Sir William Crookes

The advances which were effected in the last quarter of the 19th century in regard to the conduction of electricity through liquids, considerable though these advances were, may be regarded as the natural development of a theory which had long been before the world.

It was otherwise with the kindred problem of the conduction of electricity through gases: for although many generations of philosophers had studied the remarkable effects which are presented by the passage of a current through a rarefied gas, it was not until recent times that a satisfactory theory of the phenomena was discovered.

Some of the electricians of the earlier part of the eighteenth century performed experiments in vacuous spaces; in particular, Hauksbee[35] in 1705 observed a luminosity when glass is rubbed in rarefied air.

But the first investigator of the continuous discharge through a rarefied gas seems to have been Watson,[36] who, by means of an electrical machine, sent a current through an exhausted glass tube three feet long and three inches in diameter.

He wrote:

“It was a most delightful spectacle, when the room was darkened, to see the electricity in its passage; to be able to observe not, as in the open air, its brushes or pencils of rays an inch or two in length, but here the coruscations were of the whole length of the tube between the plates, 32 inches.”

Its appearance he described as being on different occasions “of a bright silver hue,” “resembling very much the most lively coruscations of the aurora borealis,” and “forming a continued arch of lambent fame.”

His theoretical explanation was that the electricity “is seen, without any preternatural force, pushing itself on through the vacuum by its own elasticity, in order to maintain the equilibrium in the machine”—a conception which follows naturally from the combination of Watson’s one-fluid theory with the prevalent doctrine of electrical atmospheres.[37]

A different explanation was put forward by Nollet, who performed electrical experiments in rarefied air at about the same time as Watson,[38] and saw in them a striking confirmation of his own hypothesis of efflux and afflux of electric matter.[39]

According to Nollet, the particles of the effluent stream collide with those of the aflluent stream which is moving in the opposite direction; and being thus violently shaken, are excited to the point of emitting light.

Almost a century elapsed before anything more was discovered regarding the discharge in vacuous spaces.

But in 1838 Faraday,[40] while passing a current from the electrical machine between two brass rods in rarefied air, noticed that the purple haze or stream of light which proceeded from the positive pole stopped short before it arrived at the negative rod. The negative rod, which was itself covered with a continuous glow, was thus separated from the purple column by a narrow dark space: to this, in honour of its discoverer, the name Faraday’s dark space has generally been given by subsequent writers.

That vitreous and resinous electricity give rise to different types of discharge had long been known; and indeed, as we have seen,[41] it was the study of these differences that led Franklin to identify the electricity of glass with the superfluity of fluid, and the electricity of amber with the deficiency of it.

But phenomena of this class are in general much more complex than might be supposed from the appearance which they present at a first examination; and the value of Faraday’s discovery of the negative glow and dark space lay chiefly in the simple and definite character of these features of the discharge, which indicated them as promising subjects for further research. Faraday himself felt the importance of investigations in this direction.

“The results connected with the different conditions of positive and negative discharge,” he wrote,[42] “will have a far greater influence on the philosophy of electrical science than we at present imagine.”

Twenty more years, however, passed before another notable advance was made. That a subject so full of promise should progress so slowly may appear strange; but one reason at any rate is to be found in the incapacity of the air-pumps then in use to rarefy gases to the degree required for effective study of the negative glow.

The invention of Geissler’s mercurial air-pump in 1855 did much to remove this difficulty; and it was in Geissler’s exhausted tubes that Julius Plücker,[43] of Bonn, studied the discharge three years later.

It had been shown by Sir Humphrey Davy in 1821[44] that one form of electric discharge—namely, the arc between carbon poles—is deflected when a magnet is brought near to it.

Plücker now performed a similar experiment with the vacuum discharge, and observed a similar deflexion. But the most interesting of his results were obtained by examining the behaviour of the negative glow in the magnetic field; when the negative electrode was reduced to a single point, the whole of the negative light became concentrated along the line of magnetic force passing through this point.

In other words, the negative glow disposed itself as if it were constituted of flexible chains of iron filings attached at one end to the cathode.

Plücker noticed that when the cathode was of platinum, small particles were torn off it and deposited on the walls of the glass bulb. “It is most natural,” he wrote, “to imagine that the magnetic light is formed by the incandescence of these platinum particles as they are torn from the negative electrode.”

He likewise observed that during the discharge the walls of the tube, near the cathode, glowed with a phosphorescent light, and remarked that the position of this light was altered when the magnetic field was changed.

This led to another discovery; for in 1869 Plücker’s pupil, W. Hittorf,[45] having placed a solid body between a point-cathode and the phosphorescent light, was surprised to find that a shadow was cast. He rightly inferred from this that the negative glow is formed of rays which proceed from the cathode in straight lines, and which cause the phosphorescence when they strike the walls of the tube.

Hittorf’s observation was amplified in 1876 by Eugen Goldstein,[46] who found that distinct shadows were cast, not only when he cathode was a single point, but also when it: formed an extended surface, provided the shadow-throwing object was placed close to it.

