Alternating Current Spark Potentials. 20& 



chosen so as to satisfy the case when the ion is regarded as a 

 single atom, they appear to call for some independent proof 

 of the effective existence of this new type of viscosity as^ 

 applied to the motion of the " small " ions in gases. Even 

 if were of sensible magnitude, I am unable to see how it is 

 proportional to the density for any one gas, a relation which 

 is indispensable in order to explain the experimental result 

 that the mobility varies inversely as the density. In view of 

 the fact that Dr. Franck * was recently led to support this 

 theory as a result of a number of experiments, it seems 

 desirable that j\Jr. Sutherland should furnish a theoretical 

 formula for 6 applicable to the "small " ions in a gas at any 

 pressure, so that at any rate a rough idea of its order of mag- 

 nitude could be ascertained from independent considerations. 



I am, Gentlemen, 

 Cavendish Laboratorv, 5Tours very trulv, 



Cambridge, * E. M. WELLISCH. 



10 November, 1900. 



XTX. Alternating Current Spark Potentials. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



rpHE reference- (p. 699) by Professor de Kowalski and 

 J- Dr. Rappel in your November issue to " Russell's Theory'* 



are somewhat misleading. They state (p. 707 ) that "Previously 

 it was commonly accepted that the spark would jump when 

 the maximum electric intensity between the electrodes had 

 passed certain definite limits. Since this theory failed to 

 explain all observed facts, A. Russell has added an hypothesis 

 of his own.'" May I point out that, in this country at all 

 events, it was not commonly accepted that the maximum 

 electric intensity was the governing factor in disruptive 

 discharge. Professor Schuster's paper f was considered to 

 have definitely disproved this hvpothesis. AVhen I discovered 

 that Professor Schuster had assumed that the potential of 

 one of the electrodes in the experiments he quotes was always 

 zero, I thought that it would be of value to recompute the 

 formulae and re-examine the experimental evidence, on the 

 assumption that the charges on the electrode- were equal 

 and opposite at the instant of discharge. In Paschen's expe- 

 riments with half-centimetre spheres, for example, Schuster 



* Verh. Deutsch. Phys. Ges, x\. p. 397 (1009). 

 t Phil. Mag. vol. xxix. p. 192, Feb. 1890. 





