196 
of experiments with currents which were of an order ten 
times smaller than those already described : yet their form is 
very similar. The curve A shows lesults with an 8 mm. ionisa- 
tion chamber ; curve B, 4 mm. The ionisation was due tu a 
thin layer of radium, surmounted by a set of vertical tubes, 
as described by us in the "Philosophical Magazine,'' September, 
1905. Other experimental results niay be expressed in 
terms of I4 , the saturation current for 4 turns of the screw 
(rather less than 4 mm.), and I\ the current for a potential 
gradient of 25 volts per cm,, and the same depth of chamber. 
On one o,ccasion it was found that 14/14 =117: I.^ 1'^= 1-21 ; and 
on another 1J1\=^ 1-18 : I« T'^= 1-23. In these experiments the 
radium was 5-05 cm. from the gauze. When the distance was 
6*25 it vv^as found that l4/I'4=l-27: IhT'„=1'29; and again 
l4/I'4 = 1*30, I„ I'f.= 1*30. In the latter cases the a rays did not 
all get across the chamber : possibly the small variation of the 
ratios with flistance may, in some way, be due to this fact. 
It might be argued that we ought not to find much varia- 
tion in the lack of saturation when the current is increased by 
shooting a greater number of a jjarticles across the chamber 
in one second, on the following grounds : — Each particle as 
it flies across makes something like 10'^ ions in a centimetre of 
its path. If there are only about 10* or 10'' ions in a c.cm. 
at any one time it is clear that these must be all the work of 
one particle, and that all the ions it produces are cleared 
away before the next one comes. Thus, the icns made by one 
a particle have no chance of combining with those made by 
another, and recombination cannot be proportional to the 
square of the number jDer c.c. But this consideration, though 
no doubt true, cannot furnish an explanation of the fact that 
the curves are little altered when the chamber is altered in 
dejDth. It was, indeed, in view of this argument that we 
made the experiments with the varying depths of the cham- 
ber. 
It is very instructive to comj^arc these figures with the 
results obtained by Retschinsky, and described by 
him in a paper contained in 'Drude's Anna- 
len," No. 8, 1905. Very careful measurements have 
been made by this observer of the relation be- 
tween current and potential gradient in the case when the 
currents are of an order 100 to 1,000 times great^er than those 
of the experiments described above. Curves D, E, and F, in 
Fig. 1, are plotted from the table on page 531 in his paper, 
being reduced to a saturation value 400, so as to be com- 
parable with the other curves in the same figure. It will be 
seen that in this case the curves for different widths of the 
ionisation chamber differ very widely at low potential gra- 
