinduced e.m.f. may be regarded as being connectedin series with the capacity of the aerial.'

c) By connecting a self-inductance Z in series withthe aerial, a circuit tuned to the impressed signalcan be obtained. At resonance the voltage Vz canbe many times V. '

with the wave length is l for a vertically-directedelectric vector; the e.m.f. induced by the wave willthen be V = l e. '.Adopting the above definition it is also possible

to speak of effective' height in the case of a frameaerial. The effective height of a frame aerial (flat

JULY 1937 RECORDING FIELD-STRENGTH METER 217

coil, fig. 2a) of n turns; each enclosing a surface 0whose dimensions are small as compared with thewave length, is

2nnOh = sin a, . . . . , (2)

Ä

where a is the angle between the plane of the frameand the magnetic vector. As is well 'known a frameaerial has directional characteristics. A frameaerial' suitable for use with e.g. a portable field-strength meter has 16 turns of 0.4· 0.4 = 0.16 sq. msurface, when for Ä = 300 m and a = 90, deg, ,we get h --;' 0.054 m. This height is considerably'less than the effecti~e height of a vertical aerialof .several meters.

Q>A 8

(E}I

b caFig. 2. a) Frame aerial (inductive aerial). Á magnetic field

ofunit intensity, whichis perpendicular to the plane• .' " 2:n;nOof the frame aenal, mduces a voltage V = -,-.1.-

where n is the number of turns, 0 'the area of theframe, and .1.the wave length., . \.

b) Equivalent circuit of the inductive aerial. Theinduced e.m.f. may be regarded as connected inseries with the self-inductance of the frame aerial.

c) By connecting a capacity Z in seriesWiththe frameaerial, a circuit tuned to the received signal can beobtained. At resonance the voltage Vz can bemany times V.

.The equivalent circuits of the capacity aerial andthe frame aerial with an assumed induced e.m.f. arerepresented in figs. ib and 2b respectively.

The problem therefore resolves itself into ameasurement of the voltages V indicated in figs.lb and 2b. But only the terminals A and Bareaccessible. Theoretically, it is sufficient to measure .the potential difference between A and B with a 'voltmeter, whose impedance is high compared tothe reactance of the vertical aerial or the frame,aerial. Although it is in fact not imperative, theresonance principle will be used in this case, and acircuit tuned to the signal under measurementformed by connecting an inductance or a condenserin series, the selective and amplifying characteris-tics of this circuit offering important advantages(figs: le and 2c). Either the current I which flowsthrough, the circuit can be measured, or the po-tential difference Vat the series-connected imped-ance. If at the same time the resistance of thecircuit is ascertained, Vcanhe directly calculated, for:

" • T: V = Ir = Vz -./ .. 1 ,Z

What type of aerial is provided with the field-strength meter?

The requirements of an aerial for use in con-.junction with the field-strength meter are that itshall have a specific ascertainable effective heightand at the same time be 'transportable. A frame,aerial automatically satisfies these requirements, .while a capacity aerial would assum~ àn unfavour-able form, for instance thöse mounted On a short :

. J

vertical pole fitted with a 'horizorit~l plate at thetop would have a capacity. with respect to earthwhich is high as compared with that between thevertical part and earth. It is thus reasonable toconsider a condenser aerial, but the effective heightof an aerial of this type cannot readily be madeappreciably greater than .that of a' standard frameaerial.A further .difficulty with the capacity aerial is

that it cannot he so easily tuned. as a frame aerial,for a variable selfinductan~e is required for thispurpose' (fig. le) which ~annot be realised to suitall cases. In the -more n'ormal circuit shown in'fig. 3 the gain in the voltage due to the field is

. .r, ,22:105

Fig. 3. A short vertical pole on which a horizontal plate ismounted, is an ,example of a mobile capacity aerial of well-defined characteristics. The effective height. is equal to thedistance between the plate and the top of the metal boxenclosing the measuring apparatus.

much smaller than the resonance amplificationobtainable with the frame aerial. Since, moreover,a frame aerial has directional characteristics ascompared with a capacity aerial, which in generalwill prove ~n advantage, as a standard it is to bepreferred to a condenser aerial. .However, where an aerial with a great effective

height is essential for th~ purpose of measuringvery weak signals, and if the directive action iseither dispensable or even undesirable, one or otherof the capacity aerials may he employed. Anyaerial system, whose effective height is unknown,and which is coupled in any manner to an am-plifier with an indicating instrument (e.g. an aerialinstalled in a motor car), can always be calibratedas a whole:1) By comparison with a standard aerial, -and2) By measuring the theoretically-known field of

an accurately-calibrated testing transmitter.',

218 PHILIPS TECHNICAL REVIEW Vol. 2, No. 7.

Important requirements which the field-strength equal to the calibration voltage, the deflection ofmeter must meet the voltmeter will be reduced or increased in the

