332 



MR. W. DUDDELL ON THE RESISTANCE AND 



TABLE VI. Distribution of the P.D. in a Solid Arc. 



Carbons, both 11-millims. "Conradty Noris." Arc length 6 millims. Direct current 



through Arc 9 '91 amperes. 



Conclusion. 



Ever since the arc was first assumed to have a back E.M.F., speculation has been 

 rife as to its cause and its location in the arc. So long as the value assigned to it 

 was of the order of 40 volts, and it was supposed to be located only at the crater 

 surface, there was great difficulty in offering any consistent explanation of it. It 

 remains, therefore, to consider whether the new facts set forth in this paper render 

 the matter more susceptible of a satisfactory explanation. 



So far as the resistance of the arc is concerned, there seem to be no difficulties, the 

 known relations between the size and shape of the vapour column, the size of the 

 craters, the current, and the arc length, explain the observed changes in resistance 

 when the two latter variables are altered. The magnitude of the resistance of the 

 vapour column and of the contacts between it and the electrodes are not such as to 

 offer serious difficulties, nor does the fact that they are altered by the presence of 

 foreign bodies. 



Any explanation of the back E.M.F. of the arc as a whole must, in the light ot 

 the results given in the last section, account for the existence of two unequal E.M.F.'s 

 of opposite signs, of which the larger opposes the flow of the current. The observed 

 back E.M.F. of the arc is the resultant of these two, their values being of the order 

 of 17 volts and 6 volts respectively in the solid arc. The existence of two E.M.F.'s 

 of opposite signs, situated at or near the electrodes, considerably simplifies matters, 

 since any explanation which would account for a back E.M.F. in the direction 

 carbon-to-vapour would probably also explain a forward E.M.F. vapour-to-carbon. 

 These E.M.F.'s are probably either due to a polarisation at the electrodes or to 

 thermo-electric forces. The polarisation E.M.F.'s include those due to chemical 

 changes and those which have been assumed' to be caused either by the volatilisation 