This clearly showed that the cathode rays (a term now for the first time introduced) are not emitted indiscriminately in all directions, but that each portion of the cathode surface emits rays which are practically confined to a single direction; and Goldstein found this direction to be normal to the surface.

His discovery established an important distinction between the manner in which cathode rays are emitted from an electrode and that in which light is emitted from an incandescent surface.

The question as to the nature of the cathode rays attracted much attention during the next two decades.

In the year following Hittorf’s investigation, Cromwell Varley[47] put forward the hypothesis that the rays are composed of “attenuated particles of matter, projected from the negative pole by electricity”; and that it is in virtue of their negative charges that these particles are influenced by a magnetic field.[48]

During some years following this, the properties of highly rarefied gases were investigated by Sir William Crookes.

Influenced, doubtless, by the ideas which were developed in connexion with his discovery of the radiometer, Crookes,[49] like Varley, proposed to regard the cathode rays as a molecular torrent: he supposed the molecules of the residual gas, coming into contact with the cathode, to acquire from it a resinous charge, and immediately to fly off normally to the surface, by reason of the mutual repulsion exerted by similarly electrified bodies.

Carrying the exhaustion to a higher degree, Crookes was enabled to study a dark space which under such circumstances appears between the cathode and the cathode glow; and to show that at the highest rarefactions this dark space(which has since been generally known by his name) enlarges until the whole tube is occupied by it. Ho suggested that the thickness of the dark space may be a measure of the mean length of free path of the molecules.

“The extra velocity,” he wrote, “with which the molecules rebound from the excited negative pole keeps back the more slowly moving molecules which are advancing towards that pole.

The conflict occurs at the boundary of the dark space, where the luminous margin bears witness to the energy of the collisions."[50] Thus according to Crookes the dark space is dark and the glow bright because there are collisions in the latter and not in the former.

The fluorescence or phosphorescence on the walls of the tube he attributed to the impact of the particles on the glass.

Crookes spoke of the cathode rays as an “ultra-gaseous” or “fourth state” of matter.

These expressions have led some later writers to ascribe to him the enunciation or prediction of a hypothesis regarding the nature of the particles projected from the cathode, which arose some years afterwards, and which we shall presently describe; but it is clear from Crookes’ memoirs that he conceived the particles of the cathode rays to be ordinary gaseous molecules, carrying electric charges; and by “a new state of matter” he understood simply a state in which the free path is so long that collisions may be disregarded.

Crookes found that two adjacent pencils of cathode rays appeared to repel each other. At the time this was regarded as a direct confirmation of the hypothesis that the rays are streams of electrically charged particles; but it was shown later that the deflexion of the rays must be assigned to causes other than mutual repulsion.

How admirably the molecular-torrent theory accounts for the deviation of the cathode rays by a magnetic field was shown by the calculations of Eduard Riecke in 1881.[51] If the axis of z be taken parallel to the magnetic force H, the equations of motion of a particle of mass m, charge e, and velocity (u, v, w) are

The last equation shows that the component of velocity of the particle parallel to the magnetic force is constant; the other equations give

showing that the projection of the path on a plane at right angles to the magnetic force is a circle. Thus, in a magnetic field the particles of the molecular torrent describe spiral paths whose axes are the lines of magnetic force.

But the hypothesis of Varley and Crookes was before long involved in difficulties.

Tait[52] in 1880 remarked that if the particles are moving with great velocities, the periods of the luminous vibrations received from then should be affected to a measurable extent in accordance with Doppler’s principle.

Tait tried to obtain this effect, but without success. It may, however, be argued that if, as Crookes supposed, the particles become luminous only when they have collided with other particles, and have thereby lost part of their velocity, the phenomenon in question is not to be expected.

396 Conduction in Solutions and Gases, The alternative to the molecular-torrent theory is to suppose that the cathode radiation is a disturbance of the aether. This view was maintained by several physicists,[53] and notably by Hertz,[54] who rejected Varley’s hypothesis when he found experimentally that the rays did not appear to produce any external electric or magnetic force, and were apparently not affected by an electrostatic field. It was, however, pointed out by FitzGerald[55] that external space is probably screened from the effects of the rays by other electric actions which take place in the discharge tube.

It was further urged against the charged-particle theory that cathode rays are capable of passing through films of metal which are so thick as to be quite opaque to ordinary light;[56]

It seemed inconceivable that particles of matter should not be stopped by even the thinnest gold-leaf.

At the time of Hertz’s experiments, an attempt to obviate this difficulty was made by J.-J. Thomson[57]. He suggested that the metallic film when bombarded by the rays might itself acquire the property of emitting charged particles, so that the rays which were observed on the further side need not have passed through the film.

Thomson ultimately found the true explanation. But this depended in part on another order of ideas, whose introduction and development must now be traced.

The general tendency was now to abandon the electron-theory of Weber in favour of Maxwell’s theory.

• This involved certain changes in the conceptions of electric charge.