\

A field-strength meter has to meet the followingrequirements:~) High sensi ti v i.t y in order to measure weak

signals,b) Hi g h s el e c t i v i t y to permit. weak signals

to be measured even when interference isproduced by more powerful stations on anadjoining frequency;

c) Con sta n t ch a r act er iSt ic s, . in ot~er to; avoid the need for 'frequent recalib~ati~,h ~n thelaboratory. -'.To meet requirements a) and b) a.. sensitive

amplifier is used containing a number 'of ampli-fying valves in cascade and various tuned circuits,by means of which the induced voltage is amplifiedfor measuring purposes. On the other hand thecharacteristics of the apparatus cannot be madeconstant over long periods by any simple anddirect method, for the sensitivity varies with theage of the valves and is to a great extent determinedhy the accuracy of tuning, etc. For this reason, theapparatus has been so designed that the sensitivitycan be adjusted to the required value as ~ecessary.

The scnsitivity of the apparatus ca:.::'-he checkedvery easily, viz, by observing the effect of a knowne.m.f. which has the same frequency as the fieldunder measurement and which is connected inseries with the fra:ine' aerial as' ~dicateä ill fig. 4.

corresponding ratio. By this comparison methodthe reliability of measurements made is mainly

.c: f ~---J' A

~Vi

Fig. 4. Principle of the field-strength meter. The' voltage Vinduced in the frame aerial is compared with the calibrationvoltage Vi' To measure low voltages, the voltage which isproduced by V or Vi at the condenser is amplified by an am-plifierA. :

dependent on the accuracy 'of the calibrationvoltage.

Electrical Layout of the. Apparatus

In fig. 4 the general principle of the Philips field-strength meter is illustrated and infig. 5 a detailedlayout of the various component ~ircuits. Referringto this diagram we shall first investigate how thesignal under' measurement produces a deflectionon !he measuring instrument. ,The voltage of frequency P which is induced at

the terminals of the aerial circuit, as already des-cribed above, is passed through a selective' ampli-fying stage H and then to the mixer valve M.The intermediate-frequency signalof frequency' Pm

Fig. 5. Diagrammatic layout of the signal-strength meter. 0 Calibration oscillator tofurnish the calibration voltage Vi.= IR. A' Aerial circuit; H High-frequency amplifier;. M Mixing valve; 0(. Heterodyne oscillator; VAttenuator; MF Intermediate frequencyamplifier; D Detector; Ti Indicator triode, 'which furnishes the current for the instrument;Tc monitor triode for the current passed to the headphones. -

Not only can the sensitivity ofthe voltmeter, whichin this case includes the amplifier and the ins~ru-ment, he checked by this method, but also the gainwhich occurs in the resonance circuit formed ofthe frame aerial and the condenser. The calibrationvoltage produces exactly the same effect as an inducede.mJ. of the same magnitude 1), and will thereforealso give the same deflection on the voltmeter.If the induced e.m.f. V under measurement is not

which is produced by interaction with the auxiliaryvoltage of frequency P + Pm originating from theoscillator Oh, is filtered out and is passed through avariable attenuator V of which further details aregiven below. Subsequently, the signal is amplified

1) This is not quite correct, since the frame aerial is neverentirely devoid of capacity and the induced e.m.f. is notconcentrated at .a single point. But deviations from theseideal conditions can be neglected in practical measurements.. . ~ . ". ,,',

-:' ,",

JULY 1937 RECORDING FIELD-STRENGTH METER.,.

,.

219

. "by two stages M R tuned to the frequency Pm andthen rectified in a diode detector D. The directvoltage now produced is passed tó· the grid of atriode Ti of which the anode current flows througha milliammeter. This triode, which 'we shall termthe indicator valve, is so adjusted that its anodecurrent is 5 milliamps when no signal' is impressedon it. A rectified signal makes the grid .of the triodenegative with respect to the cathode," so that theanode current drops and produces a deflection ofthe pointer from the no-signal reading. The totalamplification of the signal under measurement mustbe high enough to give .a deflection' ~asy to read,yet not so high that the anode current of the-indicator valve becomes zero. The'~amplificationstage is also controllable in addition to the atten-uation stage, by means of the mixer "valve andthe two intermediate-frequency valves, so as topermit accurate adjustment.

