Optical, chemical and capillaryoscillographs

V.J. Phillips. B.Sc.(Eng), Ph.D.. F.I.E.E.

Indexing terms: History, Measurement and measuring, Instrumentation and measuring science

Abstract: With the spread of alternating-current systems at the end of the nineteenth century, the need for aconvenient and reliable method of displaying waveforms became more urgent. Certain methods were developedto a high degree of usefulness, notably the rotating contacts, the Blondel/Duddell mechanical oscillographs andeventually, of course, the cathode-ray tube. The paper recalls three other methods which seemed, for a shorttime, to hold out great promise but which were not successful and soon faded into obscurity.

1 Introduction

As science in general and electrical engineering in particu-lar progressed throughout the second half of the nine-teenth century, the need for a reliable method forrecording and examining alternating waveforms becameever more pressing. Those experimenters who were con-cerned with the development of AC generators and motorsrealised full well that iron possessed nonlinear magne-tisation characteristics which could cause their waveformsto depart from the desired sinusoidal shape. When suchgenerators were used to supply the early electric lights, thewaveforms were very much affected by the peculiarvoltage/current properties of the electric arc. To quote onewriter of the period:

'For the most part, theories relating to alternating cur-rents are based on the hypothesis, rarely verified, that thecurrent is sinusoidal in form, whose period is controlled bythe speed and number of poles in the alternator. In reality,one knows very well that even with a sinusoidal e.m.f., theintroduction into the circuit of other equipment producesdeformation of the curve.' [1]

Without some convenient method of determining thevoltage and current waveforms with which they weredealing there was little hope of understanding the natureof the physical processes involved; still less of improvingthe performance and efficiency of their apparatus.

Some of the earliest methods of 'curve tracing' (as theprocess was often called in those days) involved rotatingmirrors, modulated gas flames and the like, methods whichhad often been used by workers in the field of sound,music and phonetics. The first real breakthrough occurredin the early 1880s with the introduction of the rotatingcontact method (sometimes called the 'point-to-point'method) which was a mechanical sampling or stroboscopictechnique [2]. This was introduced in Europe by Joubert[3, 4] and, in fact, it was very often referred to simply as'Joubert's method'. However, there is considerable evi-dence to suggest that others had used it before him [5, 6],and it also seems to have been invented quite independent-ly in the USA by Professor Benjamin Franklin Thomasand his associates [7, 8]. This very successful techniquewas widely used for thirty years or more, the originalsimple idea being incorporated in such relatively sophisti-cated instruments as Hospitalier's Ondograph [9-11] andRosa's curve tracer [12-14].

Paper 4271A (S7), received 28th February 1985

The author is with the University of Wales, University College of Swansea, Sing-leton Park, Swansea SA2 8PP, United Kingdom

In the 1890s, the mechanical bifilar oscillograph wasinvented by Blondel [15, 16] and was improved and devel-oped to a very advanced state in Great Britain by Duddell[17-19]. The Duddell oscillograph remained in use untilthe Second World War, and even after that for certainspecialised applications, and must be regarded as a tre-mendously successful instrument. The moving-iron oscillo-graph, also invented by Blondel [16, 19-22], was also quitesuccessful and is still used today in certain types of ultra-violet waveform recorder. The invention of the Hess/Brauncathode-ray tube [23] paved the way for the introductionof the oscilloscopes which we take so much for granted inour laboratories today.

The purpose of this paper, however, is not to relate thehistory of these highly successful devices, but to tell,instead, the story of three ideas which seemed for a time tohold out great promise, but which, in the event, turned outto have disadvantages which made them impracticable foruse in waveform display instruments. They were oftenreferred to as the optical method, the chemical method andthe capillary method.

2 The optical method

This method was proposed by A.C. Crehore in 1895 [24].It made use of the fact, discovered by Faraday, that, if abeam of polarised light is passed through certain liquidswhich are also situated in a magnetic field, the lines offorce being parallel to the light beam, then the direction ofthe polarisation is rotated. The effect was investigated byVedet [25] who found that the angular rotation producedwas given by

6 = <xHL

where H is the strength of the magnetic field in the direc-tion of the beam, L is the distance through which the lighttravels in the liquid and a is a constant of proportionalityknown as Vedet's constant. It is a constant which dependson the properties of the liquid itself, being particularlylarge for carbon bisulphide, ethyl iodide and benzene [26].The above formula holds good for light of any singlewavelength, but it is also a fact that the rotation varieswith the wavelength of the light, being approximately pro-portional to I/A2. Both d'Arsonval [27] and Bequerel [28]had attempted to use the Faraday effect for the measure-ment of current, this being often referred to as the 'opticalammeter'.

Crehore's apparatus is shown in Fig. 1. Sunlight reflec-ted from a mirror (a) was first passed through the Nicol

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prism (b) which polarised it. It was then transmittedthrough the tube (d) containing carbon bisulphide, finally

length here, but he found that if a diffraction grating wereused to produce the spectrum then it was possible to

Fig. 1 Crehore's optical method[CREHORE, A.C.: Phys. Rev., 1894-95, 2, Fig. 1, p. 125]

redblue

passing out through the second Nicol prism (e). The func-tion of the various other components in the system will beexplained shortly; but, first, it will be assumed that thelight is filtered before entering the system so that it ismonochromatic. The second prism could be rotated to thecrosspolarised position so that no light emerged at the farend. If current were now to be passed through the solenoidwound around the tube, the magnetic field would rotatethe plane of polarisation so that some light would emergefrom the second prism. The degree of rotation, measuredby turning the second prism so that extinction occurredonce more, could be used as a measure of the currentflowing. This was the principle of the aforementionedoptical ammeter.

