* am a 
on 
\ 
922 J. A. POLLOCK, E. M. WELLISCH AND A. B. B. RANCLAUD. 
fall of potential indicates an accumulation of positive ions 
near the cathode surface. These positive ions, in the 
growth of the discharge from the non-luminous current, 
must come in the first instance from the anode. IfJ is the 
current of negative ions, H the potential gradient at the 
anode surface and A the length of the last free run of the 
ions at the end of which they collide with the anode, the 
energy reaching the anode surface per second is IE); 
following the suggestion contained in the paper referred 
to, it is considered that for the arc to form, in the circum- 
stances here contemplated, the potential difference between 
the carbons, at the moment of reclosing the circuit, must 
have the value necessary to make the magnitude of FHA 
sufficient to raise a portion of the ancde surface to sucha 
temperature that positive ions are somewhat freely emitted. 
For this particular experiment the suggestion seems 
sufficient to account for the phenomena observed, but it 
will be seen that it is not of itself adequate to completely 
describe the features of the relation in other cases, even 
where, on reclosing the circuit, the hot is negative to the 
cooler carbon. 
4. Effect on the relighting of changes in the relative 
positions of the carbons.—The form of the relation between 
the minimum potential difference for relighting and the 
time interval between the break and the make of the 
circuit is influenced by many conditions. With the carbons 
ina vertical plane they may be situated, before the circuit 
is broken, either in the normal way, with the crater above, 
or in the reversed position with the crater below the nega- 
tive electrode; in addition, the connections to the battery 
may be reversed during the interval between the break 
and the make of the circuit, or left unaltered, so that there 
are four cases to be considered in connection with the 
relative positions of the carbons and the direction of the 
potential difference on the remake of the circuit. 
