DISCHARGE OF ELECTRICITY FROM GLOWING CARBON. PAA 
F i Or 8 (3 
Cool. 
c\ 
V V 
b Hot. b 
x 1 Hot. 
Cool. x x uy) 
é qa ‘a 
1 2 
Representation of the distribution of potential between the carbons, 
is illustrated by curve 2. As the potential difference 
between the carbons increases, the hot carbon rise of 
potential ab becomes less and less, and the proportion of the 
negative. ions carried to the cool carbon becomes greater. 
It the potential difference rose to such a value that the hot 
carbon potential rise disappeared, the current would be 
saturated; in our experiments an arc forms long before 
this stage is even approached. 
The deduction of an analytical expression for the relation 
between potential difference and current in the case under 
consideration, presents difficulties; it involves a statement 
of the circumstances of the appearance of the molecular 
negative ions near the hot carbon, and requires a knowledge 
of the relation between the mobility of the ions and dis- 
tance from the hot surface, which depends on the temper- 
ature gradient existing between the carbons. 
6. Change from the non-luminous to the arc discharge.— 
In the carbon arc there is a very characteristic cathode 
fall of potential which indicates an accumulation of positive 
ions near the cathode surface. In view of the fact that, 