Consider next what would happen if unfiltered whitelight were allowed to enter the apparatus. As the degree ofrotation depends on the wavelength, different componentsof the light would be rotated by different amounts. If thesecond prism were now rotated, a series of brilliant colourswould be seen, the colour at any particular position beingthe complementary colour of the light which was extin-guished in that position. Alternatively, if the prism wereleft in one position and the current through the coil wereto be varied, then at any particular value of current, onewavelength would be absent and again a series of colourswould be seen. In either case, if the output light were to besplit up into its spectral components by an ordinary glassprism or a diffraction grating, then a spectrum of the usualsort would be produced, save that there would be a gap ordark band where the absent component would have fallen.As the current in the coil is varied, so will the position ofthe gap move backwards and forwards along the spectrum.If the spectrum is now recorded on a photographic platemoving in a perpendicular direction to the band of light,then the current variation will appear as a dark trace on alight background.

One further feature is needed if the scheme is to workproperly. If the current in the coil is zero at any time, nolight cancellation will occur and the full spectrum appears.A thin quartz plate (c) is inserted in the light path to givean offset rotation of the plane of polarisation. The outputNicol prism is rotated so that a dark band appears in thecentre of the spectrum for zero current so that alternatingcurrent will then result in a full trace on the photographicplate (£). Since the rotation of polarisation is inversely pro-portional to the square of the wavelength, it would appearthat the relationship between the current and the move-ment of the dark band could not be a linear one. Thesubject was analysed in some detail by Dr. Crehore. Itwould not be appropriate to present his analysis at great

504

achieve reasonably good linearity in the system, therelationship between current and displacement being con-strained to a relatively small segment of a parabolic curve,remote from its origin.

This is really quite an ingenious idea, seeming to be therealisation of the dream of all meter designers; namely theinertia-less pointer.

As always, however, there are other factors to be takeninto account. To produce reasonable movement of thedark gap a fairly high magnetic field strength is needed.Except for the largest currents, this implies a coil of manyturns, and such a coil possesses a large self-inductance.Mechanical inertia has therefore been exchanged for itselectrical counterpart. Another disadvantage is that thedark gap is not very sharp. One wavelength may be extin-guished completely, but its near neighbours in the spec-trum are also diminished, and the image of the gap formedon the photographic plate will be fuzzy and ill defined.

The paper in which Dr. Crehore announced thismethod of waveform recording was confidently entitled 'Areliable method of recording variable-current waves'.When one reads the paper, one realises that it is almostentirely theoretical and speculative in nature. MonsieurBlondin, the Scientific Director of the journal L'Eclair ageElectrique, pointed out several other possible disadvan-tages [29]. Photographic plates of high sensitivity wouldbe required, and he doubted whether those available atthat time would be adequate. He also feared that tem-perature variations in the liquid would prove to be a nui-sance. He wrote, rather scathingly,

'It would be useful if the author, instead of describing amethod employed already by a number of others for otherpurposes, had given us the results of his experimentswhich, according to him, promised much for the future'.

Duddell mentioned the idea when writing a generalsurvey of oscillographic methods in 1897, and also seemedto be dubious of its practicability [30]:

'.. . the main difficulties seem to be in attaining sensi-bility without self induction, and in photographing themovement of the band. Up to the present, the writer hasseen no waveforms obtained by this method'.

Blondin's comment about other experimenters is prob-ably a reference to the work of J. Pionchon of the Uni-versity of Grenoble which was published at about the sametime [31, 32]. His apparatus also consisted of a tube filledwith either carbon bisulphide or a mixture of mercuryiodide and potassium iodide solution in the centre of asolenoid carrying the current under observation. When analternating current was passing through the coil, the light

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output from the second prism would fluctuate rapidly, but,because of the persistence of vision, there would be novisible effect. However, if a stroboscopically flashing lightwere used, the variation could be slowed down for furtherstudy. This would only work for monochromatic light,and, to obtain this, Pionchon passed white light from anarc lamp through a solution of potassion bichromate. Thestroboscopic flashes were produced by mounting two smalldiaphragms on the tines of an electrically maintainedtuning fork, each diaphragm having a narrow slot cut in it.A flash of light was produced each time the slots came intoalignment. Henri Abraham, commenting on this method,expressed the opinion that there would be two disadvan-tages [33]: the first was the difficulty of providing a mono-chromatic light source of sufficient intensity; the secondwas that the flashes of light produced by the tuning-forkarrangement were of rather long duration and would limitthe resolution of the current waveform. Abraham, assistedby H. Buisson, proposed an improvement which wouldalleviate these difficulties [34]. They illuminated the tubeby means of a spark from an induction coil, the primary ofwhich was actuated by a switch mounted on the shaft ofthe alternator whose waveform was being observed. In thisway a flash could be made to occur at the same point ineach successive cycle. The system was originally set up sothat the light from the output Nicol prism was extin-guished under no-current conditions. When the alternatingcurrent was applied light would again be seen, the extentof the rotation of polarisation depending on the value ofthe current at the instant of the flash. This rotation wascounteracted by passing a direct current through a secondcoil wound on the tube in such a way as to restore therotation to zero. This direct current, read from a meter,then gave the value of the alternating current at thatinstant. The whole picture of the waveform was then builtup point by point in the manner of a Joubert measure-ment. This idea would no doubt have worked after afashion, but the inductance of the coil was still a compli-cating factor, the apparatus was probably difficult to setup, and the whole thing was overelaborate for what itactually achieved. One might just as well have used thesimple Joubert method.

