CHAPTER XIV. 



MORE ADVANCED DEVELOPMENT OF THE COM- 

 BINED ELECTRIC AND MAGNETIC DEFLEXION 

 METHOD FOR MEASURING VELOCITY AND MASS 

 OF THE PARTICLES IN COMPOUND RAYS. 



THE methods given in Chaps. V. and VI. and Chap. 

 IX., for measuring u and e/m, made the determina- 

 tion look very simple, if the precaution is taken of 

 having the apparatus in vacua, so as to eliminate 

 the troublesome conducting power of the air and 

 obtain the electric deflexion undiluted, as J. J. 

 Thomson first found feasible. But then the simple 

 theory, there given, assumed that the quantities to 

 be measured were constant, and that the deflexion 

 to be observed in each case was a single deflexion 

 capable of accurate measurement ; but this is often 

 far from being the case, since the velocities of the 

 particles differ ; and when, as in the case of radium, 

 some of the speeds approximate to that of light, it is 

 impossible that it can be the case, for the inertia itself 

 then changes in a complicated way with the speed 

 and must be treated as variable. It is easy to forget 

 that, because it is an unusual feature in mechanics. 

 So the deflexion cannot be a simple deflexion, the rays 

 must be fanned out as it were into a spectrum (see 

 fig. 18); and, since this spectrum is continuous, 



