McCurLtitanp—TZhe Penetrating Radium Rays. 101 
surrounding the lead block gives up to the block more negative than positive 
carriers, the block thus getting a negative charge, and the surrounding gas 
retaining an excess of positive. 
It was easy to show that all the negative charge observed when only y rays 
escaped could be accounted for in this way. The experiment was therefore 
somewhat inconclusive in its present form as regards the y rays; it is described 
chiefly because it affords a simple method of demonstrating the charge carried by 
the a and 6 rays. 
One precaution is necessary in showing the charge on the a rays in this way. 
These rays are very easily absorbed by the air; and thus at a small distance above 
the block there will be an excess of positive electricity producing an electric field 
tending to drive positive ions to the lead block. This can be got over by placing 
an earthed sheet of foil a short distance above the block. When this is done, and 
the effects of contact electric force eliminated, the successive changes of sign of 
the charges got by the block containing the radium occur as described above. 
The charge in each case, of course, reaches a maximum when the rate of 
gain of charge is equal to the rate of loss through the surrounding conducting 
air. 
It is scarcely necessary to give any numbers, as they are not required for any 
calculations in this Paper. In showing the negative charge carried by the 6 rays, 
initial deflections of 100 scale-divisions in 10 seconds could easily be obtained 
with a Dolezalek electrometer. For the a rays, it was necessary, for the reason 
given above, to place an earthed plate at a short distance above the block con- 
taining the radium. The conducting gas between the plate and the block 
prevented the block from charging rapidly; but an initial negative deflection of 
50 divisions in 15 seconds could be obtained. The numbers are merely given 
to show how this simple arrangement demonstrates the charge on the a and 8 
rays. 
SECOND EXPERIMENT. 
The experiment described above is not conclusive as to whether the y rays 
carry an electric charge, because of the effect due to the surrounding ionised gas. 
This effect could of course be largely eliminated by carrying out the experi- 
ment in a high vacuum. It was more convenient, however, to modify the 
apparatus in another way. 
A tin cylinder 6, 20 cms. long and 8 cms. diameter, was placed inside a 
second tin cylinder a, 25:5 ems. long and 10 cms. diameter, and the annular 
space between flooded with liquid paraffin, which solidified, insulating the 
inner cylinder from the outer, which was to earth. The inner cylinder was 
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