Penetrating Radium Rays. 69 
rays escape, but a positive charge when only @ 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 ionized, 
the lead block got a negative charge. This is due to the 
known result that a conductor exposed to ionized 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 ionized 
air immediately 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 rays. 
One precaution is necessary in showing the charge on the 
ayvays 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 @ rays, initial deflexions. 
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 containing the radium.. The con- 
ducting gas between the plate and the block prevented the 
block from charging rapidly, but an initial negative deflexion 
-of 50 divisions in 15 seconds could be obtained. The numbers 
are merely given to show how this simple arrangement demon- 
strates the charge on the « and £ rays. : 
