On the Alternating- Current Carbon Arc. 781 



produced. Owing to the groat difference between the specific 

 mobilities o£ positive ami negative ions at arc temperatures, it 

 is only necessary to assume that through the body of the arc 

 a small Fraction of the total current (less than ^ per cent.) is 

 carried by positive ions to account tor the magnitude of the 

 observed wind. 



A fall account of these experiments will he published 

 shorily. 



I am, Gentlemen, 

 Physic* Laboratory, Yours faithfully, 



University, Bristol. A. M. TvNDALL. 



Oct. 15, 1920. 



XC Note upon the Alternating-Current Carbon Arc. By 

 Prof. W. Gr. Duffikld, D.Sc, and Mary D. Waller, 

 B.Sc* 



1 EXPERIMENTS carried out in the Physics Laboratory 

 A of University College, Reading, some years ago showedf 

 that the amount of carbon lost from the cathode of a direct 

 current arc consisted of two parts : (1) that necessary for the 

 mechanism of the arc, and (2) a quantity lost by evaporation 

 or combustion, which, though possibly affecting the voltage, 

 temperature, brightness, and the current, could only be 

 regarded as subsidiary. Chief interest centred about the 

 shortest arc, less than one millimetre in length, which it was 

 found possible to maintain; for here the loss from the cathode 

 was entirely of the first category, and such that for each 

 carbon atom lost from that pole there was a transfer between 

 the electrodes of a quantity of electricity equivalent to four 

 electronic charges. 



It was Farther found % that, though in a normal arc the 

 anode loss is considerably greater than that from the cathode, 

 it is nevertheless possible so to cool the anode by rotating it, 

 that its loss of weight may be reduced, not only below that 

 of the cathode, but nearly to zero. 



A careful examination of the contour of the poles of a 

 direct-current arc showed that the shape of the anode re- 

 mained practically unchanged, but that, even though pre- 

 viously burnt to shape, the contour of the cathode of a very 

 short arc went through a well-marked cycle of changes, 

 which were repeated again and again as the expenditure of 

 carbon proceeded. 



* Communicated by the Authors. 



• D iffield, lioy. Soc. Proc. A. xcii. p. 122(1916). 



X Dutfield & Waller, lioy. Soc. Proc. A. xcii. p. 247 ( lOlfiL 



