Resistance of Vacuum. 11 



tance*. (4) If the positive electrode was put into communi- 

 cation with the earth, the requisite tension was but little 

 greater than when both insulated electrodes were connected 

 with the electrical machine ; if, on the contrary, it was the 

 negative electrode that communicated with the earth, a still 

 stronger tension was necessary in order to call forth the dis- 

 charge. 



The authors note, at the end of their first memoir, that the 

 circumstances observed during the electric discharge in rarefied 

 gases appeared to argue the existence of a sort of resistance to 

 the passage at the surface between the electrode and the envi- 

 roning medium, obstructing the issue of the electricity from 

 the electrode. 



In the second memoir "Wiedemann communicates various 

 observations made with the view of determining the depend- 

 ence of the tension necessary for the discharge on the length 

 of the glass tube between the glass spheres furnished with 

 electrodes. The minimum of pressure of gas employed in this 

 series of observations was from OS to 9 - millim., and was 

 consequently considerably lower than in the corresponding 

 series of experiments of the first memoir. As conducted at 

 pressures for the most part lower, the series in question give 

 also as their result that the electric tension appears to be suffi- 

 ciently independent of the length of the glass tube. For 

 tubes of equal lengths but different widths, uniting the glass 

 spheres, no great difference was shown in the tension of the 

 electrodes necessary to produce the discharge. When the 

 apparatus was filled with rarefied hydrogen, the necessary 

 tension of discharge was less than when it was filled with 

 rarefied air. 



Wiedemann also made some experiments to ascertain the 

 quantity of heat evolved, under the prevalence of different 

 conditions in the tube, by the passage of the electricity. In 

 regard to this, he found that, with an equal flow of electricity, 

 the evolution of heat increased with the increase of the density 

 of the gas, and that the amount of heat evolved was, cceteris 

 paribus, nearly independent of the width of the tube. On a 

 former occasion Wiedemann had found that, when the current 

 of an induction-apparatus was passed through a Greissler tube, 

 the evolution of heat was proportional rather to the intensity 

 of the current than to the square of this, as would be required 

 by Joule's law for continuous currents and solid or liquid con- 

 ductors f. These two positions have been completely confirmed 

 by Naccari and BellatiJ, who found, on employing a Ruhm- 



* Pogg. Ann. cxlv. p. 374. f Ibid. cxlv. p. 237. 



X Beiblcitter zu den Ann. d, Phys. u. Chem. ii. p. 720 (1878). 



