Weber's theory

In Weber’s theory, electric phenomena were attributed to the agency of stationary or moving charges, which could most readily be pictured as having a discrete and atom-like existence.

The conception of displacement was at the root of the Maxwellian theory. It represented electricity as having a continuous nature.

Maxwell’s views met with increasing acceptance. The atomistic hypothesis entered on a period of decay.

Its revival was due largely to the advocacy of Helmholtz who, in a lecture delivered to the Chemical Society of London in 1881, pointed out that it was thoroughly in accord with the ideas of Faraday, on which Maxwell’s theory was founded.

When the conduction of electricity is considered in the light of this hypothesis, it seems almost inevitable to conclude that the process is of much the same character in gases as in electrolytes.

Before long, this view was actively maintained.

It had long been known that:

• a compound gas might be decomposed by the electric discharge
• in some cases the constituents are liberated at the electrodes in such a way as to suggest an analogy with electrolysis

The question had been studied in 1861 by Adolphe Perrot. He examined the gases liberated by the passage of the electric spark through steam.

• He found that while the product of this action was a detonating mixture of hydrogen and oxygen, there was a decided preponderance of hydrogen at one pole and of oxygen at the other

The analogy of gaseous conduction to electrolysis was applied by W. Giese, of Berlin in 1882, in order to explain the conductivity of the hot gases of flames.

Arthur Schuster, of Manchester presented a general theory of the discharge in rarefied gases 2 years later.

Schuster remarked that when hot liquids are maintained at a high potential, the vapours which rise from them are found to be entirely free from electrification. From this he inferred that:

• a molecule striking an electrified surface in its rapid motion cannot carry away any part of the charge, and
• one molecule cannot communicate electricity to another in an encounter in which both molecules remain intact.

Thus he concluded that dissociation of the gaseous molecules is necessary for the passage of electricity through gases.

Schuster advocated the charged particle theory of cathode rays. By extending and interpreting an experiment of Hittorf’s, he was able to adduce strong evidence in its favour. He placed the positive and negative electrodes so close to each other that at very low pressures the Crookes’ dark space extended from the cathode to beyond the anode.

In these circumstances it was found that the discharge from the positive electrode always passed to the nearest point of the inner boundary of the Crookes’ dark space—Which, of course, was in vessel; the opposite direction to the cathode. Thus, in the neighbourhood of the positive discharge, the current was flowing in two opposite directions at closely adjoining places; which could scarcely happen unless the current in one direction were carried by particles moving against the lines of force by virtue of their inertia,

Continuing his researches, Schuster[65] showed in 1887 that a steady electric current may be obtained in air between electrodes whose difference of potential is but small, provided that an independent current is maintained in the same that is to say, a continuous discharge produces in the air such a condition that conduction occurs with the smallest electromotive forces.

This effect he explained by aid of the hypothesis previously advanced; the ions produced by the main discharge become diffused throughout the vessel, and, coming under the influence of the field set up by the auxiliary electrodes, drift so as to carry a current between the latter.

A discovery related to this was made in the same year by Hertz,[66] in the course of the celebrated researches[67] which have been already mentioned. Happening to notice that the passage of one spark is facilitated by the passage of another spark in its neighbourhood, he followed up the observation, and found the phenomenon to be due to the agency of ultra-violet light emitted by the latter spark.

It appeared in fact that the distance across which an electric spark can pass in air is greatly increased when light of very short wave-length is allowed to fall on the spark-gap.

It was soon found[68] that the effective light is that which falls on the negative electrode of the gap; and Wilhelm Hallwachs[69] extended the discovery by showing that when a sheet of metal is negatively electrified and exposed to ultra-violet light, the adjacent air is thrown into a state which permits the charge to leak rapidly away.

Interest was now thoroughly aroused in the problem of conductivity in gases; and it was generally felt that the best hope of divining the nature of the process lay in studying the discharge at high rarefactions. “If a first step towards understanding the relations between aether and ponderable matter is to be made,” said Lord Kelvin in 1893,[70] “it seems to me that the most hopeful foundation for it is knowledge derived from experiments on electricity in high vacuum.”

