wax, by means of which they were sealed into the dischargetube, from softening. Another difficulty, due to the intense heat generated, was the liberation of gas from the walls of the vessel and from the electrodes with resultant increase of pressure within the vessel. To overcome this a large reservoir was sealed to the tube so that this gas emitted did not materially change the pressure in the apparatus while the arc was passing.

When the arc was first started the radiation emitted was entirely that from the gas molecules within the vessel, but the electrodes rapidly became red-hot and the characteristic radiation from the iron quickly made its appearance, the electrodes then becoming incandescent. As a rule, however, the arc was allowed to persist only for a short interval, as, once started, it is easily maintained, and it is the actual process of striking that is interesting and worthy of study. In passing, it may be noted that this method is very convenient and useful for starting and maintaining an iron arc in a very high vacuum, if it is desired to study the iron lines

in vacuo.

The lowest applied potential difference with which it was possible to strike the arc was 108 volts, but it started most consistently at 200 volts, and the current which could be passed varied from 0'4 ampere upwards. Below this limiting value it was impossible to maintain the arc. Another factor

which greatly influenced the process of starting was the type of electrical discharge employed. If the latter was feeble it was ineffective. A momentary discharge produced by temporarily switching in the induction coil commutator proved to be the best means of starting the arc. Thus a very high potential difference from the coil is necessary, and the most consistent results were obtained when the secondary discharge was unidirectional. The arc would not strike at pressures above 10 mm. of mercury, and worked better at lower pressures, particularly those within the range 10-2-10-3 mm.

In order that the arc may occur, the gas present must be ionized, and for this energy is required. This comes from electrified bodies which have acquired high velocities under the electric force from the transient electrical discharges, the initiation of the latter being due to stray electrons or electrified atoms, particularly to stray electrons. Before these electrons can ionize the gas, a time must elapse which is large in comparison with the time-interval between one collision of the electron with a molecule of the gas and the next, since before the electron can ionize it must obtain from the electric

field energy greater than the ionizing potential of the gas, and this energy must be conveyed to an electron in a molecule before an electron can be liberated. Thus there is produced a fairly copious supply of electrons which, under the further influence of the field due to the applied potential difference, form more ions by collisions, provided that the mean free path of the electrons is so large that they attain energy equivalent to the ionizing potential of the gas through which the electric arc passes. It seems, therefore, that the function of the electrical discharge is to provide those electrons which are ordinarily supplied by the incandescent electrode of the ordinary arc, or by the incandescent filament of a low-voltage arc.

These

In connexion with the initial ionization in a gas, Sir J. J. Thomson has given an account of experiments on the radiations inside a closed vessel containing gas at a low pressure through which an electrical discharge is passed. experiments indicate that the passage of the electrons through gases gives rise to Röntgen rays which may be of far higher frequency than any of the characteristic radiation of the gas. For example, he finds that, when electrons formed by a potential difference of 1500 volts are sent through hydrogen, Röntgen radiations having a frequency corresponding to 1500 volts were detected. The type of radiation at constant pressure depends upon the means used to send the discharge through the tube; for example, it depends upon the nature of the interrupter and its working. This is to be expected, as the potential difference between the electrodes often depends upon the nature of the discharge.

In addition, some of the radiation within the dischargetube is of an exceedingly absorbable type, so absorbable that it is practically stopped by the equivalent in mass of a layer of air a few millimetres thick at atmospheric pressure. He supposes that this soft radiation produces the ionization in the negative glow.

At the lower gas-pressures the electrons, owing to their greater mean free path, have large amounts of energy, and they give radiations over a larger range of frequency than those with smaller amounts, although the density of the energy is not so great. At each collision the fast electrons may not produce much more radiant energy than the slow ones, but they are able to make many collisions with atoms and molecules before their energy is so

much reduced that they are unable to generate Röntgen radiation. Hence the total amount of radiant energy produced by a fast electron will be much greater than that formed by a slow one, with the resultant increased ionizing effects. Thus we should expect a greater concentration of the electrons at the lower pressures; and this explains why the arc in the present experiments can be started more easily and consistently as the pressure is reduced. As Thomson has shown, the radiation from the cathode rays is a mixture of soft radiation with quanta represented by a few volts, together with much harder radiation whose quanta may be comparable with a hundred volts. In addition there is the softer type of radiation produced by the positive rays. Seeliger † has shown that the total number of ions produced by a positive ray does not vary much either with the speed of the ray or with the gas-pressure; and this suggests that the ionization is due, not to the energy of translation of the particle, but to energy internal to it, such as might be represented by supposing the particle to contain a limited number of undischarged radiation quanta. These excite the ionization. The experiments made by Thomson ‡ on the electrodeless ring discharge show the existence of such radiations of wave-lengths shorter than those of visible light.

