352 OCKENDEN AND GALL: ELECTRICAL MEASURING INSTRUMENTS

(15) TAGG, G. F.: "The Law of the Moving-Iron Instrument,"Journal I.E.E., 1943, 90, Part II, p. 73.

(16) DRYSDALE, C. V.: "Theory of the Force or Torque of Soft-Iron Electrical Instruments," ibid., 1943, 90, Part II, p. 79.

(17) HOBSON, A.: "Instrument Transformers," ibid., 1944, 91,Part II, p. 147.

(18) GALL, D. C : "A Direct-Current Amplifier and its Applica-

tion to Industrial Measurements and Control," ibid.,1942, 89, Part II, p. 434.

(19) HARTSHORN, L., and WILSON, W.: "An Electrical MoistureMeter," ibid., 1945, 92, Part II, p. 403.

(20) SHOTTER, G. F., and HUTCHINGS, E. E.: "Potential Indi-cators for Live Conductor Detection," E.R.A., 1940,F/T133.

Part 2. SCIENTIFIC INSTRUMENTS

By D. C. GALL.

During the last six years, the progress of scientific instrumentshas shown general tendencies in certain definite directions.Electrical methods of measurement extend into nearly every realmof physics and enable many measurements of entirely non-electrical quantities to be made with greater facility and accuracythan by more direct methods. The extension of the electricalmethods has come about very largely as the result of the use ofelectronic devices, particularly photo-electric cells, cathode-rayoscillographs and valve amplifiers.

The last six years have also seen both the range of r.f. measure-ments extended into the centimetre wavelengths and the adventof a great many new types of electronic valves for use at theseextremely high frequencies. Thermocouples for use at ultra-high frequencies, and crystal rectifiers of minute dimensionsand diode rectifiers for centimetre-wave frequencies are nowavailable for use in measuring apparatus.

CHEMICAL ANALYSISAn example of the extension of electrical methods to non-

electrical measurements is the wide application to chemicalanalysis, which is now being carried out by electrical means.The principal instruments which have been developed for thispurpose are the polarograph, absorptiometers for measurementsof absorption in the infra-red, visible and ultra-violet ranges,and fluorimeters which measure the excitation of visible radiationwhen ultra-violet is absorbed by certain substances. In addition,many new instruments have been developed for the measurementof pH-values, particularly by means of glass electrodes.

MEASUREMENT OF STRAINThe development of the resistance strain-gauge has facilitated

the measurement of very localized strain on every type of machineor structure. The device is suitable for both static and dynamicmeasurements, and has played an important part in aircraftdevelopment and vibration studies. The gauges, which varyin size from a few millimetres up to 10 cm, are made of fineresistance wire usually of about 0 001 in diameter. The wirecoil or grid is cemented to a thin paper backing with its endsbrought out to two terminal wires or strips. The complete unitis cemented with some quick-drying preparation to the surfaceof the member in which the stress is to be measured. A bridgecircuit is formed using the stressed gauge and another gauge ofexactly similar type located near but not stressed. This com-pensates for temperature changes. The other arm of the bridgecompletes the circuit and contains a variable element by whichresistance changes of a few parts in a million can be balanced.The resistance changes by about twice the strain of the surfaceto which it is cemented. Strains from one part in a million to0-5% can be measured with great facility.

STABILIZERSOne of the most notable devices which has been developed

to facilitate accurate measurement is the voltage stabilizer.

These are made for both a.c. and d.c. circuits and overcomemany of the difficulties which were previously encountered withfluctuating sources of supply. They work upon various prin-ciples, mostly electronic, and are now manufactured by a numberof different firms.

MICA CONDENSERSThe manufacture of silvered-mica condensers has shown a

great advance in quality, and these are now made in a formsuitable for precision capacitance standards. The advantage ofthe silvered-mica condensers over the hand-laid tin-foil andmica condenser is that the intimate contact between the silverand the mica results in a low temperature coefficient and a highdegree of stability, and as the capacitance of each unit is inde-pendent of external pressure, the stability of a bank of plates isgreatly improved, since changing pressure in no way affects thespacing of the condenser plates. In the condenser assembledwith separate foils, the capacitance is affected by the pressurewith which the plates were clamped together. It is importantthat the silver film be quite continuous and highly conducting.It is in this respect that most advances have been made.

AMPLIFIERSBoth a.c. and d.c. amplifiers have shown great advances.