Crehore's system was taken up by J.A. Switzer, who, in1898, published his results in a paper which was provoca-tively given precisely the same title as that used byCrehore: 'A reliable method of recording variable-currentcurves' [35]. He continued the detailed analysis of thesystem and showed that, not only did the position of thedark gap vary with the value of the current, but its widthvaried also. He carried out a series of practical experimentswith the system and published the curves shown in Fig. 2.These show the results obtained for currents of various fre-quencies. His results are best summed up in his ownwords:

'These photographs are selected from many as being thebest that were obtained. That results very much betterthan these could be obtained by this method, the writerdoubts; and, while the method presents itself as one full ofattractiveness, its limitations are necessarily so great that itis not likely to find practical application — at least to therecording of variable electric currents. Dr. Crehore hashowever applied the principle, with signal success, to quiteanother problem, that of recording the velocities of projec-tiles [36]; and it may be that still other fields of usefulnessare yet awaiting magnetic rotation of polarised light'.

The method sank into obscurity, and was heard of nomore.

^^w

:^jJK ]&& H^i; |||iiti§ £tfjj| iSfflfc : 'jj^| * * • I

Fig. 2 Traces obtained by Switzer using Crehore's methodThese show alternating currents of various frequencies.[SWITZER, J.A.: Phys. Rev., 1898, 7, Figs. 8-10, p. 92]

3 The chemical method

This method, also called the electrochemical method, reliedon the fact that certain chemical solutions undergo reac-tions and colour changes when they are subjected to theinfluence of electrical voltage and current. For example, ifa piece of paper is soaked in a solution of potassiumiodide, then the application of a potential differencebetween two electrodes touching the paper will cause abrown iodine stain to appear at the anode. This reactionwas applied practically, in 1864, in a simple communica-tion system known as Bain's Telegraph [37-39]. The dotsand dashes of the Morse code were represented by pulsesof DC voltage on the line. At the receiver a strip of papersoaked in potassium iodide solution was passed over ametal roller. A metal stylus was pressed on to the strip andthe incoming voltage was applied between the strip andthe roller. The signal was thus recorded as a line of browndots and dashes. The principle of recording the signal inthis way was also revived a few years later by Delaney[40].

The sensitivity of the system could be improved byadding starch to the potassium iodide. A mixture of 1 partpotassium iodide to 20 parts of starch paste to 40 parts ofwater was said to be best. An even more effective recordingmedium was made up of equal parts of a saturated solu-tion of prussiate of potash (potassium ferricyanide) and asaturated solution of ammonium nitrate, diluted with twovolumes of water. Ammonium nitrate is a deliquescentsubstance which was added to keep the paper permanentlydamp. If an iron or steel stylus were used, the solutionreacted at the anode to form Prussian blue (ferricferrocyanide), a particularly bright and clear trace beingproduced [41]. With all these various mixtures, the extentof the reaction, and hence the darkness of the trace,depended on the voltage applied or, alternativelyexpressed, on the current flowing through the solution.

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In 1887, P. Griitzner, who was a physiologist by pro-fession, attempted to record current waveforms in this way,during the course of some experiments to determine theeffects of electric current impulses on nerves and muscles[42]. He returned to the topic again in 1899, and on thisoccasion also investigated the behaviour of certain electri-cal circuits [43, 44]. One of his experiments is illustrated inFig. 3. D is a Daniell cell connected across a coil of wire Sp

be exceeded in order to make any mark at all; and, in thetraces just described, the gaps in the line where the voltage

•E El '" " ' " " I l l l l l l HBIll l l l l l l l l l i

Fig. 3 Griitzner's apparatus

D is a Daniell cell; E and Ev are electrodes pressing upon the impregnated paperstrip. The arrows show the current flow when the switch C is opened and closed.The sketch shows the current and, beneath, the type of trace expected.[GROTZNER, P.: Annalen der Physik, 1900,1, Figs. 13-14, p. 752]

through a switch C. The ends of the coil are in contactwith two platinum electrodes E and Ex pressing on to amoving strip of iodide/starch paper. The sketch on theright shows the voltage waveform as C is opened andclosed. When it is closed, a current is established in thedirection of the tailless arrows and after an initial over-shoot it settles to a steady value. When the switch isopened there is a kick of reverse voltage which drivescurrent in the direction shown by the arrows with tails.The paper is moving into the diagram; i.e. in a directionat 90° to the line joining E and Ev Because the darkeningof the paper is proportional to the current, the traces pro-duced were of the form sketched beneath the waveform.Fig. 4 shows several actual traces produced by Griitzner in

ISp

Fig. 4 Actual traces obtained by Griitzner

The trace labelled IISp corresponds to the situation of Fig. 3.[GROTZNER, P.: Annalen der Physik, 1900,1, Fig. 15, p. 753]

the course of his experiments, that labelled IISp corre-sponding to the situation just described.

