168 Prof. E. Rutherford on some 



The strength of the magnetic field H was constant in all 

 the experiments, and was equal to 9170 C.G.S, units. Thus 

 the value of Hp = 3-98 x 10 5 . 



The a rays from radium C are the most penetrating of all 

 those emitted from the various products of radium, and pre- 

 sumably, therefore, are expelled with the greatest velocity. 

 In a previous paper (loc. cit.) I deduced that the maximum 

 value of Hp for radium rays was 390000. This result was 

 obtained by means of the electric method, using radium in 

 radioactive equilibrium as a source of rays. Becquerel has 

 shown that the value of Hp is not a constant, if measured 

 by means of the trace obtained on a photographic plate when 

 the rays from radium passed through air at atmospheric 

 pressure. The value of Up varied between 291000 and 

 34 1000. This result is clear in connexion with the experiments 

 discussed later. The pencil of rays was not homogeneous-, 

 as Becquerel thought, but included a. particles projected with 

 different velocities, which consequently were deflected to 

 different extents. An explanation of the results of Becquerel 

 along these lines has been given by Bragg (loc. cit.). 



We know from the action of a magnetic field in a moving 



mV 

 charged body that Hp = , where m is the mass of the 



particle, V its velocity, and e its charge. Thus 



— =3'98xl0 5 . 



e 



The experiments of the deflexion of the « rays in passing- 

 through an electric field are not yet completed ; they are 

 complicated by the fact that a vacuum, sufficiently high to 

 allow of a large difference of potential between the plates 

 without sparking, has to be obtained in a very short time 

 after the active wire is introduced. 



We can, however, obtain an approximate estimate of the 

 value of e/m and of V on the assumption that the heatiu£ 

 effect of radium C is due entirely to the kinetic energy of the 

 a particles. From the experimental results given by Ruther- 

 ford and Barnes, it can be deduced that about 31 per cent, 

 of the total heating effect of radium is due to the product 

 radium C. Since one gram of radium emits heat at the rate 

 of about 100 gram-calories per hour, the heating effect of 

 radium C present in one gram of radium is 31 gram-calories 

 per hour. 



Since radium C is a direct product of radium, the number 

 of particles expelled per second from radium C, present in 



