106 THE ELECTRIC ARC 



humming is so loud as to make the arc disagreeable when 

 in a closed room. It is more noticeable with solid than 

 with cored carbons. 



The possibility of having an alternating-current arc is 

 due to the fact that the region between the carbons remains 

 conducting for an appreciable length of time after the 

 current has ceased to flow. This fact was known as early 

 as 1867 when Le Roux 1 found that he could start an arc 

 0.04 sec. after the removal of the E.M.F. 



Characteristic Curves. Characteristic curves for alter- 

 nating-current arcs show the same general shape as those 

 for direct-current arcs. With both the voltage becomes 

 greater as the length of the arc is increased and less as the 

 current is made greater. However, the mean voltage of 

 the alternating-current arc is less than that for the direct- 

 current arc of the same length and current. Heubach 2 

 gives the equation E = 20.4 + i.8/ for the alternating- 

 current arc between cored carbons, when the current is 6.5 

 amperes and E = 19.78 + 2.2 I, when the current is 4.4 

 amperes. As usual, E is measured in volts and / in milli- 

 meters. This gives, for example, a voltage of 29.4 volts for 

 an arc 5 mm. long having a current of 6.5 amperes, as 

 compared with 55 volts, the value given by Mrs. Ayrton 

 for a direct-current arc having the same current and length 

 between a cored anode and a solid cathode. The fact that 

 both of the carbons were cored in the alternating-current 

 arc no doubt explains part of the difference, but only a 

 small part. In some observations made by myself in using 

 cored carbons for both alternating- and direct-current arcs 

 it was found that the voltage for the former was from 



1 C. R., 65, 1149; 1867. 



2 Elektrot. ZS., 13, 460; 1892. 