Another type of waveform which he investigated wasthat generated by a primitive alternator consisting of amagnet rotating near a stationary coil. The waveform ofthe voltage and the form of trace expected are shown inFig. 5a; the actual traces he obtained when rotating themagnet at various speeds are shown in Fig. 5b. Clearly theamount of quantitative information about the magnitudeof the waveforms at any time available from these traces isvery limited. In addition, a certain threshold voltage must

506

Fig. 5 Griitzner's experiment with a primitive alternator

a Current waveform and expected traceb Actual traces obtained with magnet rotating at different speeds[GROTZNER, P.: Annalen der Physik, 1900, 1, Figs. 8-9, p. 749]

falls below this level in each cycle can be seen on closeexamination. (This is most obvious in the last trace.)

An improved instrument was made by P. Janet in 1894[45-49]; the principle is illustrated in Fig. 6. Assume that astylus is pressing on to a metal drum around which a piece

Fig. 6 Principle of Janet's first method

[JANET, P.: L'Eclairage Electrique, 1895, 2, Fig. 9, p. 246]

of iodide paper is wrapped. A sinusoidal voltage S isapplied between the stylus and the drum. To make a markon the paper, the voltage must exceed the threshold whichis marked X in the diagram; this would typically be about1 or 2 V. The stylus will thus leave a record of dashes cor-responding to AB, A'B' etc. Now let a positive DC bias beapplied to the sinusoid so that it moves upwards, becom-ing the curve S'. The waveform is now above the thresholdfor a longer time than before and the lengths of the dasheswill be C ^ u C\D\ etc. If we start off with the first traceAB, then we can draw beneath it the points C and Dbecause we know the voltage C^C which was the biasvoltage applied. Having thus established the position of Cand D, the process can be repeated with other positive andnegative values of bias, and other points such as C and Dcan be added so that gradually a picture of the curve canbe built up.

This was clearly a very tedious procedure, and Janetwent on to construct a piece of apparatus which wouldbuild up a picture of the waveform directly without theneed for the plotting of each point individually. The singlestylus was replaced by a number of styli (actually steelsewing needles) pressing on the drum which was rotated by

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a hand crank. Each was biased by a different amount bymeans of a bank of cells as shown in Fig. 7. This diagram

p,p2F$pt

Fig. 7 Janet's improved method

[JANET, P.: L'Eclairage Electrique, 1895, 2, Fig. 10, p. 247]

shows eight needles only, but in Janet's apparatus therewere fifteen or more. The voltage to be observed was con-nected between M and N. The short lines between the styliin the diagram are meant to represent the biasing cells,each of 4 V EMF. When the drum rotates, each stylusdraws a line of different length as in Fig. 8, and the actual

Fig. 8 Trace produced by Janet's improved method

[JANET, P.: L'Eclairage Electrique, 1895, 2, Fig. 11, p. 247]

wave shape can be seen as an outline or silhouette of thelines. This is an efficient method of tracing the waveform,but suffers from the disadvantage that one has to providefourteen bias batteries. Blondel suggested replacing them

left right

le f t

with a resistive dividing chain [50-52], although the exactcircuit configuration he suggested and the analysis he gave

ro-

F i g. 9 M urphy 's chemical method

a Side elevation (10 represents paper band moving from spool 11 between rollers12)b Front elevation (schematic)c Equivalent circuit (6 is an alternator)[MURPHY, E.J.: US Patent 713479, 1902]

of it are rather dubious, and his circuit would have causeddistortion of the wave shape revealed by the lines.

An interesting variant of the chemical method isdescribed in a US Patent granted to E.J. Murphy in 1902;see Fig. 9. In this instrument [53, 54], a thin band ofiodide-starch paper is caused to pass between two sets of

right

R 1

R2

point of equalpotential

point of equalpotential

Fig. 10 Principle of Murphy's method

a Distribution of potential along resistors when generator voltage is zero b Distribution of potential when upper terminal of generator is positive c Type of trace produced

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styli arranged in comb formation, one set on top of theband, and the other directly opposite on the underside.The paper band is marked 10 in the side and front sche-matic views of Figs. 9a and b. The styli are connected atregular intervals along two resistors (marked 1 and 2) andthese, together with 3 and 4, form a bridge circuit con-nected to the battery (5) as in Fig. 9c. Assume for simpli-city that all the resistors are equal in value, that point 7 atthe junction of 1 and 2 is at zero potential and that thebattery voltage is Vb. The distributions of voltage acrossthe resistors will then be as shown in Fig. 10a, takingaccount of the fact that current flows from left to right in 1and from right to left in 2. Every opposing pair of styli willthus have a potential difference between them save for thetwo centre ones which will be at the same potential. Underthese conditions, if the paper is drawn through past thepoints each pair will make a dark line except for the centrepair. As the darkening only appears at one electrode, halfof the lines will nominally appear on the top side of thepaper, the rest on the bottom surface. However, the paperwas made very thin so that the traces were clearly visiblefrom either side. Now let an alternating potential wave-form be applied across the other corners of the bridgecircuit. In the diagram, an alternator (6) is shown con-nected in this way. If, at some instant of time, the top ter-minal of the alternator is positive, this will superimpose acurrent which opposes that in resistor 1 but adds to that inresistor 2, so that the potential distribution now appears asin Fig. 10b. The position where the opposing styli have nopotential difference between them has now shifted from thecentre, and so the 'no trace' line on the record will alsohave moved. As the alternator voltage varies, so will the'no trace' position oscillate across the paper so that a tracesuch as that of Fig. 10c will result, the waveshape beingrevealed by the gaps in the lines.

