An Unfortunate Experiment: Hertz and the Nature of Cathode Rays

An Unfortunate Experiment: Hertz and the Nature of Cathode RaysAuthor(s): George ThomsonSource: Notes and Records of the Royal Society of London, Vol. 25, No. 2 (Dec., 1970), pp. 237-242Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/530878 .

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237

AN UNFORTUNATE EXPERIMENT

HERTZ AND THE NATURE OF CATHODE RAYS

By SIR GEORGE THOMSON, F.R.S.

N making sense of a complicated set of scientific facts, or claimed facts, it often happens that one turns out to be wrong, but it is rare for the villain

to be an experiment by as great a physicist as H. Hertz, the first man to prove experimentally that electromagnetic waves exist and can be propagated in free space.

The experiment in question had nothing to do with these waves, it con- cerned the nature of cathode rays. These are one of the many strange phenomena which occur when electricity is made to pass through highly rarified gases.

I When for example the air in a glass tube is reduced to a density about 3

of the normal an electric current can be passed through it with far less than the voltage which would be needed if most of the air had not been pumped out, though if the density is reduced by still another factor of Io or so it again becomes harder to pass a current. Some gas is needed, in fact, but not too much.

Soon after the stage when the density allows the discharge to pass most easily the brilliantly coloured display of lights in the tube begins to show a new feature, called the cathode rays. These get their name because they come from the (conventionally) negative of the two terminals which lead the current in and out, which Whewell, then Master of Trinity College, Cambridge, called 'cathode' after Faraday had asked him for suggestions.* They appear as straight lines diverging from points on the cathode and show up as phosphorescence when they reach the glass walls. This phosphorescence gets stronger as more gas is pumped out but the lines in the gas get fainter.

In the year 1897, thirty-eight years after the rays had been discovered by Plicker, their nature was still hotly contested, almost on nationalistic lines. Most English and French physicists thought they were electrified particles while the Germans mostly-but not unanimously-supported the view that they were some sort of waves in the aether, perhaps allied to those discovered by Hertz or perhaps the missing longitudinal waves which an aether in theory ought to be able to transmit.

* Actually this referred to the analogous case of currents in a conducting liquid



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In support of those who believed in particles was the well-known fact that the rays are deflected by a magnet placed near the discharge, just as would be a flexible wire carrying a current away from the cathode. A moving particle with a charge should behave like the wire. But then Faraday had shown that a magnet could rotate the plane of polarization of light waves in glass. Perhaps this was an analogous though different property of electronic waves?

Then again in favour, Perrin had recently collected the rays into a hollow metal cylinder and shown that they gave it a negative charge. However, years before, Crookes had done a rather similar experiment and got a positive charge. The probable explanation of Crookes' result, which is very briefly described without illustration, is that a fast cathode ray going into a solid releases secondary electrons. At grazing incidence these may exceed the primary electrons in number so that the solid gets on balance a positive charge.

An objection of those favouring waves was to say that no doubt charges accompanied the cathode rays but that the connexion was accidental and that the charge was only faintly connected with the rays, like the flash of a cannon with the shell.

In the year we are describing, I897, J. J. Thomson improved Perrin's experiment by showing that when the cylinder was not directly in line with the cathode rays it still received a charge if, but only if, the rays were deflected into it by a magnet. This pretty well disposed of the last argument.

Another objection to the charged particle, taken very seriously at the time though it now seems trivial, was derived from an experiment by Hertz (not the main subject of this paper) which showed that the rays could pass through thin metallic foil, gold, silver or aluminium, without making a visible hole, and produce phosphorescence in a piece of glass behind the foil. Lenard had used this discovery to extract the cathode rays from the discharge tube into the atmosphere or even into another tube exhausted or filled with a different kind of gas. He found the rays were stopped equally by equal masses of different substance from hydrogen to gold, a great range of density.

In those days, and much later, the idea of anything material going through a solid without leaving a trace was startling. Rayleigh in his life ofJ. J. Thomson records how surprised both J.J. and Rutherford were as late as I900 that , rays could be electrons and yet penetrate half a millimetre of metal.

