512 On the Pressure on the Poles of an Electric Arc. 



is imparted to the electrode, the stream expanding^ under 

 ordinary experimental conditions over a large volume. It is 

 therefore easy to see in view of the above considerations that 

 in this case ions will produce a reaction also on the poles of 

 opposite sign. 



Whatever the theoretical considerations may be, ihe expe- 

 riments carried out by Chattock and his pupils, Zeleny and 

 myself, undoubtedly show that such a reaction does exist in 

 reality. The pressure of the electric wind upon a sensitive 

 vane may be detected when the ionization current passing 

 through the gas is as small as 10~ 10 amp. (Phil. Mag. November 

 1916), and it appears to be proportional to the current. When 

 the current through the gas is of the order of 10~ 5 amp. the 

 pressure of the electric wind may impart large deviations to 

 a vane suspended by a rigid brass wire '1 mm. in diameter, 

 and is at the lowest estimate as large as 25 dynes per cm. 2 

 In an electric arc, where several amperes pass through the 

 gas, the expected pressure should be sufficiently strong to 

 smash at once the carbon poles of the arc. 



1 have noticed, however, long ago the remarkable effecr, 

 that the pressure of the electric wind increases with the 

 current only to a certain limit, and then tends to diminish. 

 Thus, in a discharge-tube the wind pressure appears to be 

 disproportionately small when a current of one milliampere 

 passes through the tube and disappears completely when 

 this current is increased to *5 ampere. Now it appears from 

 Prof. Duffield's experiments that also in an electric arc the 

 electric wind for some reasons does notarise, since, supposing 

 even the whole pressure effect observed by him to be due to 

 the electric wind, it is negligibly small compared with the 

 effect to be expected. 



These results are very striking, since the electric wind 

 originates solely in collisions between the ions and neutral 

 molecules in gases and can only increase with the density 

 of the current. There seems to be no other alternative to 

 explain it than to suppose that when the current density is 

 large, the carriers of electricity do not collide with the 

 gaseous molecules — an assumption which seems incredible. 



I believe that further investigation of this phenomenon 

 may throw fresh light on the nature of discharge of electricity 

 through gases. 



I am, Gentlemen, 



Yours faithfully, 

 The Physical Laboratory, g, Ratnek. 



Owens College. 

 Manchester, July 12th, 1920. 