Compared with the optical method, the chemicalmethod seems to have no severe disadvantages; at least asfar as the delineation of low-frequency AC mains wave-forms is concerned. It would have been difficult to deter-mine the precise positions of the ends of the marks on themoist paper; the stains would certainly have diffusedsomewhat through the solution. No doubt the apparatuswould have been rather messy to prepare and to use. Thepresent author has never seen any waveforms obtained byJanet's or Murphy's method reproduced in the literature ofthe period. In any case, the mid-1890s saw the develop-ment of the more convenient oscillographs of Blondel andDuddell, and the chemical method never really came intoeveryday use.

4 The capillary method

This method was proposed by G.J. Burch in 1896, and wasbased on a type of electrometer which had been introducedby Lippmann in 1874 [55-58]. In this electrometer, a finecapillary tube containing mercury in its upper part dipsdown into a bath of dilute sulphuric acid, being soassembled that there is no bubble of air in the tube and sothat the mercury and the acid form an interface in thetube. If a potential difference is applied between themercury and the acid, there are changes in the surface ten-sions which cause the interface to move up and down inthe capillary. The system is very sensitive, and it wasclaimed by Burch that a movement corresponding to aslittle as 1/30000 V could be observed with the aid of amicroscope. This method of voltage measurement wasquite widely used for a number of years by physical chem-ists and by physiologists [e.g. see References 59 and 60].

Burch's idea was that the voltage waveform under obser-vation should be applied across the electrometer, theresulting movement of the mercury being optically project-ed on to a moving plate and recorded photographically[61]. As we shall see later, there are several complicatingfeatures which have to be accounted for if the method is tobe of any use. In his published paper, Burch gave elaborateinstructions regarding the construction of the apparatus;in particular, how the capillary was to be set up. He triednumerous forms, but eventually recommended that shownin Fig. 11. The capillary is actually formed in a thick-walled glass tube H, one half of which is ground away so

B

Fig. 11 Burch's improved version of the Lippmann electrometer[BURCH, G.J.: Electrician, 1896, 37, Fig. 4, p. 382]

that a trough is created. This is closed with a thin flat plateof glass so that a mercury/acid column with a D-shapedcross-section is created. In this way, the capillary can beilluminated from the rear and photographed without theoptical distortion which a completely cylindrical tubewould have produced. The mercury pool B at the bottomwas simply used to establish electrical contact with theacid in the capillary.

In his first model, the movement of the photographicplate was accomplished by the 'hydraulic motor' shown inFig. 12. A is a brass tube, slotted at the top, and inside thistube there are two pistons B-B connected by a thin hollowrod at the centre of which is a small tap C. If this tap isopen, water can pass freely through the rod and the wholeassembly can be moved from side to side along the brasstube. The tap G at the left is connected to a mains watersupply. If the small tap C is closed, the water pressurecauses the pistons to move from left to right. A photogra-phic plate is fixed to a slide D which is in turn fastened tothe piston rod. To operate the device, the two mains water

Fig. 12 Hydraulic motor for movement of plate-holder (D) at constantspeed[BURCH, G.J.: Electrician, 1896, 37, Fig. 5, p. 435]

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taps F and G are closed and the tap C is opened. The plateholder can then be moved easily to the left. C is closed. Fis opened, and then G is opened suddenly at the requiredmoment, the result being a steady movement of the plateto the right. The speed of movement is controlled bythe degree of opening of the tap F, speeds in the range0.3 cm/s to 5 cm/s being attainable. This arrangement was,according to Burch's account, quite successful but waseventually superseded by the pendulum arrangement seenin Fig. 13. The stated reason for this changeover was toobtain higher speeds of movement of the plate, but other

Fig. 13 Burch's pendulum motor for movement of plate-holder (C)[BURCH, G.J.: Electrician, 1896, 37, Fig. 8, p. 436]

disadvantages of a water-operated system are not difficultto imagine!

Referring to Fig. 13, a plate holder S was fixed to thetop of the pendulum A, pivoted at 0. The weight Battached to the lower end was carefully adjusted so thatthe pendulum would rest in equilibrium in any position. Itwas first swung so that the pin G was caught beneath thehook at the lower right-hand side of the mounting table. Aweight E was then hung from the lever D, so that, whenthe handle was pulled up to release the hook, the pendu-lum would accelerate and move the plate holder to theright. It was so arranged that when the plate reached theposition where the image of the mercury capillary was tobe projected on to it the weight would hit the table, remo-ving the accelerating force so that movement would con-tinue with constant velocity. The final plate speed was afunction of the size of the accelerating weight, and, with aweight of 2 kg, a speed of 150 cm/s was said to be pos-sible. Having reached the other extremity of its swing, thependulum was caught and arrested by the hook H. If atransient waveform was to be investigated, the lever M wasarranged to lift the contact K out of a mercury bath, totrigger the transient at the correct time.

