750 Prof. Townsend on the Potential 
Note on the Potential required to maintain a Current in a Gas. 
By Professor TOWNSEND. 
It is interesting to investigate the phenomena which occur 
when the sparking point is reached, and to explain why it is 
that the potential required to maintain a current in a gas is 
much smaller than the sparking potential. The subject may 
be investigated on the lines which are here briefly indicated, 
but a fuller discussion of this application of the theory of 
ionization by collision must be reserved for a future paper as 
the necessary experimental investigations have not yet been 
carried very far. 
According to the principles which have been already 
explained, a continuous current should pass through the gas 
when the values of a and @ corresponding to the pressure 
and electric force satisfy the condition a= log oa B 
ees 
a is the distance between the electrodes. The theory requires 
that the field of force should be uniform so that the current must 
be small. When the current is large, the charge carried by 
the ions disturbs the electric field in such a way as to facilitate 
the passage of the current. ‘This effect comes into action 
gradually, and gives rise to the large fall of potential at the 
negative electrode when the current is sufficiently great. As 
the velocity of positive ions is smaller than that of negative 
ions, a positive charge accumulates in the gas. This charge* 
is greatest in the neighbourhood of the negative electrode 
since all the positive ions pass through that region, so that as 
the current increases the electric force becomes greater near 
the negative electrode than at other places in the gas. The 
ionizing power of the positive ions as shown by the values of 
6 is much increased in passing through the field of strong 
electric force, so that the supply of ions required to maintain 
the current is kept up when the fall of potential between the 
electrodes is less than the sparking potential. 
As an example of some experiments on the difference 
between the potential required to maintain a current and the 
sparking potential, we may take those made with air at a 
pressure of 8 millimetres and a distance of 4°31 millimetres 
between the plates. The sparking potential given in the 
, where 
* It was first suggested by Professor Schuster (Bakerian Lecture, 
Proc. Roy. Soc. xlvi. 1890, p. 541) that this variation of the electric 
field in the neighbourhood of the negative electrode is due to the 
difference between the velocities of the positive and negative ions, 
