168 Prof. E. Rutherford on Radioactivity 



was connected to the + pole of a battery of small lead accu- 

 mulators of 300 volts, the other pole of which was to earth. 



A thick layer of thorium oxide was placed in the bottom 

 of the vessel and covered with several thicknesses of paper. 

 A brass tube D was fixed in the side of the vessel and 

 metallically connected with it, A fine platinum wire AB 

 was fixed on the end of a stouter brass rod BC. The brass 

 rod was fixed centrally in the cylinder D and insulated from 

 it. The end of the brass rod B was placed well inside the 

 cylinder D. The conductor AC was connected to earth. 



The fine wire is thus the only body exposed in the field- 

 with a charge, and, under the influence of electric forces, the 

 active particles are carried to the wire AB and adhere to its 

 surface. 



The same general results are obtained whether the surface 

 of the thorium oxide is bare or covered with paper or thin 

 layers of metal-foil. 



Two or three layers of paper almost completely cut off the 

 ordinary radiation * from thorium ; so the effect cannot be 

 due to the direct radiation from its surface. 



In this way I have been able to cause a piece of platinum 

 wire of length 1 cm. and diameter '018 cm., i. e. with a 

 surface area of *056 cm., to give more than 20 times the rate 

 of discharge given by a thick layer of uranium oxide of 

 25 sq. cms. area. A rate of movement of an electrometer- 

 needle of 200 divisions in 5 seconds is quite easily obtained 

 from the action of such a small active surface. (1 volt gave 

 a deflexion of 40 divisions on the electrometer-scale, and the 

 capacity of the whole circuit was about 50 electrostatic units.) 



I have spoken of using a platinum wire, but any other 

 metal wire will serve equally well. Using large electro- 

 motive forces and a large surface of thorium oxide, it would 

 be quite possible to increase the radioactivity of unit area of 

 the conductor to more than 20 times the value cited in the 

 above case. So far as the results obtained indicate, there is 

 no limit to the amount of increase, since w T e can suppose the 

 area of the — charged conductor diminished and the amount of 

 thorium increased. In practice, however, a limit would soon 

 be reached, as it would be difficult to cause all the radio-active 

 particles to move to the small conductor without very large 

 electric forces. 



* E. Rutherford. Phil. Mag. Jan. 1900. 