All this would seem to be very simple, but, alas, thingswere not nearly as straightforward as they seemed. Burch

made a very detailed study of the dynamic behaviour ofthe electrometer under various types of voltage excitation[62, 63]. We cannot possibly discuss all his detailed find-ings here, but, briefly, owing to the inertia of the mercurycolumn, the response was by no means instantaneous, butwas gradual as shown in the trace of Fig. 14. Note that in

Fig. 14 Trace produced by application of step voltage to the electrome-ter

Note: time runs from right to left; trace is also inverted — the dark area representsthe shadow of the mercury column.[BURCH, G.J.: Electrician, 1896, 37, Fig. 11, p. 515]

this trace time runs from right to left, and it is also upsidedown due to the optical projection, the shadow of themercury being shown at the bottom. He also discoveredthat the electrometer exhibited capacitive properties,retaining its charge and deflection for some time after thevoltage which caused it had been removed. In fact, hisoverall conclusion was that the applied voltage was 'usedup' in two ways, one part being proportional to the deflec-tion, the other part to the velocity with which the interfacewas moving. Thus the measurement of the voltage at anytime involved the determination of the deflection of theinterface and its velocity at that time. Of course, the pho-tographic plate moved in the arc of a circle, but it turnedout that this was rather more convenient than a conven-tional Cartesian plot would have been. In the diagram ofFig. 15 the rectangle represents the photographic plate, z is

c /

Fig. 15 Method of measuring displacement and rate-of-change of dis-placement from recorded trace[BURCH, G.J.: Electrician, 1896, 37, Fig. 10, p. 473]

the trace recorded on it as it moves around its circularpath and AB is another track recorded at constant radiusfor ease of measurement. The first component of theapplied voltage, that which was proportional to the dis-placement, was found by measuring the radial distancebetween the two tracks Ar as, for example, at point p. Thevelocity of the interface movement is proportional to theslope of the trace; i.e. to the tangent to the curve. It can beshown that the so-called polar subnormal o-n is directlyproportional to the required velocity. The second com-ponent of the applied voltage can thus be obtained bymeasurement of distance o-n ( = D). We need not here

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concern ourselves with the details of the constants of pro-portionality of the system; we may simply note that, at anytime, the required applied voltage is given by

V = kAr + LD

where k and L are the aforementioned constants which canbe determined.

Fig. 16 is an example published by Burch and showspart of the waveform of the trilled sound V. It was clearlya complicated matter to extract from photographic recordsof this sort the original waveform, and the special plottingtable of Fig. 17 was constructed to aid in this task. The

Fig. 16 Trace produced by trilled Y sound spoken into a microphone(bottom trace)The middle trace is the stationary reference. The upper trace would normally beused in conjunction with a tuning fork to provide a time calibration.[BURCH, G.J.: Electrician, 1896, 37, Fig. 20, p. 517]

B CD

Fig. 17 Measuring tablea Elevationb Plan[BURCH, G.J.: Electrician, 1896, 37, Figs. 12-13, p. 515]

photographic plate is first clamped upon the carrier Bwhich is pivoted at 0, and its precise mounting is adjustedcarefully by means of the screws C and D so that it rotatesin the same way, and with the same radius as it did whenexposed on the pendulum. The thread F attached to thepivot and kept taut by the small weight is used to align theinstrument. The image of the horizontal wire G is castupon the underside of the plate by light reflected from theinclined mirror through the lens H. The thread lies in asmall notch at F. The image of the wire is adjusted so thatit coincides with the thread, and thereafter the thread canbe removed. A glass scale is provided so that the radialmovement of the trace can be measured as the plate isrotated.

A measuring block which is shown in detail in Fig. 18 isthen placed upon the plate. This block has a hole drilled

M

JV.7

Ml

KFig. 18 Detail of measuring block shown on top of table at L in Fig. 17b[BURCH, G.J.: Electrician, 1896, 37, Fig. 14, p. 515]

through it, and visible through this hole are two lines atright angles to each other which are inscribed on the glassplate which forms the base of the block. The continuousline is set so as to be tangential to the trace at the point ofmeasurement. On the side of the block is a mirror M andon its surface is a pointer. Referring back to Fig. lib, theslider J moves along a horizontal rod E-E. This slidercarries a vertical pin and this is moved along the rod untilthe pin, its image seen in M and the pointer are all inalignment when viewed from the left-hand side of theapparatus. The distance OJ, measured along a scale, isthen the polar subnormal referred to earlier.

If the trace of Fig. 16 is examined in detail, it will beseen that there are in fact three traces recorded on theplate. The lower one is the Y sound, and the centre trace isthe fixed one from which measurements of the radialmovement are measured. The upper trace would normallyhave had superimposed upon it the waveform of a vibrat-ing tuning fork for purposes of time calibration. In thisparticular case, the sound was being spoken into a micro-phone situated near the apparatus, and so to avoid inter-ference with the sound the fork has been switched off.However, it can be inferred from Burch's remarks that theplate had been moving at a speed somewhere between 2and 3 m/s and that the total time represented on the traceis of the order 1/3 s.

The procedure adopted was first to determine and tabu-late the radial movement and the subnormal at eachrequired instant on the trace, and then to convert theseinto voltages using the formula quoted in this Section. Thisobviously involved considerable time and labour (not tomention patience), and in the development of this instru-ment Burch had obviously had to expend much ingenuityin overcoming the basic deficiencies and complications ofthe method. In his own words:

'It has qualities of its own, distinct from any other formof electrometer or galvanometer and these will make it avaluable addition to the most perfectly appointed labor-

510 IEE PROCEEDINGS, Vol. 132, Pt. A, No. 8, DECEMBER 1985

atory. I have described the method by which these specialproperties may be utilised, and some of the apparatus Ihave designed for the purpose during the nine years I haveworked with the instrument in order, if possible, to bring itinto general use. For the same reason, I have refrainedfrom patenting any of it, preferring to present my dis-covery and invention to the scientific public'

He also claimed that one of its chief merits was that ofcheapness:

'It is essentially the instrument for those who have tothink twice before spending a shilling'.