The ratio between two signals of the same 'fre-quency can be directly read off 'on thé Instrument.without altering the adjustment. This ratioicannotbe determined if it is greater than 10, but with tlieattenuation stage, the amplification .~f one, signalcan he reduced or increased in a' definite ratio ascompared with the amplification of the other:The maximum attenuation is 1000, so.that voltages.in a ratio up to 1 : 3000 can still be compared with. each other.

Naturally, at all practical adjustments, the am-_.plification ratio must be independent of the am-plitude. Both the maximum amplification in eachstage and the method by which the amplificationis varied are so calculated that the maximum de-flection is obtained on the instrument before anyother part of the apparatus becomes overloaded.

/,z24a8

:tFig. 6. Asymmetrical frame-aerial circuit, If one end of theaerial winding is earthed, the electric field will cause currentsto flow,which, independently of the direction of the frame aerial,will produce a voltage at the terminals of the amplifier. Thearrows indicate the currents which are produced by the ver-tical electric field between the frame aerial and earth. It isseen that part of these currents flow through. the condenser C,'and hence produce an alternating voltage at the input ter-minals of the amplifier.. . . .

We shall now give a description of the variouscomponents of the field-strength meter.Aerial circuit: Frame aerial. If. a frame aerial is. . "connected up as shown diagrammatically in fig. 4,the capacity of the aerial and that of the apparatuswith respect to earth (fig. 6) will; as a whole,exhibit the same characteristics as a' capacityaerial, so that a different type of signal will begenerated that would he deduced f~om equation (2).This follows from the fact that the signal does notfall to zero, even when the plane of the frame isparallel to the magnetic vector. This undesirablecharacteristic, the socalled "aerial 'effect", may beeliminated by, mounting the frame symmetrically,when the currents generated by the capacity effectbalance each other (fig. 7). .

r--III

~,I

ol.-rI

, I,.1.,,,\-,

t/

.228oa

Fig. 7. Symmetrical frame-aerial circuit, Since the centre ofthe aerial winding is earthed, .the currents' induced by theelectric field counteract each other, No current flows throughthe condenser C. .

The frame aerial is tuned with a variable con-. '

denser C. The measuring equipment is providedwith a series of interchangeable frame aerials.Reception with capacity aerial. For reception witha capacity aerial, the rotatable frame' holder can hereplaced by coils which automatically make thenecessary contacts, so that a circuit of the typeshown in fig. 8 is obtained. Each coil is fitted with

Fi.g 8. Connection of the aerial circuit when using the signal-strength meter with a capacity aerial.

a terminal to which the aerial IS connected; theapparatus is earthed.The calibration. signal. The potential difference atthe resistance R (fig. 5) is used for calibration and

220 PHILIPS TECHNICAL REVIEW . Vol. 2, No. 7.

is produced by an alternating current I of therequired frequency furnishe'd by a special oscillator(0). As already indicated in fig. 4, the 'calibration, . ,

voltage is in series with the frame aerial. Theresistance is only of the order of lj8 ohm, so thatfor I = 8 milliamps, Vi , 1 millivolt. The sensitivityof the thermo-couple which is used for currentmeasurement and the resistance R are independentof the frequency, hence also the calibration voltage.The oscillator through its five adjustable ranges 'furnishes all frequencies between 150 and 30 000kilocycles and is so rated that the maximumcurrent flowing' through the thermo-couple willnever overload the couple, This maintains thecharacteristics of the thermo-couple constant andalso raises the reliability of the apparatus as anabsolute measuring instrument.

High-frequency Amplifier. The function 'of thisamplifying stage, which consists of an amplifyingvalve with tuned anode circuit, is twofold, viz:

1) To raise the high-frequency selectivity in orderto' suppress those signals with a frequency')I+2 ')Im which may give rise to a disturbing inter-mediate-frequency signal, and

2) To make the ratio of the signal strength to theinterfering noises as satisfactory as . pOssible.By using a high-frequency amplifying valvewith very low mush the noise level can bereduced to the 'unavoidable thermo-electricfluctuations in the first circuit. Even when usinga small frame aer~al a signal due to a field ofonly a few microvolts per m will still be 'suffi-

'T' 'ciently audible above the mush.