A.C. amplifiers, which are widely used for a.c. bridge measure-ments in conjunction with cathode-ray oscillographs or tuningindicators, are supplied in complete form by a number of manu-facturers. They fall into general classes and can be made tocover a very wide frequency range with tuned or untuned circuitswith controllable amplification, capable of dealing with a fewmicrovolts input with gains of up to 120 db, with good stability.In the class of d.c. amplifiers, a number of purely electronicamplifiers have been developed, which depend either uponinterrupting the d.c. input and amplifying it as alternating current,afterwards rectifying it and using negative feedback for stabiliza-tion, or upon non-linear input circuits which are balanced foralternating current, but are unbalanced by the applied directcurrent, thereafter following conventional amplifier lines. Afurther d.c. amplifier has been developed, using a galvanometerand photocell input, and giving a rectified a.c. output propor-tional to the applied input. All electronic devices require arelatively large voltage to operate them, and when d.c. voltagesof the order of a few microvolts or less have to be amplified, thegalvanometer is still the most sensitive form of detector. Photo-cell amplifiers have been developed in which one galvanometerunbalances a pair of photocells and the outcome of this un-balanced circuit is observed on a second galvanometer. Thisavoids all valves and other complications; amplification up to asmuch as 2 000 times can be obtained in this way. D.C. ampli-fiers operating recorders are now available giving continuous inkrecords of a few microvolts with relatively high input resistanceor alternatively recording current as low as 10~8 amp with low

OCKENDEN AND GALL: ELECTRICAL MEASURING INSTRUMENTS 353

input resistance. One amplifier has been described giving a full-scale record with an input of only 0-2 microvolt. This employsa double galvanometer. Thus amplifiers are used for recordingtemperature, radiation, sound, magnetic effects, conductivity andmany other physical phenomena which were previously beyondthe range of the continuous ink recorder.

STROBOSCOPIC DEVICESThere have been very considerable developments in strobo-

scopic measuring apparatus, particularly in the direction ofhigher-intensity neon lamps, which are now made by a numberof firms. This stroboscopic apparatus is often operated fromvalve oscillators of variable frequency, but it can be coupled toany standard source of frequency, such as a tuning fork, orcontact on a machine of which the moving parts have to beobserved. The flash time of these lamps has to be of the orderof microseconds if high definition is to be obtained, and lampsbehaving in this way are now available.

VALVE VOLTMETERSValve voltmeters for the measurement of small a.c. voltages

over a very wide range of frequencies have been developed ingreat variety by a large number of firms and applied to manyspecial purposes where the voltage across a circuit is a measureof some quality, such as the dryness of a fabric or timber orgrain, and the voltmeter can thereby be made to indicate themoisture content, or some other property. There is still difficultyin obtaining stable valves for the more sensitive types, such as areused in the low-voltage range with reflecting galvanometerswhere the scale length allows reading deflections to a high degreeof accuracy. Balanced circuits are used, the variation in onevalve tending to cancel those in the other. The performance ofthis type of precision valve voltmeter is still limited by theinstability of the valves. One interesting type is used in irontesting in place of the purely electrostatic instrument. It consistsof a single- or double-stage valve amplifier operating an ordinarymoving-iron a.c. voltmeter. Negative feedback from the volt-meter circuit raises the input impedance to several megohms with-out loss of accuracy. The robust a.c. voltmeter thus becomes ahigh-impedance low-loss circuit.

TIME BASESMany developments in special time-base mechanism have been

made for investigating such things as switching phenomena anddevices for selecting any point on an a.c. wave. One devicegives a sequence of operations timed electrically to operatemechanical apparatus, through relays. Any selected differenceof the order of 10~4 to 10"1 sec can be set between operations.Another type is designed to close a single-phase circuit withcurrents up to 50 000 amp at 11 kV at any point in the wave

' with a time accuracy of ± 2D at 50 c/s. Many other devicessuch as impulse generators involving accurate timing have beendeveloped for the investigation of switchgear problems whichhave obviously much wider uses.

DffiLECTRIC MEASUREMENTSInvestigations in dielectric phenomena have introduced many

new methods as the ranges of frequency and voltage have ex-tended. These include the resonant type of dielectric tests, inwhich the losses are determined from the shape of the resonancecurve, accurately up to 150Mc/s and less accurately up to3 000 Mc/s.