One must admire his motivation and his principles in re-fraining from making a profit from his labours, but, asregards the last comment, one wonders whether the cost ofconstructing this elaborate apparatus had really been fullytaken into account.

5 Concluding remarks

None of the three oscillographic methods which have beendescribed here can possibly be considered to have been agreat success. Each of them seemed to its supporters tohave considerable merit, but each made only a briefappearance before it was consigned to the storehouse of'inventions which never made it'. Nevertheless, they illus-trate very well the readiness of experimenters in those daysto investigate the possibilities of any physical effect whichseemed to lead to that most important goal; namely a con-venient and reliable method for displaying alternatingwaveforms. They also demonstrate (particularly the capil-lary instrument) the lengths to which people were preparedto go, and the patient and meticulous attention to detailwhich would be given in the attempt to achieve their aims.

6 References

1 ARMAGNAT, H.: 'Forme des courants', L'Eclairage Electrique, 1897,12, pp. 346-353

2 PHILLIPS, V.J.: 'Point-to-point'. IEE weekend meeting on thehistory of electrical engineering, Brighton, July 1982, Paper 15

3 JOUBERT, J.: 'Sur les courants alternatifs et la force electromotive del'arc electrique', J. Phys., 1880, 9, pp. 297-303

4 Report on Joubert's experiments, Electrician, 1880, 5, pp. 151-1525 BOWERS, B.: 'Sir Charles Wheatstone' (HMSO, 1975), pp. 164ff6 LENZ, E.: 'Ueber den Einfluss der Geschwindigkeit des Drehens auf

den durch magneto-elektrische Maschinen erzeugten Inductionss-trom', Poggendorff's Ann., 1849, 76, pp. 494-523, and ibid., 1854, 92,pp. 128-152

7 Discussion on THOMPSON, M.E.: 'A study of an open coil arcdynamo', Trans. Amer. Inst. Elect. Engrs., 1891, 8, p. 393

8 THOMAS, B.F.: 'Notes on wiping-contact methods for current andpotential measurements', ibid., 1892, 9, pp. 263-270

9 HOSPITALIER, E.: 'The slow registration of rapid phenomena bystrobographic methods', J. IEE, 1903, 33, pp. 75-94

10 HOSPITALIER, E.: 'M. Hospitalier's Ondograph', Electr. Rev., 1902,50, pp. 969-971, 1040-1041 and ibid., 1903, 53, pp. 1006-1007

11 'Plotting alternating current waves', The Electrical Engineer, 1901, 28,p. 363

12 ROSA, E.B.: 'An electric curve tracer', Electrician, 1897-98, 40, pp.126-128,221-223,318-321

13 ROSA, E.B.: 'An electric curve tracer', Phys. Rev., 1898,6, pp. 17-4214 ROSA, E.B.: 'Curve tracer of electrical measurements', British Patent

no. 1872, 189815 BLONDEL, A.: 'L'inscription directe des courants electriques vari-

ables', Rev. Gen. ScL, 1901, 12, pp. 612-62616 BLONDEL, A.: 'Sur l'inscription directe des courants variables'.

Congres Int. de Physique, Paris 1900, Vol. 3. (published by Gauthier-Villars), pp. 264-295

17 DUDDELL, W., and MARCHANT, E.W.: 'Experiments on alternatecurrent arcs by aid of oscillograph', J. IEE, 1899, 28, pp. 1-107

18 DUDDELL, W.: 'Improvements in oscillographs or apparatus forindicating or recording rapidly varying electric currents or potentialdifferences'. British Patent 5449, 1898

19 'Oscillographs', Nature, 1900, 63, pp. 142-14520 SOLIER, A.: 'Nouveaux modeles d'oscillographes BlondeF, Rev.

d'Electr., 1904, 40, pp. 167-17221 BLONDEL, A.: 'Oscillographs for the investigation of slow electric

oscillations', Electrician, 1893,30, pp. 571-57222 BLONDEL, A.: 'Oscillographs: nouveaux appareils pour l'etude des

oscillations electriques lentes', Comptes Rendus, 1893, 116, pp.502-506courants variables', L'Eclairage Electrique, 1897,12, pp. 131-132

24 CREHORE, A.C.: 'A reliable method of recording variable-currentwaves', Phys. Rev., 1894-95, 2, pp. 122-137

25 THOMPSON, S.P.: 'Elementary lessons in electricity and magnetism'(Macmillan, 1900), p. 566

26 KAYE, G.W.C., and LABY, T.H.: 'Physical and chemical constants'(Longmans Green, 1941, 9th edn.), p. 91

27 D'ARSONVAL, J.A.: 'Les amperemetres optiques', La LumiereElectrique, 1884, 12, pp. 156-157

28 BEQUEREL, H.: 'Methode optique pour mesure l'intensite absolud'un courant optique', ibid., 1884,12, pp. 321-323

29 BLONDIN, J.: 'La methode description des courants variables deM. Crehore', L'Eclairage Electrique, 1895, 2, pp. 337-341

