E L E C T.li I C I T Y [DISRUPTIVE DISCHARGE, Hih Minimum strength. Strength of va- cuum. other words, that the disintegration of the electrode played no essential part in the discharge. The quantity of electricity required to effect a discharge, other things being equal, increases with increasing pressure. This increase is at first rapid, then slower, and at high pressures it is nearly proportional to the increase of pressure. It was found that y could be expressed with sufficient accu racy in terms of the pressure p by the empirical formula, y = A + B/> - C/> 2 , in which the constants A, B, C depend on the size and insulation of the electrodes, their distance apart, and so on. They arrange the gases in the following order of dielectric strength : hydrogen, oxygen, carbonic acid, air, nitrogen. It is not a little remarkable that this is the order given by Faraday in the second column (the best) of the results we quoted above. They find, in agreement with Faraday, that a greater quantity of electricity is required to bring two unequal spheres to the discharging point when the small one is positive than when it is negative. When two equal spheres are used, the value of y is least when both are insulated, greater when the positive sphere is uninsulated, and very much greater when the negative one is uninsulated. All this is in accordance with theory, provided we assume with Faraday that the limiting tension is greater at positive than at nega tive surfaces. For example, suppose the surface densities correspon ding to the limiting positive and negative tensions to be P and N (P>N), and consider the case of two equal spheres of radius a, at so / a 3 great a distance c apart that ( - j may be neglected, then by taking three consecutive images the reader will easily find that the charges which must be given to either ball in the case where both spheres are insulated and equally charged, and to the negative ball in the case where the positive ball is uninsulated, and to the positive ball / a 2 when the negative ball is uninsulated, must be ( 1 - 3 ^ Uwa-N ,
c /
4rra 2 N , 4?ra 2 P , respectively, in order to produce discharge. The discharge begins at the negative ball in the first two cases, and at the positive ball in the third, and the quantities are obviously in ascending order of magnitude when P is >N. The dielectric strength goes on increasing when the pressure is raised above the atmospheric pressure. Cailletet 1 found that a powerful induction coil worked by eight large Bunsen cells was powerless to effect discharges across ^ mm. of dry gas at a pressure of 40 or 50 atmospheres. On the other hand, however, the dielectric strength does n ut diminish indefinitely as the pressure decreases, but reaches a minimum. Morren and De la Rive 8 have sought to determine this minimum dielectric strength by measuring by means of a galvanometer the mean intensity of the current sent through the gas by an incluc- torium so arranged that only the direct induction current passes ; they thus obtain what they call a minimum resistance. Morren gives the pressures corresponding to this minimum for various gases; they lie between O l and 3 mm. It may be questioned whether any very definite meaning can be attached to results of this kind; for the discharge is discontinuous, and resistance in the proper sense of the term cannot be spoken of. It is clear, however, that a minimum dielectric strength must exist ; for, if we go on improving our vacuum, we n( j th^ our ordinary machinery fails to send electricity through any considerable length of the exhausted space. Morgan 3 seems to have been the first to discover that the electric spark would not pass in a vacuum. Having carefully boiled the mercury in a barometer tube, so as to remove the last traces of moisture, he found that the inductive discharge caused by electri fying a piece of tinfoil on the outside of the tube would no longer pass to the mercury, and cause the luminous phenomena usually seen under such circumstances. Masson repeated this experiment in a more satisfactory form. Gassiot 4 greatly improved the exhaus tion of vacuum tubes by filling them with C0 2 , pumping out as usual, and then absorbing the residual gas by fusing a piece of KHO previously inserted into the tube. He constructed tubes in 1 Mascart, t. i. 187 8 Phil. Trans., 1785. 2 Wiedemann Bd. ii. 952 4 PhU. Trans., 1859. this way which had sufficient dielectric strength to insulate the pole of his great battery of more than 3500 Zn. Aq. Cu. cells. Hittorf and Geissler 5 have constructed vacuum tubes (by pumping with a Geissler s pump, and heating the whole to 400 to 500 C.) in which the opposition to the discharge of an interval of mm. between two platinum electrodes was greater than that offered by 15 or 20 centimetres of ordinary air. Different Forms of the Discharge in Gases. We have said Progr that the subsequent progress of the disruptive discharge of dl . s when once begun is influenced by a great variety of circum- ^ l stances. The beginning of the discharge evolves heat, charg- which rarefies the neighbouring air, and therefore weakens its dielectric strength. Owing to this cause the discharge once started tends to go on. Again, if any considerable quantity of electricity escapes into the ruptured dielectric at the first burst, this relieves the tension at the surface of the conductor. On the other hand, the progress of part of the electricity towards the opposing conductor raises the tension at the surface of the latter, so that disruptive dis charge is provoked or helped there. If the initial tension is considerable, or the quantity of electricity which passes to begin with very great, glowing metal particles are shot forth into the dielectric, causing a reduction of its strength, which will be very different in different directions. Motions of the air play a great if not a preponderating part in many forms of the discharge. The electrification, &c., of the walls of the tube, and the form of the electrodes and of the tube, both in the neighbourhood of the electrodes and at a distance from them, are as important in their influence on the continuance of the discharge as they are on its start. And, last but not least, much depends on the way the electricity which produces the discharge is furnished, on the nature of the electromotor, in short. Although we have not yet exhausted the influencing conditions, we have pro bably said enough to convince the reader that little aid is to be hoped for in this matter from considerations a priori. There is a great deficiency even in proximate principles to guide us in the maze of experimental detail; and although most of the experiments are beautiful beyond all conception, yet the mere narration would scarcely interest the reader. Our description of the department will, therefore, consist simply in going round the boundary. The luminous appearances may be roughly classed under the forms of spark, brush, glow and convective discharge, and dark discharge. At the ordinary atmospheric pressure the disruptive dis- Sparl charge between two conductors at a moderate distance apart takes place in the form of a brilliant sharply-bounded streak of light, whose apparent breadth is in general small. For small distances the spark is straight, and has the appearance of being thicker, or at least more brilliant, at the ends than in the middle. When the distance is considerably increased the spark assumes the characteristic zig-zag form seen in forked lightning. It seems occasionally to be absolutely broken by perfectly dark spaces. The duration of the dis charge in this form, more especially when the resistance of the discharging circuit is very small, as tested by a rotat ing mirror, appears to be exceedingly short. We have taken photographs of the sparks of a Holtz s machine by simply moving the camera containing the sensitized plate vertically upwards past the electrodes of the machine. The result is a column of perfect photographs, quite unblurred by the jarring, &c., of the camera stand. Again, if a disc painted with white ana black sectors be caused to rotate very rapidly, it appears in ordinary light to have a uniform grey colour; but when it is viewed by the light of an electric spark the sectors are seen exactly as if the disc were at rest, which proves that the illumination lasts for a very short time. Masson founded on this experiment a beautiful method for measuring the intensity of the light given out by the spark. A description of his apparatus, with an account of his results, will be found in Mascart. The colour of the spark in air is bluish, 6 but at the same Pogg. Ann., 1869.
6 Faraday, Exp. Res., 1422.