Within the two following years considerable progress was effected in this direction. J.J. Thomson,[71] by a rotating-mirror method, succeeded in measuring the velocity of the cathode rays, finding it to be[72] 1·9 x 107 cm./sec.; a value so much smaller than that of the velocity of light that it was scarcely possible to conceive of the rays as vibrations of the aether.

A further blow was dealt at the latter hypothesis when Jean Perrin,[73] having received the rays in a metallic cylinder, found that the cylinder became charged with resinous electricity. When the rays were deviated by a magnet in such a way that they could no longer enter the cylinder, it no longer acquired a charge. This appeared to demonstrate that the rays transport negative electricity.

With cathode rays is closely connected another type of radiation, which was discovered in December, 1895, by W. C. Röntgen.[74] The discovery seems to have originated in an accident: a photographic plate which, protected in the usual way, had been kept in a room in which vacuum-tube experiments were carried on, was found on development to show distinct markings.

Experiments suggested by this showed that radiation, capable of affecting sensitive plates and of causing fluorescence in certain substances, is emitted by tubes in which the electric discharge is passing; and that the radiation proceeds from the place where the cathode rays strike the glass walls of the tube.

The X-rays, as they were called by their discoverer, are propagated in straight lines, and can neither be refracted by any of the substances which refract light, nor deviated from their course by a magnetic field; they are moreover able to pass with little absorption through many substances which are opaque to ordinary and ultra-violet light—a property of which considerable use has been made in surgery.

The nature of the new radiation was the subject of much speculation.

Its discoverer suggested that it might prove to represent the long-sought-for longitudinal vibrations of the aether; while other writers advocated the rival claims of aethereal vortices, infra-red light, and “sifted” cathode rays. The hypothesis which subsequently obtained general acceptance was first propounded by Schuster[75] in the month following the publication of Röntgen’s researches. It is, that the X-rays are transverse vibrations of the aether, of exceedingly small wavelength.

A suggestion which was put forward later in the year by E. Wiechert[76] and Sir George Stokes,[77] to the effect that the rays are pulses generated in the aether when the glass of the discharge tube is bombarded by the cathode particles, is not really distinct from Schuster’s hypothesis; for ordinary white light likewise consists of pulses, as Gouy[78] had shown, and the essential feature which distinguishes the Röntgen pulses is that the harmonic vibrations into which they can be resolved by Fourier’s analysis are of very short period.

The rapidity of the vibrations explains the failure of all attempts to refract the X-rays. For in the formula of the Maxwell-Sellmeier theory,[79] n denotes the frequency, and so is in this case extremely large; whence we have

i.e., the refractive index of all substances for the X-rays is unity. In fact, the vibrations alternate too rapidly to have an effect on the sluggish systems which are concerned in refraction.

Some years afterwards H. Haga and C. H. Wind,[80] having measured the diffraction-patterns produced by X-rays, concluded that the wave-length of the vibrations concerned was of the order of one Ångstrom unit, that is about 1/6000 of the wavelength of the yellow light of sodium.

One of the most important properties of X-rays was discovered, shortly after the rays themselves had become known, by J. J. Thomson,[81] who announced that when they pass through any substance, whether solid, liquid, or gaseous, they render it conducting. This he attributed, in accordance with the ionic theory of conduction, to " a kind of electrolysis, the molecule of the non-conductor being split up, or nearly split up, by the Röntgen rays."

The conductivity produced in gases by this means was at once investigated[82] more closely.

It was found that a gas which had acquired conducting power by exposure to X-rays lost this quality when forced through a plug of glass-wool; whence it was inferred that the structure in virtue of which the gas conducts is of so coarse a character that it is unable to survive the passage through the fine pores of the plug.

The soon as conductivity was also found to be destroyed when an electric current was passed through the gas—a phenomenon for which a parallel may be found in electrolysis.

For if the ions were removed from an electrolytic solution by the passage of a current. the solution would cease to conduct as sufficient electricity had passed to remove them all; and it may be supposed that the conducting agents which are produced in a gas by exposure to X-rays are likewise abstracted from it when they are employed to transport charges.