Mixer, Valve and Auxiliary Oscillator. An ordinary~er hexode is used as a mixing. valve, and aspecial triode which has five switched wave-lengthranges similar to. the calibration oscillator is con-nected up as an oscillator. The tuning condenseris coupled mechanically to the condenser in thehigh-frequency stage, so that for tuning the ap-paratus only one other knob in addition to thatfor the aerial condenser has to be adjusted. The gain'in the mixer valve is regulated by means of thevariable negative grid bias.

Attenuator. Tlie attenuator V is coupled to themixer valve by a pair of circuits tuned to theintermediate frequency signal. By. means of aswitch the attenuation factor can be adjusted. asrequired to 1, 3, 10, 30, 100, 300 or 1000. Theattenuator consisting of wound wire resist anceswhich have been very carefully calibrated, isbalanced so that the input and output resistanceshave exactly the same values at all adjustments. ,

The attenuated .signal passes to the first inter-mediate-frequency amplifier through two coupledtuning, circuits.

Intermediate Frequency Amplifier. This amplifierhas the following components and characteristics:Two amplifying valves, four tuned circuits, and.controllable gain by variation of the negative' gridbias. In addition, the grid bias of both valves canbe automatically controlled, by the impressedsignal, such that strong signals are amplified lessthan weak signals, which' may be desirable whenrecording signal strengths subject to 'markedfluctuation.

Rectijication: The amplified intermediate fre-quency voltage is passed to the two diodes of a·duo-diode-triode. One of the diodes serves ex-clusively for generating a direct voltage for theautomatic gain control just referred to, the wholeor part of this voltage being applied to the gridof the intermediate frequency amplifiers. Rectifi-cation by the othe~'diode furnishes a low-frequencyalternating voltage with a D.e. voltage component,the hitter being passed to the indicator valvedescribed above. The alternaring voltage is passed

. to the triode grjd 'of the duo-diode-triode. Receptionc~n be followed audibly by means of headphonesconnected to the anode circuit of this triode.'

Indicating Instrument. The milliammeterwith arange of 5' milliamps, which gives readings' of the·anode current, is connected up in such a mannerthat the pointer is to the right of the scale when nocurrent is flowing. When a current öf 5 milliampsflows the pointer -will . give a reading to, the leftof the scale opposite the scale zero, In the absenceof a signal, the anode current can be adjusted to thecorrectvvalue by regulating the potential differencebetween the cathode and the grid. The scale hasbeen calibrated by experiment in s'uch a way thatwith a constant sensitivity, i.e. when not using theautomatic gain control, the deflection on the in-strument is proportional to the input signalstrength. 'This division of the scale is determined principally

by the la -Vg characteristic of the triode, so thatit is important for the operating characteristicsof this valve to remain constant. The anode currentis, therefore, maintained automatically constantby means of a neon lamp and is thus unaffected hyany', fluctuations ~ the voltage supply. A second.instrument, e.g. .a recorder, can be connected inseries with the ...milliammeter incorporated in the

~ .•.• "_ .01., ~.~'",~" r~ _... •

apparatus. ,., .',.:,"~ ':":.' . ,Th~. field-strength meter with frame aerial and

battery box is shown i~ fig. 9. I

JULY 1931 RECORDING FIELD-STRENGTH METER 221

Variable Selectivity. Although the meter has tellcircuits, the selectivity has not been made exces-sively sharp, since the eight intermediate-frequencycircuits are connected in pairs as band filters. Thecoupling of two pairs of these circuits has beenmade variable, so that the selectivity can be in-creased by making the coupling looser.

Current Supply. The apparatus is designed forconnection to either a direct-voltage supply (bat-teries) or a 50-cycle A.C. mains supply.

Fig. 9. Philips' field-strength meter with battery box. Theapparatus is equipped with a frame aerial for wave lengthsfrom 28 to 75 m. On the left of the front panel is the meterfor the calibration voltage; on the right is the meter whichindicates the output current. The large circular tuning scale isprovided for frequency calibration in each of the five frequencyranges. Above and to the left of the tuning scale is the knobfor the attenuator.

