Il8 THE ELECTRIC ARC 



condensing vapor than in the carbon arc and that this may 

 hinder the starting of the arc, but as yet we have no direct 

 evidence of such action. 



Alternating-current Arc between Unlike Terminals. - 

 When an alternating current is passed between two unlike 

 terminals, the current is in general greater in one direction 

 than in the other. This is most noticeable when one ter- 

 minal is a metal and the other carbon, but it is possible 

 to observe it even with two carbon terminals, if they are 

 of different sizes, or if one is cored and the other solid. 1 



Jamin and Maneuvier 2 seem to have been the first to 

 observe this. They found that when two carbons are 

 of different sizes the current from the larger is greater 

 than that in the opposite direction. The difference be- 

 tween the two currents is greater the greater the differ- 

 ence in the size of the carbons. They found also that with 

 short arcs between a metal and carbon there is little 

 difference between the currents in the two directions. The 

 excess is greatest when mercury is used for one of the 

 electrodes. 



When an alternating-current arc is maintained between 

 a carbon rod and a rotating carbon with an E.M.F. of 

 220 volts and 50 alternations per second about 30 per cent 

 of the energy is rectified, the rod being positive with respect 

 to the carbon. 3 



A large number of photographs which showed the curves 

 of current and potential difference with the alternating- 



1 Sahulka, Wien. Sitzungbr., 103, 2 A., 925; 1894. 

 Gold, Wien. Sitzungbr., 104, aA, 814; 1895. 

 Von Lang, Wied. Ann., 63, 191; 1897. 



Eichberg and Kallir, Wien. Sitzungbr., 107, 2A, 657; 1898. 



2 C. R., 94, 1615; 1882. 



3 Sahulka, Elektrot. ZS., 29, 949; 1908. 



