﻿778 Mr. B. B. Baker on the Path oj an 



exchanges between the kinetic energy o£ the electron and 

 the radiant energy should conform to this relation, it is 

 necessary that the atom should contain a mechanism which 

 is such that an electron approaching an atom will induce in 

 the atom what may be called a " magnetic current " ; the 

 model which he has suggested to typify such a structure 

 consists o£ a number of elementary bar magnets lying in 

 a plane and rigidly connected like the spokes of a wheel, 

 so that they rotate together in the plane, each magnet 

 having one pole at the centre of the wheel and having its 

 direction at every instant radial from the centre ; it is, in 

 fact, such a structure as Sir Alfred Ewing has proposed to 

 explain induced magnetism *. 



If we suppose such a magnetic wheel to be placed with its 

 centre at the origin and its plane in the plane of yz 9 and 

 suppose an electron to be projected towards it along the axis 

 of a?, then the electron, by its motion, creates a magnetic field 

 which will cause the magnetic wheel to rotate, and the rota- 

 tion of the magnetic wheel will set up an electric field which 

 will retard the motion of the electron. Denoting the radius 

 of the magnetic wheel by a, the magnetic moment of one of 

 the elementary bar magnets by fia, the sum of the values of 

 /ul for all the elementary bar magnets composing the wheel 

 by M, the moment of inertia of the wheel about its axis by A, 

 the charge on the electron and the mass of the electron by e 

 and m respectively, then Whittaker has shown that if the 



. . 2eM 



velocity of projection u of the electron is less than • 1 — ■ 



V Aw 

 the collision between the electron and the wheel is in the 

 nature of an elastic impact, i. e., the electron is stopped at a 

 certain point and forced to return along its path, the mag- 

 netic structure giving back to the electron the energy it had 



previously received from it; but that if u >^ — . — - , the 



— v Am 



electron is able to pass completely through and away from 



the magnetic structure so as to be free from its influence, 



and the magnetic structure is left in rotation. In this latter 



case, the amount of energy U lost by the electron and 



gained by the wheel, is given by the equation 



2e 2 W 

 A ' 

 and the absorbed energy appears in the atom as a magnetic 



* Cf. Ewing, " On Models of Ferromagnetic Induction," Proc. Rov. 

 Soc. Edin. xlii. pp. 97-128 (1922). 