Exemples of various measurements

The application of the field-strength meter isillustrated by the following practical examples ofits use;

a) Field-Strength ofa Local Transmitter. One of themost important uses of the field-strength meteris for measuring the field distribution in the areaabout a transmitting station. The apparatus des-cribed here is particularly suitable for this purpose,since it is capable of measuring signal strengthsover a very wide range of values.Assume that the signal strength of the Hilversum

station, transmitting on a wave length of 300 mwith an aerial output of 15 kilowatts, has to bemeasured in the neighbourhood of Muiden. 'As already stated above, the effective height of

the frame aerial provided for this wave length,when suitably orientated, is 0.054 m at maximumsignal strength. By accurate tuning and satis-factory orientation of the frame aerial, and usingan attenuation of e.g. 1/30, without automaticgain control, the pointer can be adjusted to read5 by regulating the amplification. The frame isnow turned into the position corresponding to theminimum signal strength (there will remain aresidue of e.g. 1 per cent), and the calibration os-cillator is switched in. After tuning and adjustingthe current at the low resistance to 8 milliamps(calibration voltage = 1 millivolt), a reading of6.5 is obtained with an attenuation of 1/10, allother adjustments remaining unaltered. Hence theeffective value of the signal originating from thefield radiated by Hilversum IS

30·5Veff = 1· = 2.3 mY.

10·6.5

The effective value of the field strength IS thus

2.3-- = 42.2 mY/m.0.054

Where the field strength is to be explored over alarge area, the field-strength meter can be installedin a motor van. It is then an advantage to use, inplace of the frame aerial, a vertical aerial 0.5 to 1 min height, which has no directional characteristicsat all and which does not have to be taken downwhen moving from place to place. The absolutecalibration of the mobile field-strength meter iscarried out with a field strength previously exploredby a frame aerial by the method described above.b) Signals -radiated from radio receivers. It is

essential to suppress as far as possible the radiationof signals from auxiliary oscillators in superhetero-dyne receivers. in view of the mutual interference

222 PHILIPS TECHNICAL REVIEW Vol. 2, No. 1

which they cause in reception. In a particular case,it was stipulated that the field radiated by a re-ceiving set equipped with a particular aerial shouldnot exceed 5microvolts per m at a distance of 25 m.

To investigate whether a particular set satisfiesthis condition on a wave length of 30 m, a frameaerial is used which is adapted to this wave lengthand has two annular windings of 40 cm diameter.The effective height in the optimum position is0.0525 m. The field radiated by the receiver underinvestigation is so small that the calibration signalis much more powerful than the induced signal,so that the frame aerial before calibration doesnot have to be turned into the position for whichthe received signal has a minimum strength.

very great distances has been the subject of system-atic investigations during recent years. Fig. 10reproduces an oscillogram of the signal strengthof the L.R.I. transmitter at Buenos Aires, whichwas recorded at Eindhoven during the night hours.

For this measurement a vertical aerial 12 m inheight was used, which owing to the lack of topcapacity had an effective height of only 6 m. Themeasuring equipment was supplemented by arecording milliammeter. As the -automatic gaincontrol was in circuit, the deflection of the pointerincreasèd more slowly than the intensity of thereceived signal; this is shown in fig. 10. In caseswhere the field strength is expected to show verylarge variations, the amplitude can be controlled

.22437

11- 'r-30- L.R.f. Buenos Aifles10- 50 kW. 1070kç/sec9-8- I

7-'r--20- I

1 }I

6-5-- 'r-12,3 il"4-v=to3- m I. E2- 1--6

1111'r--31-1--2-

0-'--0

pV

19 23 27 31 3'-., 39 43 47 51 55Fig.' 10. Oscillogram of the field strengths of the L.R.l. transmitting at Buenos Aires,recorded on December 15, 1935, at Eindhoven. The scale 0 to 30 was obtained by plottingthe deflection for calibration signals of 1 and 0.5 millivolt using different settings of theattenuator. A station with a field strength of 450 micro-volts per m and about the samefrequency thus gives a deflection of 12.3 ,= 4.5 micro volts per m when using an amplifi-cation 100 times smaller than employed in the according; this deflection gives the absolutecalibration of the scale.

The amplification is so adjusted that a readingof 10 is obtained with a calibration voltage of 500microvolts and 1/1000 attenuation.

After switching off the calibration signal, adeflection of only 3.1 is found for the inducedsignal without attenuation. The signal strength istherefore

Veff = 500·10·1000

0.155 fLV.

and the effective value of the field strength is:

0.155Eeff = 0.0525 = 2.95 fLV/m,

c) Recording the signal strength of a distant trans-mitter.

The transmission of electromagnetic waves over

still more effectively than was done in the presentcase.

