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CII. Manometric Observations at the Poles of the Electric 

 Arc. By H. E. G. Beer, 31. Sc. (Bristol), and Professor 

 A. M. Tyndall *. 



THE recent work of Duffield, Burnham, and Davis (Phil. 

 Trans. A. vol. 220. p. 109, 1919 ; Proc. Roy. Soc. 

 A. vol. xcvii. p. 326, 1920) on the pressure on the poles of 

 the electric arc has revived interest in the problem of the 

 mechanism of the transmission of current in the arc. In their 

 first paper, when discussing the possible causes of the effect 

 which they obtained, they drew attention to some early 

 experiments of a different character by Dewar in 1888. 

 Dewar made observations on the hydrostatic pressure de- 

 veloped at carbon electrodes drilled centrally along their 

 length. He found that a positive pressure of the order of 

 1 or 2 mm. of water was developed at the anode, whereas 

 that at the cathode was either absent or slightly negative. 

 As, however, further quantitative data were not given, it 

 was felt that a full investigation of the effect might be 

 profitable and possibly give some information on the relative 

 importance of positive and negative ions as carriers of 

 current in the arc. The following is an account of experi- 

 ments that we have carried out on the problem. 



From the start we suspected that the effect was an electric 

 wind phenomenon and that its small magnitude was evidence 

 of the fact that both signs of ion take part in the discharge, 

 in which case the wind-pressure would be the difference 

 between the effects of each ionic stream taken separately. 

 The results have substantiated this view. 



Now if throughout the arc the ionic flow is viscous and 

 the current carried by each sign is strictly proportional to 

 its mobility, then there will be no space-charge in the arc 

 and consequently no wind-pressure, If, however, in any 

 region an excess of one sign of ion is present, a wind will 

 be set up by the motion of these ions to the appropriate 

 electrode. The experimental study of the relation between 

 wind-pressure, arc-length, and current has enabled us to 

 show that the wind-producing regions are confined to the 

 vicinity of the electrodes, and that therefore the main body 

 of the arc has no net space-charge. It is therefore presumably 

 a region in which intense ionization and recombination is 

 going on. 



Moreover, the wind-pressure method enables a very 

 sensitive test to be made of the perfection of the balance 



* Communicated by the Authors, 



