212 
Mr Campbell, The /3 Rays from Potassium. 
It did not seem possible to use the ionising effect of the ify- a ys 
for detecting the magnetic deflection : for in order to obtain sUC h 
an effect of appreciable magnitude a considerable area the 
active material has to be employed. To procure a magneti . field 
of sufficient intensity over so great an area would require b tl use 
of special magnets and larger currents than are conveniei^gn It 
was therefore decided to attempt to observe the deflection o^ V) the 
rays in an electrostatic field — a process which appeared to preset, o -f 
fewer practical difficulties. However, the accuracy and rapidity 
with which measurements can be made was found to be so much 
greater than was expected, that I think now that it would have 
been perfectly possible to employ the magnetic effect. 
§ 4. The use of the electrostatic deviation has the great 
disadvantage that it does not permit quantitative measurements 
of the velocity of the rays to be made. A layer of active material 
emits rays in all directions : if over such a layer is placed a pair of 
vertical plates oppositely charged, the electric field between the 
plates, unlike a magnetic field, does not cause the rays to travel in 
paths of constant curvature, but merely impresses on them a 
velocity in the direction of the field. When there is no field 
acting, the rays which emerge at the top of the plates are those 
which have been emitted by the layer at a small angle with the 
vertical : the effect of the field is to substitute for these rays, those 
which have been emitted at a larger angle with the vertical. The 
ionising effect of these rays will be less, both because they are 
fewer in number and because the vertical component of their 
velocity is less ; but unless the rays are homogeneous in velocity 
and the relation between the intensity of the rays and the angle 
at which they are emitted from the layer is known, it is impossible 
to calculate their velocity from an observed decrease of the 
ionising effect. A limitation of the rays emitted by the layer to 
those in the vertical direction by suitably arranged slits would 
reduce so greatly the intensity of the rays that they could hardly 
be detected. However, it was thought that by comparing the 
effect of the field on the potassium rays with that on uranium rays, 
the velocity of which is known, sufficient information could be 
obtained for the purpose of the research. 
ii 
Apparatus. 
| 5. The testing vessel in which the ionising effect of the , 
potassium rays was observed was a rectangular box 40 x 40 x 22 
cms. : it was lined with lead sheet with the hope of increasing the 
effect due to the rays by adding to it that of the secondary 
radiation excited at the walls. One of the larger sides which 