The division of the relative scale from 0 to 30was carried out with a constant calibration signaland the atteIiuator, by keeping the adjustment ofthe automatic gain control unaltered and varyingthe intermediate 'frequency signal in the ratios of1, 3, 10, 30, etc. and then determining the deflectionon the oscillogram. The scale value in microvoltsper m was determined by picking up a stationtransmitting on approximately the same frequencyand having a constant field strength, the latterheing measured by the method described under a)above.If no interference is experienced in reception,

field strengths of less than 0.1 microvolt per m can

JULY 1937. .

RECORDING FIELD-STRENGTH M;ETER 223

REVIEW. OF-RECENT SCIENTIFIC PUBLICATIONS OF THE N.V. PHILIPS'GLOEILAMPENFABRIEKEN

be measured by this method. In the recording stripreproduced, the interference levellay between 1and. 2 microvolts per m. r. ,

Use of the Apparatus as a Voltmeter. In conclusion, reference should be made to ~n

important use to which this meter can be put, viz., .as a selective v6ltmete:.: with adjustable sensitivityand high aperiodic irïput impedance for measuringsinusoidal alternating voltages. After removing theframe aerial or aerial coil, the alternating voltage

. 'to be measured is applied to the grid and cathodeof the first amplifying' valve, using the specialterminal provided for' this purpose. With a switchthe unknown voltage .Vx can be compared to the

No. 1172: M. J. Druyvesteyn and N. War-mol t z: Ein neuer Dunkelraum in derNähe ein~r·. Glühkathode in eine~Bogenentladung (Physica 4, 51 - 68,Jan. 1937).: .

A new dark space round the oxide-coated cathodeof an are discharge, which has already been brieflydescribed in Abstract N~. 1117, is discussed indetail in this paper. :This. dark layer is about 20times the thickness of. the space charge sheathclose to the cathode and occurs at current densities, above 0.1 amp per sq cm 'ilnd at pressures between10-2 mm and 3 mm. At lower pressures the new darksheath disappears and a bright sheath with the same, dimensions appears in its place. Starting from thecathode, a' sharp increase' of, the concentration anda decrease of the mean energy of the fast electronsoccur ..at the boundary between this layer and thebright part of the discharge. This may be accountedfor by a scattering of' the fast electrons whichleave the boundary of the, space charge' sheathclose to the cathode. 'This scattering increases withincreasing current. A dark or bright sheath is ob-served according as the increase in concentrationof the fast electrons outside this layer or the reduc-tion in mean energy of these electrons has thegreater effect on t!Ie, emission of light. . .

No. 1173: . K.. F,. Ni e s s en. Über die Wirkungeine~~vertikalen Dipolsenders auf ebe-

. :ner Erde in einem Entfemungsberèichvón der. Or!inung' einer Welle:n,länge(Ann. Phys., 28; 209 - 224, Jan. 1937).~..

.-

voltage' Vi of the calibration oscillator (fig. 11).The sensitivity of the instrument as voltmeter isapproximately 1millivolt .

"vVx

Fig. 11. Ciréuit using the meter as voltmeter. The ,unkno~ ,. voltage V", is compared to the voltage Vi of the calibrationoscillator. '

Formulae which are sufficiently accurate forapplication up to distancés of the order of a wavelength are deducèd for the' intensity of the horizo~tal. componentiof the magnetic field and the ve~ti~àlcomponent Of the electric field of a vertical dipolet~ansmiitér.· These formulae are then simplifiedfor types óf soil and wave 'lengths for which theSom m ~r fe I-d numerical distance is small com-,pared with unity.

No. 1174: .H. ·C. Hamaker: A general theory of"lyophobic' colloids. n. Rec. Trll;v."chim, Pays Bas, 56, 3 - 25, Jan. 1937).

Continuing the analysis reviewed. in AbstractNo. 1154" a .fonImla is deduced which gives' thetotal energy' of, interaction petween two colloidalparticles; expressed approximately as a functionof the dis'tànc.e 'apart of the particles; it also con-..tains as" independent parameters the charge E of acolloidal particle and the electrolyte concentration c..In an E" - c .diagram the different states of a colloidcan be represented as a function' ~f E and c. Avariety of phenomena, such as peptisation, rever-'sible and irreversible flocculation and thixotropy,can be analysedin a clear and simple way by meansof the curves in the E • c diagram. .

No. 1175: J. Á. M. van Liempt and P:Le y den s:· Die Farbenwiedergabeb~im '.Photographieren mit Neonlicht(Rec. Trav .. chim. Pays. Bas, 56,.2,6 ~ 28, Jan. 1937) .

. .Colour reproduetion in photography with neonlight is investigated for different negative materials;