l6 THE ELECTRIC ARC 



giving the relation for steady conditions have been called 

 by Simon 1 " statical" characteristic curves; those giving 

 the relation with changing conditions were called "dynam- 

 ical" characteristic curves. 



The dynamical curves are found by comparing the 

 potential differences with the currents for different phases 

 of an alternating-current arc, or when an alternating cur- 

 rent is superimposed on a direct current which is main- 

 taining an arc. The discussion of such curves will, there- 

 fore, be considered in connection with alternating-current 

 arcs. 



E.M.F. Required as Distinct from Voltage of Arc. Arc 

 lights are commonly maintained by constant current 

 dynamos. Under such conditions there is no need of put- 

 ting resistance in series with the arc in order to insure its 

 stability. But occasionally an arc is connected to some 

 source maintaining a constant E.M.F., as, for example, a 

 constant potential dynamo. When this is done, it is neces- 

 sary to put resistance in series with the arc, as was pointed 

 out by Blondel. 2 The curves which have been given show 

 the potential difference between the electrodes for different 

 currents, but they do not show the required E.M.F. of a 

 source having a constant voltage. Such E.M.F. can be 

 found as follows: 



Let us assume that the potential difference is E when 

 the current is 7 for any given length /. If now the dynamo 

 gave exactly E volts, and there was no resistance in the 

 circuit, the system would be in a condition of unstable 

 equilibrium. For if there were any fluctuations either 

 in the dynamo or in the arc which gave a momentary 



1 Phys. SZ., 6, 302; 1905. 



2 Lum. Elec., 42, 621; 1891. 



