218 J. A. POLLOCK, E. M. WELLISCH AND A. B. B. RANCLAUD. 
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 differ- 
ence 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 gradu- 
ally diminishing current, rather than with any abrupt 
change in the nature of the discharge. 
The current curve is unsymmetrically placed with refer- 
ence 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 oppo- 
site 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 nega- 
tive 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 were arranged to 
open and close the circuits of two electromagnetic scribers 
which marked a smoked plate fixed to the pendulum; the 
records for various distances between the switch levers 
