emanation given off from radium is very small, the electrical conductivity produced by the emanation in the gas, with which it is mixed, is often very large, and offers a ready means of measuring the emanation present.
Some experiments have been made by Miss Brooks and the writer23 to determine the rate of the diffusion of the radium emanation into air, by a method similar to that employed by Loschmidt24 in 1871, in his investigations of the coefficient of inter-diffusion of gases.
Fig. 56 shows the general arrangement. A long brass cylinder AB, of length 73 cms., and diameter 6 cms., was divided into two equal parts by a moveable metal slide S. The ends of the cylinder were closed with ebonite stoppers. Two insulated brass rods, a and b, each half the length of the tube, passed through the ebonite stoppers and were supported centrally in the tube. The cylinder was insulated and connected with one pole of a battery of 300 volts, the other pole of which was earthed. The central rods could be connected with a sensitive quadrant electrometer. The cylinder was covered with a thick layer of felt, and placed inside a metal box filled with cotton wool in order to keep temperature conditions as steady as possible.
In order to convey a sufficient quantity of emanation into the half-cylinder A, it was necessary to heat the radium slightly. The