the Relighting of the Carbon Arc. 451 



The conditions associated with a change from the non- 

 luminous to the luminous discharge, in the case of the 

 ordinary carbon arc, are seen in the wave-forms of current 

 and potential-difference in connexion with alternating-current 

 arc-lamps. 



Fig. 1 (PL IX.), showing curves of the volts at the brushes 

 of the machine, of current in the circuit, and of the potential- 

 difference between the carbons of the lamp, is copied from 

 fig. 14 of a paper by Mr. Duddell and Professor Marchant 

 on Experiments on Alternate Current Arcs by aid of 

 Oscillographs*, in which many other illustrations will be 

 found. 



The curves may be described by saying that the potential- 

 difference between the carbons rises from zero, while the 

 current keeps low and non-luminous, until the potential- 

 difference reaches the value, p, necessary to change the state 

 of the current to that of the arc-discharge. The current 

 then rises very rapidly, while the potential-difference falls 

 so that a greater electromotive force may be available along 

 the rest of the circuit, a necessary condition if the increase 

 in the current is to be maintained. On the falling side of 

 the wave, the second maximum of the potential-difference 

 seems to be connected with the gradually diminishing 

 current, rather than with any abrupt change in the nature 

 of the discharge. 



The current curve is unsymmetrically placed with re- 

 ference to the zero points of the potential-difference curve, 

 because on the rising side of the wave the change is from a 

 non-luminous to a luminous discharge, when on account of 

 the smallness of the previous current the temperatures are 

 low, whereas on the falling side the change is in the opposite 

 direction, when, owing to the previous larger current, the 

 temperatures are higher. 



2. Experimental Detail. 



For all the experiments Conradty carbons, Marke C, were 

 employed ; both positive and negative were solid, each 13 

 millimetres in diameter, the lamp being hand-fed. A heavy 

 pendulum, operating two switches when allowed to swing, 

 opened and again closed the circuit ; the distance between 

 the switch levers could be readily altered. The time interval 

 between the opening and the reclosing of the circuit for 

 different lengths between the levers was carefully determined 

 by separate experiments carried out as follows: — The switches 



* Duddell Sc Marchant, Journ. Inst. Elect. En?, xxviii. p. 1 (1899). 



2 12 



