CHAPTER VI. 



ALTERNATING-CURRENT ARCS. 



THE preceding discussion has been on arcs produced by 

 direct currents. These are more efficient than alternating- 

 current arcs and would probably be used in every case, if 

 it were always convenient to have direct currents. Since 

 it is not, alternating-current arcs have a somewhat limited 

 use. The differences between the two kinds of arcs are in 

 general such as might well be expected. There is, however, 

 very great difficulty in maintaining alternating-current arcs 

 between metals, as will be shown later. 



Alternating-current Arc between Carbon Terminals. 

 With carbon terminals the appearance of the alternating- 

 current arc is much the same as that of the direct-current 

 arc, except that the two electrodes appear alike in the former 

 case. This, of course, is due to the rapid change of polarity 

 of the terminals. However, with the frequencies commonly 

 used, the light appears to the eye to be continuous, but it is 

 easy to show with a revolving mirror that the luminosity 

 of the region between the carbons fluctuates greatly, the 

 vapor being comparatively non-luminous for a short period 

 between each alternation. 1 The carbons remain luminous, 

 although they also pass through quite appreciable changes 

 with each alternation. 



This arc is accompanied by a humming sound corre- 

 sponding to the frequency of alternations. In general, this 

 1 Lum. Elec., 42, 551 and 618; 1891. 

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