Field of Radium B on Recoil from Radium A. 881 



deflected. The distance between the two maxima was taken 

 as a measure of the deflexion of the " recoil-stream " while 

 passing through the electric field. Although this procedure 

 does not give accurate values, it serves to show the order of 

 magnitude of the deflexion. 



Table II. 



Voltage between 

 plates. 



Displacement of 



maximum observed, 



in millimetres. 



Displacement of 



maximum calculated, 



in millimetres. 



930 

 1680 

 1860 

 1890 

 1920 



20 



39 

 4-5* 

 4-0 

 4-1 



1-62 

 2-94 

 3-25 

 3-31 

 3-36 



* In this case the readings were small and correspondingly more 

 uncertain than in the other experiments. 



If the velocity of the " recoil-atoms " of radium B is 

 assumed to be 3*27 x 10 7 centimetres per second, as calculated 

 on p. 878, it is possible to deduce the deflexion to be expected 

 on certain simple assumptions. For the particles projected 

 from the wire will have a parabolic path as they pass between 

 the two charged plates, and will proceed along the tangent 

 to the parabola at the point at which they emerge from the 

 electric field until they ultimately strike the strip mounted 

 to receive them. From the known dimensions of the 

 apparatus it is a matter of no great difficulty to calculate 

 the displacements to be expected on the strip for different 

 voltages applied between the plates, assuming that the value 



of — for the hydrogen ion in electrolysis is 9*63 x 10 3 on 



the electromagnetic system, that the charge carried by the 

 atom of radium B is the same as for the hydrogen ion in 

 electrolysis, and that its mass is 214. 



There is, however, one source of uncertainty in making the 

 calculation ; for since the length of the plates was only 

 9*4 millimetres, which is not very great compared with their 

 distance apart, which was 1*17 millimetres, the field must 

 spread out appreciably at both ends. The corrections for the 

 end effects in cases similar to that of our experiments have 