HARMONIC ANALYSERSHarmonic analysers have shown considerable development.

The direction has been towards great speed in obtaining results

from records or from the wave itself. The principles employedare all based upon Fourier analysis, but great ingenuity has beenused to apply the theory and to obtain the components offrequency without calculation. A number of electrical calculatingbridges have been developed by which operation of the dials of acomplex network solves certain types of equations which areotherwise very tedious to solve.

NON-LINEAR CONDUCTORSThere have been a number of very interesting developments

in connection with semi-conductors of unique electrical pro-perties. These materials are usually sintered or built up frompowders, use being made of their peculiar non-metallic conduc-tion. One such material is Metrosil, which has a very steepnegative voltage/current characteristic, so that, while actingalmost as an insulator at low voltages, it becomes a very goodconductor at higher voltages. In this way it can be used forspark suppression at contacts and for many other purposeswhere a non-linear conductor has special application.

In the same class can be considered such materials as thethermistor, which has a very large negative temperature/re-sistance coefficient, and can therefore be used for temperaturemeasurement and control, and for many other purposes wherethis principle is essential. The temperature coefficient of thismaterial is such that its resistance halves with a 20-deg tem-perature rise above normal room temperatures.

SOME OTHER DEVELOPMENTSMagnetometers have been developed for measuring and

recording. In one case use is made of two lengths of Mumetalwire forming part of an a.c. bridge. The bridge is unbalancedby the magnetic field. Fields of the order of 10~3 gauss can bedetected. In another type a suspended magnet is held inequilibrium by a Mumetal inductor bar. The magnet is part ofa photo-electric amplifier system with magnetic feedback givinglinear output proportional to the magnetic field measured.

The application of photocells to scientific measurements isso wide that there is hardly a branch of measurement into whichthey have not penetrated, either as part of the measurementdirectly, or indirectly as part of the associated detecting apparatus,such as amplifiers or temperature-controlling devices associatedwith the measurement.

High-frequency a.c. bridges have shown a steady developmentin accuracy and in frequency range, and bridges are now avail-able for the measurement of capacitance and losses up to60 Mc/s.

Standard condensers, forming part of other measuring equip-ment, have shown a steady development in the direction of betterinsulators and very fine mechanical construction. For use athigh frequencies and high voltages, condensers sealed in glassand evacuated have been developed. This enables much highervoltage gradients to be used between the condenser plates; italso keeps them entirely free from dust.

The developments in portable cathode-ray oscillographs havebeen very great and highly specialized for certain purposes, suchas radar. For general laboratory purposes, cathode-ray oscillo-graphs with self-contained amplifiers are now available withfrequency ranges from 0-1 c/s to 4 000 c/s in the one amplifier,and up to several megacycles with other types of amplifiers.Photographic records can be made of a single sweep at 30 Mc/swith scaled-off oscillographs. Multiple records can also bemade.

One of the most spectacular instruments which has nowbecome available is the electron microscope. It operates at50 kV and has a magnification of 50 000. It is being applied toa very wide range of technical problems.

354 OCKENDEN AND GALL: ELECTRICAL MEASURING INSTRUMENTS

Galvanometers have shown little fundamental change exceptfor a gradual improvement in simplicity and stability. Newmagnet steels such as Alcomax have enabled smaller magnetsto be used, and have replaced electromagnets where these wereformerly used to provide intense fields.

Acceleration-proof galvanometers have been developed for useon aircraft and on board ship, the general principle employedbeing dynamical balancing of the coil system, very heavy dampingand short periodic time.

High-speed amplifier recorders have made possible the exami-nation of a wide range of phenomena which vary rapidly, suchas the cooling curves of metals, reverberations in buildings andmechanical vibrations. The immersion-thermocouple methodof recording the temperature of molten steel and other metalshas been made possible by their use, the measuring of temperaturebeing recorded in a few seconds while the thermocouple isimmersed. This type of recorder has been developed alongconventional lines, using a siphon ink-pen with a very powerfulmoving-coil milliameter to operate it. Full-scale records in0 • 2 sec are possible in this way on roll charts. Still higherspeeds are possible using specially prepared electrical oxida-tion paper. In this case voltages up to 500 V are maintainedbetween the pen arm and the paper, giving a trace as the

stylus moves across. This type of recorder is used in echosounding and similar applications where very fast records arerequired.

