Jbdecfromefer*^ 



Fig. 1. 



This quite satisfied the purpose for which it was intended, and 

 we were therefore provided with the means of testing the effects 

 of ay stream fairly free from jS rays. We used a magnetic 

 field of about 2,500 units. The details and dimensions of the 

 apparatus are shown in fig. 1. The radium was placed at 

 the bottom of a conical hole made in a massive lead block. 

 Plugs of various materials and different thicknesses were 

 turned to fit exactly in the hole. The screens were brought 

 to the position p,p by means of a sliding frame, which passed 

 through an opening in the side wall of the ionization-chamber. 

 This was done to avoid the necessity of opening up the cham- 

 ber, an operation which often makes readings unsteady for a 

 short time. The electrode consisted of two concentric circles 

 of thick copper- wire, connected by short crosspieces, as shown. 

 The chamber itself was kept at 400 volts. The radium was a 

 fine specimen, which Dr. Herman Lawrence, of Melbourne, 

 was so very kind as to lend us. It was contained, when we 

 received it, in a small brass vessel covered with mica, over 

 which a little wax had been run. Being very anxious to avoid 

 any trace of emanation escaping into our apparatus, and at 

 the same time to run no risk of injuring the specimen, we 

 covered it still further with asbestos, placed the whole in a 

 thin brass vessel, just big enough to hold it, and soldered down 

 the lid. Thus the radium was at all times sufficiently screened 



