0. W. Richardson

The 18th century philosophers knew that the air near an incandescent metal acquires the power of conducting electricity.

The subject continued to attract attention at intervals.

The process of conduction in gases came to be better understood, the conductivity produced in the neighbourhood of incandescent metals was attributed to the emission of electrically charged particles by the metals.

But it was not until the development of J.J. Thomson’s theory of ionization in gages that notable advances were made.

In 1899, Thomson determined the ratio of the charge to the mass of the resinously charged ions emitted by a hot filament of carbon in rarefied hydrogen, by observing their deflexion in a magnetic field.

The value obtained for the ratio was nearly the same as that which he had found for the corpuscles of cathode rays; whence he concluded that the negative ions emitted by the hot carbon were negative electrons.

The corresponding investigation[121] for the positive leak from hot bodies yielded the information that the mass of the positive ions is of the same order of magnitude as the mass of material atoms.

There are reasons for believing that these ions are produced from gas which has been absorbed by the superficial layer of the metal.[122]

If, when a hot metal is emitting ions in a rarefied gas, an electromotive force be established between the metal and a neighbouring electrode, either the positive or the negative ions are urged towards the electrode by the electric field, and a current is thus transmitted through the intervening space.

When the metal is at a higher potential than the electrode, the current is carried by the vitreously charged ions: when the electrode is at the higher potential, by those with resinous charges.

In either case, it is found that when the electromotive force is increased indefinitely, the current does not increase indefinitely likewise, but acquires a certain “saturation” value. The obvious explanation of this is that the supply of ions available for carrying the current is limited.

When the temperature of the metal is high, the ions emitted are mainly negative; and it is found[123] that in these circumstances, when the surrounding gas is rarefied, the saturation-current is almost independent of the nature of the gas or of its pressure.

The leak of resinous electricity from a metallic surface in a rarefied gas must therefore depend only on the temperature and on the nature of the metal; and it was shown by 0. W. Richardson[124] that the dependence on the temperature may be expressed by an equation of the form

where i denotes the saturation-current per unit area of surface (which is proportional to the number of ions emitted in unit time), T denotes the absolute temperature, and A and b are constants.[125]

In order to account for these phenomena, Richardson[126]adopted the hypothesis which had previously been proposed[127] for the explanation of metallic conductivity; namely, that a metal is to be regarded as a sponge-like structure of comparatively large fixed positive ions and molecules, in the interstices of which negative electrons are in rapid motion.

Since the electrons do not all escape freely at the surface, he postulated a superficial discontinuity of potential, sufficient to restrain most of them. Thus, let N denote the number of free electrons in unit volume of the metal; then in a parallelepiped whose height measured at right angles to the surface is dx, and whose base is of unit area, the number of electrons whose x-components of velocity are comprised between u and u + du is

• m denotes the electron’s mass
• T is the absolute temperature
• q is the universal constant previously introduced.

An electron whose x-component of velocity is u will arrive at the interface within an interval dt of time, provided that at the beginning of this interval it is within a distance udt of the interface. So the number of electrons whose x-components of velocity are comprised between u and u + du which arrive at unit area of the interface in the interval dt is

If the work which an electron must perform in order to escape through the surface layer be denoted by φ, the number of electrons emitted by unit area of metal in unit time is therefore

The current issuing from unit area of the hot metal is thus

where ε denotes the charge on an electron. This expression, being of the form

agrees with the experimental measures; and the comparison furnishes the value of the superficial discontinuity of potential which is implied in the existence of φ.[128]

A few years after the date of this investigation, a plan was devised and successfully carried out[129] for determining experimentally the kinetic energy possessed by the ions after emission.

The mean kinetic energy of both negative and positive ions was found to be the same for various metals (platinum, gold, silver, etc.), and to be directly proportional to the absolute temperature, and the distribution of velocities among the ions proved to be that expressed by Maxwell’s law. The ions may therefore be regarded as kinetically equivalent to the molecules of & gas whose temperature is the same as that of the metal.

By the investigations which have been recorded, the hypothesis of atomic electric charges has been, to all appearances, decisively established.

But all the parts of the theory of electrons do not enjoy an equal degree of security; and in particular, it is possible that the future may bring important changes in the conception of the aether.

The hope was formerly entertained of discovering an aether by reference to which motion might be estimated absolutely; but such a hope has been destroyed by the researches which have sprung from FitzGerald’s hypothesis of contraction, and in some recent writings it is possible to recognize a tendency to replace the classical aether by other conceptions, which, however, have been as yet but indistinctly outlined.

In the last decades of the 19th century, the following profoundly changed:

• the discoveries themselves
• the conditions of scientific organization and endeavour

The investigators who advanced the theories of aether and electricity, from the time of Descartes to that of Lord Kelvin, were, with very few exceptions, congregated within a narrow territory:

• from Dublin to the western provinces of Russia
• from Stockholm to the north of Italy

These may be circumscribed by a circle of no more than 600 miles radius.

But throughout Kelvin’s long life, the domain of culture was rapidly extending:

• the learning of the Germanic and Latin peoples was carried to the furthest regions of the earth
• new universities were founded
• inquiries into the secrets of nature were instituted in every quarter of the globe.