68 ‘Prof. J. A. McClelland on the 
rays are charged particles is to endeavour to detect directly 
the charge carried by the rays, if any. | 
First Experiment. 
A block of lead was taken anda hole drilled to a small 
depth in the centre of one of its faces; 50 milligrams of a 
good sample of radium bromide inclosed in an air-tight vessel 
with a very thin mica top was placed in the hole. The rays 
from the radium could thus escape upwards from the hole, 
the block being so large that no 2 and 8 rays and very few 
y rays could escape in any other direction. This lead block 
was carefully insulated on paraffin, screened by an earthed 
conductor from outside electrical forces, and joined to a sen- 
sitive Dolezalek electrometer. The electrometer was first 
connected to earth, then insulated, and the charge it gradually 
got was observed. Observations were made (1) when the 
hole in the lead block was covered with only a very thin sheet 
of foil, so that a, 8, and y rays could escape ; (2) when the 
hole was covered with such a thickness of foil that only 8 and 
y rays could escape ; (3) when the hole was so covered that 
only y rays escaped. | 
It is obvious that care must be taken, in screening the block 
of lead with an earthed conductor, to guard against effects due 
to contact electric forces. The air between the lead block 
and the surrounding screen being ionized, a current will exist 
between the block and the screen if they are not of the same 
metal ; and this will produce a deflexion of the electrometer 
when insulated. This was avoided by wrapping the block and 
the wire leading to it with tinfoil, and making the surrounding 
screen also of tinfoil. 
When the effects of contact electric forces are eliminated, 
the effects observed are as follows :-— 
(1) When a, 8, and y rays are allowed to escape, the lead 
block gets a negative charge. 
(2) When @ and y rays escape, the block gets a positive 
charge. 
(3) When only y rays escape, the block gets a negative 
charge. 
The results in cases (1) and (2) are what we should expect. 
The a rays carry away positive electricity, and the @ rays 
carry away negative electricity. The @ rays are only pro- 
duced in the final stages of disintegration, so that we should 
expect the negative electricity carried away by the B rays to 
be less than the positive carried away by the « rays. We 
thus get a negative charge on the block when both « and @ 
