254 
DR. HAROLD A. WILSOX OX THE 
pressure, ljut witli 250 volts it rises rapidly to a maximum value at nearly 3 millims., 
and then falls with increasing pressure. The temperature in this case was 1483° C. 
The alx')ve results on the variation of the current with the P.D. and pressure can 
be explained if it is assumed— 
(1.) That the number of negative ions coming from the surface of the platinum 
per second is independent of the air pressure and so remains constant 
during each series of measurements, and 
(2.) That the negative ions can produce ionization l^y collisions with the air 
molecules when moving with sufficiently great velocities. 
It has been shown by Professor Townsend* that negative ions produced in air and 
other gases at low pressures produce ionization by collisions with the molecules when 
moving under the influence of a sufficiently large electric intensity. He finds that if 
a is the number of negative ions produced by one negative ion in going 1 centim., 
then a where j) is the pressure, X is the electric intensity, and /denotes 
a function whose values he has determined. 
The following table gives the values of /(X/^j) for air found experimentally by 
Townsend {‘ Phil. Mag.,’ vol. 5, 1903, p. 395) for several values of X/p :— 
X 
f' 
NE^) 
X€-'W. 
70 
0-25 
0-07 
100 
0-73 
0-33 
200 
2-5 
2 '2 
400 
5-8 
5-8 
600 
7-9 
8-0 i 
800 
9-4 
9-4 1 
1400 
12 
11-6 ! 
‘Phil. Mag.,’ Fehraary, 1901, June, 1902, April, 1903, and ‘Electrician,’ April 3, 1903. 
t Townsend has determined a large number of values of a with various values of p and X, and he 
finds that they can be very accurately represented by an equation of this form. 
