ALTERNATING-CURRENT ARCS 107 



15 to 20 volts lower than that for the latter, the current and 

 length of arcs being the same in both cases. 



The characteristic curves for the flaming arc with alter- 

 nating current are nearly identical with those for the same 

 arc with direct current. One curve taken by myself for 

 a flaming arc 10 mm. long with alternating current was 

 identical throughout the greater part of its length with 

 that taken with the same carbons and length where a 

 direct current was used, as shown in Fig. 16. 



Current and Potential Difference at Different Phases. 

 As far as I have been able to learn the first published work 

 on the alternating-current arc was by Joubert, in iSSo, 1 

 who showed the values of the potential difference and the 

 current at different phases of the alternations. His method 

 was the one which is well known in the work on alternating 

 currents, where a measuring instrument is connected to 

 the circuit for a brief length of time during each alternation, 

 the connection being always made at the same phase of 

 the alternation. He found the curve for the current to be 

 nearly a sine curve, while that for the potential difference 

 was greatly deformed. This observation was confirmed 

 by Tobey and Walbridge. 2 The form of these curves has 

 been found to vary with the kind of dynamo used, the 

 character of the carbon and the length of the arc. 3 



The preceding method gave the average value of some 



1 c. R., 91, 161; 1880. 



2 Amer. Inst. Elec. Eng., 7, 367; 1890. 



3 Rossler and Wedding, Elektrot. ZS., 15, 315; 1894. 

 Fleming and Petavel, Phil. Mag., (5), 41, 315; 1896. 

 Frith, Phil. Mag., (5), 41, 507; 1896. 



C. S. Smith, Lond. Elec., 39, 855; 1897. 



Beckit Burnie, Lond. Elec., 39, 849; 1897. 



Eichberg and Kallir, Wien. Sitzungbr., 107, 2A, 658; 1898. 



