280 Possible Magnetic Polarity of Free Electrons. 



if the medium through which it passes is susceptible to 

 magnetization. For the introduction of such a doublet will 

 induce magnetization in the surrounding medium just as a 

 bar magnet induces magnetization in a neighbouring mass of 

 iron. If the atmosphere acts paramagnetically, the magnetic 

 field at the doublet due to the induced magnetization has the 

 same direction as the magnetization of the doublet. Con- 

 versely, the induced magnetic field due to a diamagnetic 

 atmosphere is opposite -in direction to the doublet's axis. 

 This induced magnetic field will clearly have the same effect 

 on the motion of the electron as would an externally applied 

 field of the same intensity. That is, the beta ray will move 

 in a helical path whose axis is parallel with the magnetic 

 axis of the beta particle. The path may have the form of 

 either a right- or left-handed helix, according as the north 

 or south pole of the beta particle is foremost. This is in 

 accord with observation, which shows paths of both kinds. 

 Furthermore, if the induced magnetic field does not decrease 

 while the velocity of the beta ray diminishes, the tracks of 

 the particles will be converging helices such as appear in the 

 photographs. 



The intensity of the magnetic field induced at a beta 

 particle due to the effect of its magnetic moment on the 

 part of the medium at a distance greater than r from the 

 doublet may be shown to be 



tj _ SlT/jLS 



nr ~ ~W ' 



where jjl is the magnetic moment of the doublet and s is the 

 effective susceptibility of the medium. If the cathode 

 particle is moving at a speed corresponding to a drop through 

 10,000 volts, the minimum distance of its approach to an 

 electron at rest is according to usual theory about 10 -10 cm. 

 Taking this as the value of r, using for s the value 3 x 10~ 8 of 

 the magnetic susceptibility of air, and for /jl the moment 

 10~ 20 of an electron with an angular momentum h/2ir, this 

 expression indicates that the intensity of the magnetic field 

 induced at the electron should be of the order of 3000 gauss. 

 This is approximately the field which would be required to 

 produce the observed curvature. 



The impulsive torque exerted by the beta ray doublet on a 

 magnetic electron at rest when passing it at a distance 

 of 10" 10 cm. has, however, a period corresponding to X-ray 

 frequency. It is clearly necessary, therefore, to take into 

 account the inertia of the elementary magnetic doublets of 

 which the medium is composed. Furthermore, the magnetic 



