440 



Sir J. J. Thomson on the Origin of 



When the electric force changes from plus to minus, as 

 we have supposed it to do inside the atom, the amount of 

 energy which must be communicated to an electron to 

 enable it to get from one place P to another Q may be 



greatly in excess of j ~Rds, where R is the force on an 



electron and ds an element of its path. For the potential 

 distribution may then be represented by a curve such as 

 that, in fig. 1, when an electron could not get from P to Q 



Fiff. 1. 



unless it surmounted the peak R. To do this it must receive 

 energy represented by EM, which may be much greater 



than 1 Hds, which is represented by QN. In such a case 



as this the electron ejected from P will arrive at Q with a 

 finite amount of kinetic energy, measured by the difference 

 between RM and QN. 



In a periodically varying electric field of the type we are 

 considering we might expect the maximum value of RM to 

 be approximately twice QN. For the work done between 

 two places of equilibrium is equal to Bx — B 2 when Bi and 

 B 2 are the values of the magnetic induction at the two 

 places. 



Now, if the numerical value of the magnetic force at P is 

 not less than that at any other point nearer the surface of 

 the atom, the greatest possible negative value of B 2 will be 

 — B], and since B l5 to a rough approximation, varies har- 

 monically, B 2 may be expected to approach this value. Thus 

 Bj — B 3 will have as its maximum value 2B l5 i. e. the 

 energy which must be communicated to the electron to 

 enable it to surmount an obstacle like the peak at R is twice 

 the increase in potential energy it gains by going to Q, 

 where B 2 is assumed to vanish. Thus the electron will 

 possess, when it emerges from the atom at Q, kinetic energy 

 equal to half the energy communicated to it at P. 



