30 IONIZATION THEORY OF GASES [cH. 
until the rate of production of fresh ions is exactly balanced by the 
recombination of the ions already produced. On application of a 
small electric field, the positive ions travel to the negative electrode 
and the negative to the positive. 
Since the velocity of the ions between the plates is directly 
proportional to the strength of the electric field, in a weak field 
the ions take so long to travel between the electrodes that most of 
them recombine on the way. 
The current observed is consequently small, With increase of 
the voltage there is an increase of speed of the ions and a smaller 
number recombine. The current consequently increases, and will 
reach a maximum value when the electric field is sufficiently 
strong to remove all the ions before appreciable recombination has 
occurred. The value of the current will then remain constant even 
though the voltage is largely mcreased. 
This maximum current will be called the “saturation !” current, 
and the value of the potential difference required to give this 
maximum current, the “saturation P.D.” 
The general shape of the current-voltage curve is shown in 
Fig. 2, where the ordinates represent current and the abscissae 
volts. 
Current 
Saturation Curve 

Volts 
Fig. 2. 
1 This nomenclature has arisen from the similarity of the shape of the current- 
voltage curves to the magnetization curves for iron. Since, on the ionization 
theory, the maximum current is a result of the removal of all the ions from the gas, 
before recombination occurs, the terms are not very suitable. They have however 
now come into general use and will be retained throughout this work. 

