1881.] on the Plmwmeua of the Electric Discharge. 488 



Tlic discluirge, in the bell jar, was pliotograplHsd on ono occasion 

 and tlic central spindle or arc })r()per was measured on the photograj)!!, 

 and a caleulation made of wliat its temperature must have been on the 

 supposition that the sudden dilatation might be dua to it, and the 

 result was IGOOO^ Cent. Experiments were also made to ascertain 

 the temperature of different })arts of the arc, and it was found that 

 l^latinum wire , „\y^ of an inch in diameter was innnediately fused, 

 but there was no vaporisation of the platinum, wliich certainly would 

 have occurred had such a teni2)eraturc as 16000° Cent, existed. It 

 was ultimately concluded, from a number of experiments and con- 

 siderations, that the enormous and sudden dilatation could not be 

 attributed to a sudden increase of temperature, but must be caused 

 by the scattering of the gas molecules away from the terminals, and 

 their projection by electrification against the walls of the containing 

 vessel. 



We have proved experimentally that the discharge in a vacuum 

 tube does not differ essentially from that in air and other gases at 

 ordinary atmospheric pressures ; it cannot be considered as a current 

 in the ordinary acceptation of the term, nor as at all analogous to 

 conduction through metals, and must consequently be of the nature of 

 a disruptive discharge, the particles acting as carriers of electrification. 

 For example, a wire having a given difference of potential between its 

 ends, can permit one, and only one current to pass ; whereas, we 

 have found by accurate measurements that with a given difference of 

 2)otential between the terminals of a vacuum tube, currents of strength 

 varying from 1 to 135 can flow. 



We have found, moreover, that the resistance of a vacuum tube, 

 unlike that of a wire, does not increase in the ratio of the distance 

 between the terminals. As an exam2ile may be cited that, in a 

 Spottiswoode tube (Fig. 20) with one shifting terminal, which can 

 be 2^1iT'Ced at any required distance from the other, for seven times 

 the distance between the terminals the resistance was found to be 

 only twice as great. Moreover the fall of j^otential is not uniform 

 for equal increments of distance between the terminals of a vacuum 

 tube as it is for equal increments of the length of a wire. In order 

 to determine this we used a tube with seventeen rings inserted in it 

 at equal distances (Fig. 20) ; to these were attached wires which 

 projected through the tube, and were soldered to it. One pole of 

 the battery was connected to No. 1 ring of the tube, and the last 

 ring as well as the second pole of the battery were connected to 

 earth and stood at zero. By means of an electrometer, shown in 

 Fig. 21, the induction jdate of which could be made to communicate 

 with each ring successively, it was found that the difference of potential 

 between the first pair of rings, reckoning from the terminal connected 

 with the battery, was five times as great as that between the eighth 

 and ninth ; again, that between the sixteenth and seventeenth it was 

 twice that between the eighth and ninth. If I, by way of illustra- 

 tion, suspend a number of intli balls to a wetted thread, one end of 