The same idea may be applied to explain another property of gases exposed to X-rays. The strength of the current through the gas depends both on the intensity of the radiation and also on the electromotive force; but if the former factor be constant, and the electromotive force be increased, the current does not increase indefinitely, but tends to attain a certain “saturation” value. The existence of this saturation value is evidently due to the inability of the electromotive force to do more than to remove the ions as fast as they are produced by the rays.

Meanwhile other evidence was accumulating to show that the conductivity produced in gases by X-rays is of the same nature as the conductivity of the gases from fames and from the path of a discharge, to which the theory of Giese and Schuster had already been applied. One proof of this identity was supplied by observations of the condensation of watervapour into clouds.

It had been noticed long before by John Aitken[83] that gases rising from flames cause precipitation of the aqueous vapour from a saturated gas; and R. von Helmholtz[84] had found that gases through which an electric discharge has been passed possess the same property. It was now shown by C. T. R. Wilson,[85] working in the Cavendish Laboratory at Cambridge, that the same is true of gases which have been exposed to X-rays. The explanation furnished by the ionic theory is that in all three cases the gas contains ions which act as centres of condensation for the vapour.

During the year which followed their discovery, the X-rays were so thoroughly examined that at the end of that period they were almost better understood than the cathode rays from which they derived their origin. But the obscurity in which this subject had been so long involved was now to be dispelled.

Lecturing at the Royal Institution on April 30th, 1897, J. J. Thomson advanced a new suggestion to reconcile the molecular-torrent hypothesis with Lenard’s observations of the passage of cathode rays through material bodies.

“We see from Lenard’s table,” he said, “that a cathode ray can travel through air at atmospheric pressure a distance of about half a centimetre before the brightness of the phosphorescence falls to about half its original value. Now the mean free path of the molecule of air at this pressure is about 10-5 cm., and if a molecule of air were projected it would lose half its momentum in a space comparable with the mean free path.

Even if we suppose that it is not the same molecule that is carried, the effect of the obliquity of the collisions would reduce the momentum to hall in a short multiple of that path.

“Thus, from Lenard’s experiments on the absorption of the rays outside the tube, it follows on the hypothesis that the cathode rays are charged particles moving with high velocities that the size of the carriers must be small compared with the dimensions of ordinary atoms or molecules.[86]

The assumption of a state of matter more finely subdivided than the atom of an element is a somewhat startling one; but a hypothesis that would involve somewhat similar consequences—viz. that the so-called elements are compounds of some primordial element—has been put forward from time to time by various chemists.”

Thomson’s lecture drew from FitzGerald[87] the suggestion that “we are dealing with free electrons in these cathode rays”—a remark the point of which will become more evident when wo come to consider the direction in which the Maxwellian theory was being developed at this time.

Shortly afterwards Thomson himself published an account[88] of experiments in which the only outstanding objections to the charged-particle theory were removed. The chief of these was Hertz’ failure to deflect the cathode rays by an electrostatic field.

Hertz had caused the rays to travel between parallel plates of metal maintained at different potentials; but Thomson now showed that in these circumstances the rays generate ions in the rarefied gas, which settle on the plates, and annul the electric force in the intervening space. By carrying the exhaustion to a much higher degree, he removed this source of confusion, and obtained the expected deflexion of the rays.

The electrostatic and magnetic deflexions taken together suffice to determine the ratio of the mass of a cathode particle to the charge which it carries. For the equation of motion of the particle is

where r denotes the vector from the origin to the position of the particle; E and H denote the electric and magnetic forces; e the charge, m the mass, and v the velocity of the particle.

By observing the circumstances in which the force eE, due to the electric field, exactly balances the force e[v.H], due to the magnetic field, it is possible to determine v; and it is readily seen from the above equation that a measurement of the deflexion in the magnetic field supplies a relation between v and m/e; so both v and m/e may be determined.

Thomson found the value of m/e to be independent of the nature of the rarefied gas: its amount was 10-7 (grammes/electromagnetic units of charge), which is only about the thousandth part of the value of m/e for the hydrogen atom in electrolysis.

If the charge were of the same order of magnitude as that on an electrolytic ion, it would be necessary to conclude that the particle whose mass was thus measured is much smaller than the atom, and the conjecture might be entertained that it is the primordial unit or corpuscle of which all atoms are ultimately composed.