14 THE ELECTRIC ARC 



Mrs. Ayrton found that with the solid carbons which 

 she used the relation between the potential difference, 

 current and length of arc could be expressed by the equa- 

 tion 



11.66 + 10.54 1 



E = 38.88 + 2.07 



where E is the potential difference between the terminals 

 in volts, / the current in amperes, and I the length of the 



6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 

 CURRENT IN AMPERES 



FIG. 6. 



arc in millimeters. For cored carbons the relation could 

 not be expressed by a simple equation. She believed that 

 this was due to the change which occurs when the crater 

 of the anode begins to cover more than the core of the 

 carbon. 



The power used in the arc is, of course, equal to the 

 potential difference times the current, so that the equation 

 for the power used in the arc with solid carbons according 

 to Mrs. Ayrton's data is 



P = (38.88 + 2.07 /)/ + 11.66 + 10.54 /, 