30 DUDDELL, W.: 'Oscillographs', Electrician, 1897, 39, pp. 636-63831 PIONCHON, J.: 'Sur une methode optique d'etude des courants

alternatifs', Comptes Rendus, 1895,120, pp. 872-87432 'Sur une methode optique d'etude des courants alternatifs',

L'Eclairage Electrique, 1895, 3, pp. 232-23333 ABRAHAM, H.: 'Sur le rheographe a induction Abraham-Carpentier

et les differentes methodes d'enregistrement des courbes de courantsalternatifs', Bull. Soc. Int. Electriciens, 1897, 14, pp. 397-434

34 ABRAHAM, H., and BUISSON, H.: 'Nouvelle methode optiqued'etude des courants alternatifs', L'Eclairage Electrique, 1897, 12, pp.221-222

35 SWITZER, J.A.: 'A reliable method of recording variable currentwaves', Phys. Rev., 1898, 7, pp. 83-92

36 CREHORE, A.C., and SQUIER, G.O.: 'Experiments with a newpolarizing photo-chronograph as applied to the measurement of thevelocity of projectiles', ibid., 1895-96, 3, pp. 63-70

37 BAIN, A.: 'Transmitting and receiving electric telegraphic communi-cations: apparatus connected therewith'. British Patent 11480, 1846

38 PREECE, W.H., and SIVEWRIGHT, J.: 'Telegraphy' (LongmansGreen, 1914), pp. 140-141

39 MARLAND, E.A.: 'Early electrical communication' (Abelard-Schumann, 1964), p. 121

40 HESS, A.: 'La telegraphie', L'Eclairage Electrique, 1897, 13, pp. 385-390, 445-458

41 PREECE, W.H., and SIVEWRIGHT, J.: 'Telegraphy', (LongmansGreen, 1905), pp. 77ff

42 GRUTZNER, P.: 'Ueber die Reizwirkungen der Stohrer'schen Mas-chine auf Nerf und Muskel', Pfluger's Arch. PhysioL, 1887, 41, pp.256-281

43 GR0TZNER, P.: 'Ueber die elektrostatische und elektrolytische Auf-zeichnung elektrischer Strome', Ann. Phys., 1900, 1, pp. 738-757

44 Report in L'Eclairage Electrique, 1900, 24, pp. 37-3845 JANET, P.: 'Determination del la forme des courants periodiques en

function du temps de la methode d'inscription electrochimique',Comptes Rendus, 1894, 119, pp. 58-61

46 JANET, P.: 'Determination de la forme des courants periodiques enfunction du temps au moyen de la methode d'inscription electrochi-mique', La Lumiere Electrique, 1894, 53, pp. 92-94

47 JANET, P.: 'Sur une methode d'inscription electrochimique des cou-rants alternatifs', L.Eclairage Electrique, 1895, 2, pp. 241-248

48 JANET, P.: 'Sur une methode d'inscription electrochimique des cou-rants alternatifs', Bull. Soc. Int. Electriciens, 1895,12, pp. 6-17

49 JANET, P.: 'Inscription autographique directe de la forme des cou-rants periodiques en moyen de la methode electrochimique', ComptesRendus, 1894,119, pp. 217-218

50 BARBILLON, L.: 'Manipulations et etudes electrotechniques'(Dunod, Paris, 1904), pp. 194-197

51 BLONDEL, A.: 'Remarques sur la methode electrochimiqued'inscription des courants alternatifs', Comptes Rendus, 1894, 119, pp.399-402

52 BLONDEL, A.: 'Remarques sur la methode electrochimiqued'inscription des courants alternatifs', L'Eclairage Electrique, 1894, 1,pp. 83-84

53 MURPHY, E.J.: 'Recording electric waveforms'. US Patent 713479,1902

54 Report in Electrical World and Engineer, 1902,40, p. 91455 LIPPMANN, G.: 'Relation entre les proprietes electriques et capil-

laires d'un surface de mercure en contact avec differents liquids', Ann.Chimie Phys., 1877,12 (series 5), pp. 265-276

IEE PROCEEDINGS, Vol. 132, Pt. A, No. 8, DECEMBER 1985 511

56 LIPPMANN, G.: 'On the electrical and capillary properties ofmercury in contact with different aqueous solutions', Phil. Mag., 1877,4 (series 5), pp. 238-239

57 LIPPMANN, G.: 'Experiences electrocapillaires', J. Physique Theo-rique et Applique, 1874, 3, pp. 41-43

58 HOSPITALIER, E.: 'Formulaire pratique de l'electricien'. Deuxiemeanee 1884, Masson, L.Academie de Medecine, Paris, 1884, p. 97

59 WALLER, A.D.: 'On the action of the excised mammalian heart',Phil. Trans. Roy. Soc. London, 1887, 178B, pp. 215-255

60 WALLER, A.D.: 'On the electromotive changes associated with thebeat of the mammalian heart', ibid., 1889,180B, pp. 169-194

61 BURCH, G.J.: 'The capillary electrometer in theory and practice',Electrician, 1896, 37, pp. 380-382, 401-403, 435-437, 472-473, 514-517,532-535

62 BURCH, G.J.: 'On the time relations of the capillary electrometer'Phil. Trans. Roy. Soc. London, 1892,183A, pp. 81-105

63 'Sur le deplacement du menisque d'un electrometre capillaire en func-tion du temps', L'Eclairage Electrique, 1892, 44, pp. 145-146

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