Resistance of Vacuum. 13 



be proportional to rjs when the current s traverses that column. 

 Now Wiedemann's experiment has demonstrated that the 

 quantity of heat evolved diminishes with the rarefaction of the 

 gas, while the quantity of electricity which has passed is the 

 same, whence it follows that the resistance i\ of the gas dimi- 

 nishes as the gas is rarefied. 



As was said above, according to Wiedemann and Eiihlmann 

 the electric discharge in rarefied gases presents phenomena 

 similar to those which would be called forth by the existence 

 of a sort of resistance to the passage of electricity impeding 

 its escape from the electrodes. Gaugain's above-mentioned 

 observation with the tinfoil is in favour of such an admis- 

 sion. If such a resistance between the tinfoil and the sur- 

 rounding rarefied gas be not admitted, it is impossible to un- 

 derstand why the electricity passes exclusively through the 

 aperture, as the metal must be regarded as a better conductor 

 than the gas. We can cite an allied phenomenon well suited 

 to furnish an elucidation in regard to this. When the resist- 

 ance opposed by the voltaic arc to the propagation of the gal- 

 vanic current is measured, that resistance is found to be 

 composed of two parts, one of which is independent of the 

 length of the arc, and the other proportional to that length — 

 provided that, by an appropriate modification of the rest of 

 the resistance in the circuit, the intensity of the current is 

 maintained without sensible variation while the length of the 

 arc is diminished or increased. Here, therefore, the resistance 

 can be expressed by a + bl, where a and b are constants, and I 

 denotes the length of the arc*. As a remains always invari- 

 able however short the arc be made, this resistance must have 

 its seat upon the electrodes themselves, whether it be shared 

 between the two or be situated exclusively upon one only. 

 Experiment shows, it is true, that the obstacle a to the propa- 

 gation of the current is not produced by a resistance in the 

 ordinary sense of the term, but by an electromotive force 

 which diminishes the intensity of the current quite as much 

 as an ordinary resistance of the amount a. The propagation 

 of the current in the voltaic arc and in a rarefied gas assumes 

 forms so different that it is impossible to deduce a priori, from 

 that which takes place in the one case, what must happen in 

 the other. In the arc, particles are torn from one electrode 

 and transported to the other; but a similar transference is not 

 effected in rarefied gas. The intense evolution of heat which 

 was found by Naccari and Bellati in the negative electrode 



* (Efvers. Kongl. Svenska Vetemk.-Akad. Forhandl. 1867, p. 95 ; Pogg\ 

 Ann. cxxxi. p. 086. 



