Field of Radium B on Recoil from Radium A. 879 



electric field gave inconclusive results ; for it was found that 

 surfaces, even though situated so that they could receive no 

 direct radiation from a source of radium A, became active. 

 It is unnecessary here to enter into a discussion of the 

 mechanism by which this occurs. 



It thus appeared that no reliable results could be obtained 

 unless the possibility was excluded of much active matter 

 reaching the receiver by methods other than direct radiation. 

 The apparatus shown in fig. 3 was therefore designed to 

 obviate this trouble. 



$ OIwS 



The wire W was mounted between two parallel mica strips 

 2 centimetres long, coated with copper plates kept at the same 

 potential. The u recoil-stream " from the wire passed from 

 the region between the two copper plates where no electric 

 field existed into the space between the two parallel brass 

 plates A and B, 9*4 millimetres long, between which a 

 difference of potential was maintained. The plates A and B 

 fitted into the ebonite plug E and were 1*17 millimetres 

 apart. The copper cylinder K, which was 4*5 centimetres 

 long and fitted over the ebonite plug E, prevented the 

 " recoil-stream " from being subjected to an electric field 

 after leaving the space between the plates A and B. The 

 " recoil-stream " subsequently fell on a brass strip C, 2 centi- 

 metres long, situated 5 millimetres from the end of the 

 copper tube. 



To carry out an experiment the wire W was exposed to a 

 large quantity of emanation for ten minutes, transferred to a 

 vessel at 400° C. which could be quickly evacuated to remove 

 adhering emanation, and then mounted as shown in fig. 3. 

 The whole apparatus just described was contained in a glass 

 tube which could be exhausted to a pressure of about 

 1/300 millimetre of mercury within two minutes, after which 

 the electric field could be applied between the plates A 

 and B without fear of a discharge taking place through the 

 residual gas in the tube. The difference of potential between 

 A and B was determined by a direct-reading electrostatic 