But far the greatest objection to the particle theory of cathode rays came from a paper by Hertz as far back as 1883. This began with a long and careful examination of the distribution of the current in a gaseous discharge. He first produced the discharge in a flat glass box (I2 I2 X I cm inside) and traced the flow of current by the deflection of a magnetic needle held close to the surface



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of the box. He concluded, probably correctly, that the part of the current carried by the cathode was too small for him to measure, and that the rays had no close causal relation to the discharge. In fact a gas discharge of this kind is an extremely complicated system, almost comparable to a living body, in which the different parts react on one another in processes which are usually cyclic so it is difficult to separate cause and effect. Is for example the circulation of my blood a consequence or a cause of my being alive?

From this he went on to consider the effect of an electrostatic field on the cathode rays. For this he used a more ordinary cylindrical discharge tube with an anode, partly of wire net, which surrounded the cathode. He also used a small induction coil instead of the 2000ooo-volt battery he had used before. He admits that the induction coil (which of course produces an intermittent instead of a steady current) is a less stable device and gives less accurate measurements. He does not explain why he used it, perhaps because it gave a higher voltage, but says that in spite of its imperfections the main conclusions are reliable.

He then proceeded to test whether cathode rays are deflected by an electric field such as might be expected to exist between a pair of metal plates connected to the poles of a battery or of an induction coil (with the proviso that in the second case the field would be intermittent). Since this experiment is described immediately after that with the round tube I assume that this was what he used, though in some ways the flat one would have been more convenient. First he put a pair of plates one on each side of the tube through which the rays were streaming. No deflection was produced, nor was there any when, to avoid possible electrification of the glass, he moved the plates inside the tube (no picture is given). Satisfied with this he stopped.

This experiment was a very strong argument against charged particles. Such a particle must be acted on by an electric field and if moving across such a field its path must be bent by the field. Hertz drew the inevitable conclusion from his experiment: cathode rays are not charged particles! One cannot wholly blame him yet strangely, as we shall see later, he had just guessed the reason why the experiment failed when the plates were outside; and other physicists had made observations, of which he knew, which were in fact instances of the bending of cathode rays by an electric field. Though their authors did not see them in that way, J.J. was to quote them in support when he at last in 1897 guessed the reason.

The earliest and most persistent of these was Goldstein who found in I876 that using a discharge tube whose double cathode was composed of two parallel wires resulted in two parallel patches on the glass walls where the phosphorescence was weakened. Goldstein correctly interpreted this result as due



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to the repulsion of the cathode rays from one wire by a force due to the other wire. He wrote:

At the negative electrode is the seat of a repulsion (abstossung) which gives each ray of the discharge passing near the cathode a deflection from the cathode. Rays from one part of a cathode may be deflected from another part of the

same cathode. Goldstein did not believe that cathode rays were particles and continued to regard the 'abstossung' as some special kind of force. In a later paper (i88i) on the 'reflection' of cathode rays he rejects the action of electric force as a cause for the rays bending and ascribes it to a diffuse scattering. In many cases this is correct. He explained experiments by Crookes as modified by Wiedemann and Ebert in terms of the Abstossung. This is correct if 'abstossung' is interpreted as the result of a charged particle entering an electric field.

Why cathode rays, if electrical, are deflected by the field near a cathode but not by the field between Hertz's parallel plates was first realized by J.J. and explained by him in a lecture to the Royal Institution on 30 April I897 and printed in The Electrician of 2I May. It depends on the fact that when a discharge is passing most of the gas is a very good conductor; most of the fall of potential, at least at the pressures at which cathode rays are formed, is concentrated at the cathode in the Crookes dark space which surrounds it. It is here that the cathode rays are accelerated and given a kinetic energy nearly corresponding to the voltage on the tube. Thomson had worked for many years on the electrodeless discharge and been struck by the very high conductivity that comes when the discharge is re-entrant and the current doesn't have to get into or out of metal electrodes.

