564 Sir E. Rutherford and Mr. H. Robinson on Mass and 



the risk o£ puncturing the a-ray tube, or fogging the photo- 

 graphic plate by a premature application of the electrostatic 

 field. 



Generally about an hour was required to complete the 

 exhaustion, and then the exposure with the electric field 

 lasted about six hours. The length of this exposure, which 

 was necessary even with a practically constant source of 

 about 150 millicuries, shows the impossibility of performing 

 a similar experiment, using an activated wire as a source. 



A photograph showing the electrostatic deflexion of the 

 a rays is given in Plate XII. fig. 1. The voltage applied was 

 1435 volts, and the time of exposure 6 hours in all. The 

 reproductions are enlarged 3*75 times. The central band is 

 the result of the direct action of the a rays before the appli- 

 cation of the field, w r hile the three bands on each side show 

 the traces produced on reversal of the field by the a, rays 

 from radium C, radium A, and the emanation respectively. 

 The radium C lines having the highest velocity are, of course,, 

 the least deflected. 



It is easily seen from the photograph that the inside edges 

 of the bands are exceedingly sharply defined. The reason 

 of this peculiarity, which was of great assistance in making 

 accurate measurements, will be discussed later. 



When the a-ray tube is used as a source, it is necessary to 



in v 

 determine directly the value of -^ for the issuing rays by 



measuring the deflexion of a pencil of rays in a magnetic 

 field. The reduction in velocity of the a rays due to their 

 passage through the walls of the «-ray tube could be esti- 

 mated from the observed stopping power of the tube, but for 

 our purpose it was preferable to make the measurement 

 directly. The a-ray tube was therefore attached to the 

 apparatus used in the magnetic experiments (p. 555), and 

 photographs taken for exposures of about 10 to 20 minutes, 

 the magnetic field being reversed at intervals. The magnetic 

 field was of exactly the same intensity as that employed in 

 determining the deflexion due to a line source of homogeneous 

 rays. 



A reproduction of one of the photographs is showm in 

 Plate XII. fig. 2. As in the previous case, three distinct bands 

 due to the three types of rays are clearly seen, but, as is to 

 be expected, the separation of the bands from one another is 

 less marked than in the case of the electrostatic deflexion. 

 The sharpness of the inside edges of the lines, as in the 

 electrostatic experiments, is again very noticeable, as well as 

 the actual narrowing of the radium C trace on the plate. 



