Mr Campbell, The ft Rays from Potassium. 
211 
The /3 Rays from Potassium. By Norman Campbell, M.A., 
Trinity College. 
[Read 6 May 1907.] 
§ 1. In a recent paper Mr Wood and I described some ex- 
periments proving that the salts of potassium and rubidium emit 
ionising rays which appear to be similar to the (3 rays from 
uranium. The observations about to be described were undertaken 
with the object of investigating further the nature of these rays. 
It seemed of the first importance to determine whether the 
rays carry an electric charge. The great penetration of the rays 
indicates that they cannot be positively charged a rays, but it does 
not prove decisively whether they are /3 or y rays. At first it 
was hoped that it might be possible to observe a deflection of 
the rays in a magnetic field by making use of their photographic 
action, and experiments were made to establish beyond doubt 
that the photographic action which had been observed previously 
was not spurious. 
§ 2. In the most convincing of these experiments a brass 
plate 03 cm. thick was pierced by a number of holes 0'4 cm. in 
diameter forming two distinct geometrical patterns. The plate 
was supported a short distance above a photographic plate* 
wrapped in light-tight envelopes : covering the holes of one 
pattern was placed a shallow paper dish containing potassium 
sulphate, over those of the other a similar dish containing sodium 
sulphate. After the lapse of eight weeks the plate was developed 
and it was always found that the pattern covered by the potassium 
salt was clearly distinguishable, while of that covered by the 
sodium salt no sign could be seen. This photographic action is 
interesting, for it can be detected with the simplest apparatus and 
seems to establish beyond doubt the claim of potassium to be 
considered one of the radioactive elements. 
§ 3. In order to observe the deflection of the rays by a 
magnetic field it is necessary to interpose a fine slit between 
the potassium and the plate. Such a slit cuts off such a large 
proportion of the rays coming from the active material that it was 
found, by analogous experiments with uranium, that an exposure 
of many years would be required for the potassium rays to give 
an impress capable of measurement. This means of determining 
the presence of a charge on the rays had to be abandoned. 
* In the previous experiments Cadett Spectrum plates had been used : unfortu- 
nately these plates are no longer obtainable. Five other brands were employed, but 
with all of them an exposure of eight weeks was required to give a visible impres- 
sion in place of the three weeks which were sufficient for the Cadett plates. 
