172 Mr. W. Makower on the Number and Absorption 



the rod when insulated in a perfect vacuum could be 

 measured by a quadrant-electrometer. As a result of his 

 experiment, Rutherford concluded that " the total number o£ 

 ft particles expelled bv 1 gram of radium per second is 

 7-3 xlO 10 "*. 



In the light of recent investigations on secondary 

 radiation, it seemed not impossible that in the above- 

 described experiments errors might have arisen on account 

 of secondary radiation of the ft type set up when the rays 

 from the active deposit impinged on the lead rod and on the 

 containing cylinder. As it is difficult to estimate to what 

 extent the value obtained was vitiated b}^ this circumstance, 

 it seemed desirable to repeat the experiments in such a 

 way as to reduce the possibility of errors due to secondary 

 radiation to a minimum. It was with this object, and at 

 the suggestion of Professor Rutherford, that the following 

 measurements were undertaken. 



Method of Experiment. 



As somewhat large quantities of radium would have been 

 required to carry out the experiments satisfactorily, it was 

 thought advisable to work, not with radium itself but with 

 the emanation. The emanation from a considerable quantity 

 of radium was therefore collected in a fine glass tube, about 

 1 millim. in diameter, the walls of which were exceedingly 

 thin. When the emanation had been transferred into the 

 tube, it was sealed off so that the emanation could not 

 escape. After sufficient time had elapsed for radioactive 

 equilibrium to be established, the quantity of emanation 

 which had been collected was determined by comparing the 

 7 radiation emitted from the tube with that from a standard 

 quantity of radium. The quantity of radium emanation in 

 the tube at any subsequent time could then be deduced from 

 the well-known rate of decay of the emanation. The tube, 

 which was about 7 centims. long, was then covered with 

 thin aluminium-leaf to render it conducting, and introduced 

 into the measuring apparatus, the arrangement of which can 

 be seen from fig. 1. The tube A containing the emanation 

 was attached by sealing-wax to the brass tube B, which was in 

 turn cemented to the glass tube C fused on to the stopper D 

 of an internally silvered glass vessel of 4*4 centims. diameter. 

 By inserting the stopper D, the tube A could be made to lie 

 along the axis of the glass vessel. The other end of the glass 



* Taking the value 4*65 X 10- 10 for the charge carried by the j3 particle, 

 instead of 34x10— 10 , which is probably too low, this value should be 

 5-33x1010. 



