190 
tion current being 200- This is recognised in the usual 
formulae. For example, Langevin finds that — 
IT ^ ' (T J 
wliere Qq is the saturation current per sq.cm. of electrode, 
and Q is the current when sucn a potential is applied that <r is 
the density thereby caused to exist on each sq.cm. of the 
electrode. When (^> and (>„ are both small compared to a, 
it follows that : — 
. Qo~Q _^Qo 
Thus, the relative lack of saturation, viz. (Qi)- (J,/^ i Qo^ i^ 
proportional to Qo, which itself depends on the depth of the 
chamber. Other formulae show the same dependence. 
But experiment shows that when the density of the ions 
is small the depth of the ionisation chamber has very little 
eft'ect on the degree of saturation. This may be illustrated ],y 
the following experiments : — 
Five mm.g- of radium bromide were so placed that tlie 
a rays passed upwards through an aperture in a lead plate 
and crossed the gauze of the ionisation chamber. The rays 
formed a cone whose vertical angle was about 20°. The 
apparatus used was the same as that of the previous experi- 
ment described, but the currents were so strong that a capa- 
city of 1,070 cms. had to be 2)ut in parallel with the electro- 
meter. Determinations were then made of the strengths of 
the current at various potentials: — (1) When the ionisation 
chamber was 3 mm. wide; (2) when 6 mm. wide; and (3) when 
9 mm. wide. The values obtained were then reduced so that 
the saturation current in each case was set at the same value. 
Comparison then showed that the curves were almost identi- 
cal except at low potentials, and this was in agreement with 
the hypothesis now put forward. For at all but low potentials 
aii'^ was so small as to be negligible. When the potential 
was very low, one or two volts per centimetre, then the ions 
moved so slowly that a was larger, and a?r was not negligible, 
and under those circumstances the curve showed a difference 
of the right kind. That is to say, the 9 mm. curve was fur- 
ther from being saturated than the others. The currents 
were specially made not too weak in order to bring out this 
contrast between the effects at low and at high potential*. 
The figures are given in the following table: — 