The main potential fall is near the cathode, if there is one. Here the fields are large and provide Goldstein's 'abstossung' on any negatively charged particles. Hertz's parallel plates in a highly conducting part of the discharge would indeed have sparked over but for the difficulty of getting the current in and out of them. Put in a slightly different way, the positive ions are attracted to the negative plate and accumulate there, the negative go to the positive plate, the layers of ions almost neutralize the charges on the plates which are trying to produce the field. This is therefore very small in the region between the layers. In addition J.J. may have been helped to realize what was happening by the work he and Rutherford had recently published on the ionic conduction due to X-rays. J.J. in his R.I. lecture demonstrated the deflexion of a ray from one of a pair of cathodes by the field round the other and referred to others of Goldstein's experiments.



24I

In the same lecture he made the first determination of elm using a thermopile to measure total energy received, a cylinder to collect and measure total charge, and magnetic deflexion to measure mv/e. He also postulated that the particles, 'corpuscles' as he called them, are universal constituents of all matter thus explaining Lenard's experiments on the absorption of cathode rays.

Later in the year, in the October Philosophical Magazine, he amplified the lecture and used the electrostatic deflexion of the cathode rays in the well- known measurements of elm.

Is there a moral to be drawn for the would-be discoverer? Perhaps Goldstein lost by being too cautious, strictly scientific he probably called it, in not trying to explain his abstossung. If he had tried other theories he might have found the obvious one much the best, even if it did mean acknowledging that cathode rays are charged particles. He seems to have considered that other parts of the discharge are subject to abstossung, possibly they are, but the discharge is so complicated that the only way to advance is to dissect it somehow. He was a very good experimenter.

Hertz one feels spent too long on the not-very-conclusive plotting of the lines of current flow which hardly justify the conclusions he drew since they do not satisfy the boundary conditions, as in one place he admits.

He had pretty well made up his mind before he got to it that the result would be negative, for if as he thought he had proved that the rays did not exert electric force it was almost impossible that they should be deflected by it. But he had indeed realized that with the plates outside the experiment might be falsified by a build-up of charges on the glass, and it is perhaps strange that he did not go on to consider whether there might be a similar falsification when the plates were inside by charges of opposite sign surrounding them.

It is safe to say that no-one has spent a lifetime on physical research without sometimes arriving at a misleading result. Fortunately for science few have such important consequences.

NOTES

The references to E. Goldstein's papers quoted here are: Mber. K. Preuss. Akad. Wiss., May I876, pp. 280-295. The 'abstossung' is introduced

on p. 285. Ibid., 7 July I88I. Reprinted in Ann. Phys. & Chem., 15, 246 (1882); deals with the

diffuse reflection of cathode rays. Verh. Phys. Ges. Berlin, I6 Dec. 1892. Reprinted in Ann. Phys. & Chem., 48, 787 (1893).

There are references both by Goldstein himself and others to a publication by him called Eine neue Form electrischer Abstossung. This appears to be a book published in Berlin in i880; pages are quoted up to 124. It is given as a reference in J. J. Thomson's Conduction of Electricity through Gases, Ist edition, C.U.P., 1903, p. 517. On the next page is an illustration (Fig. 174) of



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an apparatus of Goldstein's with two cathodes which must I think have been taken from this book as I can find it nowhere else. Unfortunately I have been unable to trace the book.

The references to Hertz's papers are: Ann. Phys. & Chem., 19, 782 (1883). This is a long paper on glow discharges; the part

which concerns us is from p. 809 to p. 816, which deals with the distribution of current in the discharge and the attempts to deflect cathode ray electrostatically.

Ann. Phys. & Chem., 45, 29 (1892). On the passage of cathode rays through thin metal foils.

J. J. Thomson, the relevant papers are: For the Royal Institution lecture The Electrician, 21 May 1897. Some preliminary experiments are recorded in Proc. Camb. Phil. Soc., 9, 243 (Feb.

1897). The use of electrostatic deflexion to find elm for cathode rays comes in Phil. Mag., 44, 293

(Oct. 1897). Crookes expt. on the charge on cathode rays, which was suggested by Maxwell is in Phil.

Trans., 170, 647 (1879). Another experiment of his is discussed in J.J.T.'s Conduction of electricity through Gases,

ISt edition, p. 518; 2nd edition, p. 638; 3rd edition, Vol. 2, p. I5.



An Unfortunate Experiment: Hertz and the Nature of Cathode Rays

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