Beta Rays and by Gamma Rays from Radium C. 553 



of the electroscope. Wires of 20, 30, 40, 50, and 60 cm. 

 were used, of diameter '08 cm., and a similar series of 

 diameter "048 cm. The electroscope, of unknown eapacit} r C, 

 had its vertical system charged to potential V, and when the 

 insulated vertical wire touched it, the potential fell to V. 

 Thus 



CV=(C-fC'-M)V', 



where C is the capacity of the wire found by calculation (as 

 for half a wire of double length), and where x is a small 

 correction, apparently varying, approximately, inversely as 

 the length. A large number of simultaneous equations were 

 determined from the experiments, and these were plotted 

 and solved graphically. A further check on the result was 

 obtained by bringing up to the top end of a wire, when in 

 contact with the central axis, spheres of known radii. The 

 value of the capacity was found to be 2'90 cm., with a possible 

 error of about 2 per cent. 



Measurements, 



Using the well-known relation 



? S<?=CV/300i*, 



it follows that q = 292 divisions/minutes, where g is the 

 number of ions per cm. 3 per second in the electroscope 

 corresponding to a given leaf-measurement in divisions a 

 minute. 



If a quantity of radium Q is placed at a distance r from 

 the electroscope, 



where q is the number of ions produced per cm. 3 per second 

 by the <y rays from the radium C in equilibrium with the 

 radium. When r is only 2 or 3 metres, we may omit e* r , 

 because /// is so small. 



In order to find the constant K', specimens of radium of 

 strengths *23, '52, 1*6, and 8*5 rag. were used. A specimen 

 was placed at distances varying from 75 to 300 cm., and was 

 closely surrounded by cylinders of zinc or lead, between the 

 poles of a large electromagnet. In this way the /3 rays were 

 absorbed and deflected, but the correction for the absorption 

 of the 7 rays was as small as possible. The strengths of the 

 radium specimens were determined by comparison with a 



* Rutherford's ' Radioactivity/ p. 87. 



