100 McCietitanp— The Penetrating Radium Rays. 
the radium could thus escape upwards from the hole, the block being so large 
that no a 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 sensitive 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, andy rays could escape; (2) when the hole was covered with such thickness 
of foil that only 6 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 ionised, a current 
will exist between the block and the screen if they are not of the same metal ; 
and this will produce a deflection 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 6 rays carry away negative electricity. 
The 6 rays are only produced in the final stages of disintegration, so that we 
should expect the negative electricity carried away by the 6 rays to be less 
than the positive carried away by the arays. We thus get a negative charge 
on the block when both a and 6 rays escape, but a positive charge when only 
B rays escape. The result obtained when only y rays escape appeared at first 
sight to show that these rays carried a positive charge. This, however, is 
not necessarily so. It was found that, when the radium was taken out of the 
block and placed outside the screen, so that the air surrounding the block was 
still ionised, the lead block got a negative charge. ‘This is due to the known 
result that a conductor exposed to ionised air gets a small negative charge, owing 
to the fact that the negative carriers travel with a slightly greater velocity than 
the positive carriers under the same electric force. The ionised air immediately 
