Field of Radium B on Recoil from Radium A. 877 



field was obtained by testing the cross-piece in a similar 

 manner. 



The results of a series of experiments made in this way 

 with different voltages between the plates led us to believe 

 that at least some of the " recoil-atoms " of radium B 

 projected from the wire were positively charged. 



To test the correctness of this conclusion with certainty 

 the following experiments were made. 



Direct Determination of the Quantity of Radium B deflected 

 by an Electric Field. 



The active wire W (fig. 2) was placed symmetrically between 

 two pairs of plates 1*4 millimetres apart and 4 centimetres 

 long adjusted to be in line with each other and contained in 

 a glass vessel which could be evacuated as in the previous 



Fig. 2. 



experiments. The " recoil-stream " from the wire W was thus 

 projected through the gaps between the two pairs of plates, 

 and fell upon the cross-pieces A and B. Between one pair of 

 plates an electric field was maintained, while the other plates 

 were kept at the same potential. If the "recoil-stream" 

 were charged on leaving the wire, it would be deflected by 

 the electric field, and the plate B should therefore receive less 

 activity than A. Since the radium A on the wire might not 

 have been deposited uniformly by exposure to the emanation, 

 the wire was kept constantly rotated by attaching it to a 

 ground -glass stopper turned by hand and fitted into the 

 containing vessel. After an exposure of ten minutes in 

 vacuo to the radiation of the wire, the two cross-pieces A 

 and B were removed and tested by an a-ray electroscope. 



The number of " " recoil-atoms " reaching the plate A 

 through the uncharged plates was always found to be greater 

 than the number reaching B through the electric field. The 

 reduction in the number reaching B depended on the field- 

 strength, being greater the greater the field applied ; but 

 although the quantity of radium B reaching the cross-piece B 

 was reduced in this way by the field between the plates, some 

 activity was always found there even with the greatest field 

 used, indicating that part of the " recoil-stream " projected 

 from the wire was undeflected. In view of some subsequent 

 experiments on the magnetic deflexion of radium B it seems 



