THE TRANSFORMATIONS OF ELEMENTS 527 



them radioactive. The history of science shows no parallel 

 with the advance made in this subject. It is comparable with 

 half a century's work in other directions of research. The 

 advance is due in great part to the work of Prof. Ernest 

 Rutherford, who has co-ordinated the facts by putting forward, 

 and in part proving, the first reasonable theory to explain the 

 great mass of data which is even yet accumulating. 



The electrical measurements of radioactivity are made with 

 some form of electroscope or electrometer. The electroscope 

 devised by C. T. R. Wilson, and used largely for measuring 



Fig. i. 



small amounts of activity, may be taken as typical. The 

 description is that in Rutherford's Radioactivity, p. 86. A brass 

 cylindrical vessel (or very frequently a tin can) is stoppered 

 at the metal mouthpiece m (fig. 1) by an ebonite cork or an 

 ordinary cork covered with sulphur. Through this passes a 

 metal rod r, supporting a sulphur bead s, which supports and 

 at the same time insulates a narrow brass plate b some two or 

 three inches in length. To the upper end of the brass plate is 

 attached a strip of gold, or better, aluminium, a. The sliding 

 metal rod cc' is used to charge the leaf system, usually by touch- 

 ing it with a charged rod of sealing-wax while it is in contact 

 with b. r is earth-connected and c revolved to break the 