New portable cardiographs have been developed, the chiefimprovements being in the amplifier systems employed and thecompactness of the general design. These instruments makeuse of both cathode-ray recording and moving-coil oscillographrecorders.

The non-destructive testing of the hardness of steel has beendeveloped by two electrical methods, one by measuring itsmagnetic properties and the other its resistivity. Both havebeen successfully used for armour-piercing shells.

Sensitive pressure gauges using electrical methods have beendeveloped. A diaphragm varies the inductance of a coil whichin turn can be measured or made to modulate the frequency of acircuit for transmission to a distance.

It is inevitable that such a brief review should miss much thathas taken place during a period when scientific methods havebeen applied with unrestricted intensity to the infinitely complexproblems of war, and by so many countries. Much is inaccessible,and some of the most elegant developments may remain so untiltime has disarmed their potency in war but widened their use inpeace.

DISCUSSIONS AT INFORMAL MEETINGS OF THE INSTITUTION

262ND INFORMAL MEETING, 25TH FEBRUARY, 1946.

Chairman: Mr. H. C. Waters, Associate Member.Subject: "Practical Experience of Arc-Suppression Coils."

(Introduced by Mr. H. Willott Taylor, Associate Member, and Mr.L. B. S. Golds, Member).

Mr. Willott Taylor said it had been estimated that roughly75 per cent of network faults started as earth faults even thoughthey might finally develop into phase-to-phase faults if left onlong enough. If apparatus were installed which could success-fully deal with the earth-fault conditions so that interruptionswould be prevented, an even greater measure of continuity ofsupply to consumers would be achieved than was now given.This was precisely what the Petersen coil purported to do. Notonly would the coil deal with transient earth faults, but by itsability to reduce the current through the fault to a very smallvalue, it enabled the fault to be held for a considerable timewithout the trouble spreading.to another phase.

The number of coils necessary for a given system, and theirlocation, depended on the size of the network. In small net-works, the simplest and most satisfactory arrangement was tohave one coil capable of dealing with the whole network, butprovided with suitable tappings to enable correct tuning to beeffected if part of the network was switched out for any purpose.

On large systems, whilst it was possible to manage with onecoil, it was usually found advantageous to increase the numberof coils and locate them at different points in the network.This, although rather more expensive, was beneficial from theoperating point of view, as it permitted the network to be splitup more easily. Growth of networks could be more easilycatered for and a smaller coil could be kept as a spare.

Normally the coil should be under-tuned to avoid resonancewhich could occur with accurate tuning and the presence of anasymmetric voltage on the system. It also left the coil bettertuned should a section of the system be switched out.

Whether full advantage should be taken of a coil's ability tohold earth faults and, if so, for how long, depended mainly onthree factors: the type of system, the possibility of telephone

interference and safety of the public. With a rural networkcomposed chiefly of overhead lines, an earth fault could generallybe held without difficulty. Such faults had been held for aslong as 24 hours. With an urban network consisting chiefly ofcables, and with alternative supplies available, there was not thesame need to hold the fault if there were other advantages to begained by isolating it quickly. Although Post Office engineersheld that a fault should be isolated as quickly as possible, anoverall view should be taken so as to include the maximumbenefit to the consumers not only of the telephone service butalso of the electric supply. On the question of public safety,there was little doubt that on a network chiefly composed ofcables, little danger was likely to arise by holding an earth fault.With overhead lines the most serious case was that of a brokenconductor lying on the ground, and then it was undoubtedlysafer to isolate it immediately.

Continuing the introductory remarks, Mr. Golds said thatsome form of indication, either an ammeter or a voltmeter witha visual and audible alarm, was necessary to indicate when afault had occurred. The voltmeter was particularly useful togive an indication of the asymmetric voltage under healthy con-ditions. The ammeter was connected to a current transformerin series with the coil, and its reading would indicate the tappingon the coil when it was operating. The voltmeter was con-nected either to a secondary winding on the coil or to a voltagetransformer measuring the neutral-to-earth voltage. He pre-ferred a recording voltmeter and ammeter in place of the indi-cating instruments, as records were then obtained of transientfaults, provided that the recorder had a sufficiently high writingspeed.

In the general discussion which followed, most speakers werein favour of the use of Petersen coils, and the suggestion wasmade that having regard to the experience obtained with them,the Electricity Commissioners should now recommend their use.Emphasis was laid on the necessity for tuning the coil accurately,