THOMSON'S METHOD OF CHEMICAL ANALYSIS 55 



deflected at right angles to the plane of the paper a distance y 

 given by the equation : 



He 



k x k 2 being constants depending solely on the dimensions and 

 form of the apparatus used. 



If a continuous stream of particles, all of the same mass, 

 carrying the same charge (or what amounts to the same thing in 

 this case, having the same ratio mje of mass to charge) and 

 moving with the same velocity, strike the screen shown in 

 plan in fig. 4 — which is covered with a layer of powdered 

 Willemite, a substance that fluoresces strongly under the influ- 

 ence of the rays— a bright patch of light is produced at the 

 point B, due to undeflected rays, when neither the potential 



Fig. 4. 



nor the magnet is on. The plates P P' being vertical, if the 

 electric field only be on, the spot will be deflected to C; if the 

 magnetic field only be on, the spot will be deflected to D but 

 if both are on together to a point p of which the horizontal 

 and vertical displacements are x andjy respectively. It is there- 

 fore only necessary to measure x andjy and from the equations 

 given above it follows that x is inversely proportional to the 

 kinetic energy of the particle y and inversely proportional to 

 its momentum y and that 



yjx is a measure of the velocity of the particle ; 



yi* m 



m 



or the ratio of mass to charge. 



Now e can only exist as a multiple (and in general only a 

 small multiple) of the charge on a single corpuscle and all 

 the evidence up to now shows that this is invariable and 



