The Mechanism of the Electric Arc. 



413 



length of arc varies with the frequency of the alternating current, and 

 it is pointed out that even if the arc has as large a back E.M.F. as is 

 usually supposed, the true resistance cannot be measured with an 

 alternating current of lower frequency than 7000 complete alternations 

 per second. 



The exact conditions under which the true resistance of the arc can 

 be measured in this way are examined, and the precautions that it is 

 necessary to take to ensure the fulfilment of these conditions are 

 enumerated. 



The Changes introduced into the Resistance of the Arc by the Use of Cored 



Carbons. 



A core in either or both carbons has a great effect on both the P.D. 

 between the carbons and the change of P.D. that accompanies a given 

 change current. It lowers the first, and makes the second more 

 positive, i.e., gives it a smaller negative or larger positive value, as 

 the case may be. It is pointed out that this might be due to the 

 influence of cores either on the cross-section of the arc, or on its 

 specific resistance, or on both. 



To see the effect on the cross-section, enlarged images were drawn 

 of 2 ram. arcs with currents increasing by 2 amperes from 2 to 14 

 amperes, between four pairs of carbons, + solid - solid, + solid 

 - cored, -f cored - solid, + cored - cored. Two sets of images 

 were drawn with each pair of carbons — the one immediately after a 

 change of current, to get the " non-normal " change, and the other 

 after the arc had become normal again. The mean cross-section of 

 the mist was calculated in each case, and its cross-section where it 

 touched the crater was taken to be a rough measure of the cross- 

 section of the vapour film. 



It was found that the mean cross-section of the mist with a given 

 current was largest when both carbons w T ere solid, less when the 

 negative carbon alone w T as cored, less still when the positive alone was 

 cored, and least when both were cored. Coring either the positive 

 carbon alone, or both carbons, had the same effect on the cross-section 

 of the vapour film as on that of the mist, bub coring the negative 

 alone only diminished this cross-section immediately after a change of 

 current, but not when the arc had become normal again. Hence it 

 was deduced that if the cores altered the cross-sections of the arc only 

 they would increase its resistance, and, consequently, the P.D. between 

 the carbons. As they lower this, however, they must do it by lowering 

 the specific resistance of the arc more than they increase its cross- 

 section. The vapour and mist of the core must therefore have lower 

 specific resistances than the vapour and mist of the solid carbon. 



When it is the positive carbon that is cored, all the vapour and mist 