3. The Effects of Foreign Bodies on the Arc.

After the arc electrodes had been used for some time, it was impossible to start the arc, and on examining the electrodes they were found to be coated with a film of oxide arising from the decomposition of carbon-monoxide and carbon-dioxide gases, which are very difficult to remove from the vessel. If the electrodes were withdrawn and scraped, the arc could be produced as before. On the other hand, it was found that when patches of fused salts, such as potash, soda, sodium chloride, and calcium carbonate, were placed at the ends of the electrodes, the arc would strike more readily and consistently, this effect being, in fact, the reverse to that produced by the film of oxide on the metal. In addition, it was noted that usually a brilliant spot appeared on the cathode, even in the absence of any fused salt, and that this spot continuously changed its position, as it does in the mercury arc.

The effect of the fused salts can be explained by an

abnormal local heating, brought about by the bombardment with the positive ions. The salt, or a small part of it, is raised to incandescence, with the result that it emits a copions supply of electrons; and we have, therefore, a thermionic source similar to that of an incandescent cathode, only on a small scale. As a result the starting of the arc is facilitated. As opposed to this effect, the oxide film prevents these positive ions from bombarding the cathode, and, accordingly, there is no local heating-effect at the small particles of impurities, which exist even in the purest of iron. Thus the arc cannot be started.

These experiments seem to prove conclusively that this phenomenon of the cold electrode arc is due mainly to the localized heating-effect at these foreign particles which gives a copious supply of electrons from such local sources. Anything that shortens the life of the electron will increase the difficulty of getting the arc, but anything that reduces the number of collisions, required by one electron to detach another from a molecule, will facilitate the arc starting. It has been found that a thin deposit of sulphur is formed on the walls of the discharge-tube after the experiments have been continued for some time, this sulphur coming from the iron electrodes. Now, sulphur is very strongly electronegative, and, presumably, a sulphur molecule would be likely to capture an electron in collision, so that, although the vapour-pressure of sulphur is exceedingly low, the presence of this element would diminish the life of the electrons. The various gases present in the tube, such as nitrogen, hydrogen, etc., combine with the sulphur, and if these products, like sulphur, possess the power of capturing electrons, then the ionization in the tube is decreased, and this precludes the starting of the arc. On the other hand, complexes formed by aggregation, such as are produced by traces of moisture, facilitate the production of the arc. This may be explained by the electron detaching an electron more easily from one of these aggregates than from a single molecule. The moisture appears to act as a catalyst for the arc as it does in many cases of chemical combination. The existence of these aggregates is suggested by some experiments of Thomson. The ordinary electrical discharge through gases produces also other modifications of the gases which only persist for a short time. In some cases, for example, with nitrogen and oxygen the existence of these modified forms is shown by the visible after-glows. We also

* Phil. Mag. iv. p. 1128 (1927).

know that the discharge and the arc can be maintained with lower applied potential differences than those required to start them, and this fact can be explained by the discharge and are producing systems which are more easily ionized than the normal molecules.

It is evident, therefore, that in the present experiments the initial electrical discharge gives rise to two effects which facilitate the starting of the arc. In the first place it produces intense local heating of impurities lying on the surface of the cathode with resulting thermionic emission, and in the second place modifications of the gas arise, these modifications being more easily ionized than the gas in the normal

state.

LXXX. The Motions of Electrons in Ethylene. By J. BANNON. B.Sc., and H. L. BROSE, M.A., D.Phil., F.Inst.P.*.

IN

N an address given by Professor Townsend at the centenary celebration of the Franklin Institute in Philadelphia, September 1924, the type of instrument used in the present research has been fully described. The instrument affords a ready and accurate means of determining the velocity W in the direction of the electric force, and the velocity of agitation U of electrons in gases. It has been found that in nitrogen, hydrogen, and oxygen †, amongst other gases, these velocities depend only on the ratio of the electric force Z to the gas pressure p. A similar result has been obtained for ethylene, especially for values of Zip greater than 5. Exceptionally tedious was the work on this gas, which appeared to alter as a result of making observations. This alteration manifested itself solely as an increase both in W and U.

In Table I. the values of Z and p, with the corresponding values of k and W, are given, where represents the ratio of the mean kinetic energy of agitation of an electron to the mean kinetic energy of a molecule of a gas at 15° C. The numbers given are the means of several experiments, which are in good agreement. For values of Z/p, equal to

* Communicated by Prof. J. S. Townsend, F.R.S.

↑ J. S. Townsend & V. A. Bailey, Phil. Mag. xlii. p. 875 (Dec. 1921); H. L. Brose, Phil. Mag. 1. p. 543 (1925).