56 



SCIENCE PROGRESS 



indivisible. Thus if we have a beam of positive rays of constant 

 mass m but moving with velocities varying over a considerable 

 range, y 2 /x will be constant and the spot of light will be drawn 

 out into a parabola pp l (fig. 5). When other rays having a 

 larger mass m l but the same charge are introduced into the 

 beam, they appear as another parabola q q 1 having a smaller 

 magnetic displacement. (If the range of kinetic energy be the 

 same for both particles the electric displacement will be the 

 same.) If any straight line p, q, n, be drawn parallel to the 



Fig. 5. 



magnetic axis O Y cutting the two parabolas and the electric 

 axis O X in p, q, n, it will be seen at once that 



tw- 

 in 



~ qti 1 



That is to say, the masses of two or more different particles can 

 be compared directly by merely measuring two lengths the ratio 

 of which is entirely independent of the form of the apparatus and 

 the experimental conditions. 



This is really the fundamental principle on which the 

 method is based. A photograph is taken in which we can 

 identify at least one parabola as belonging to a set of par- 

 ticles of known mass ; all the other parabolas can then be 

 measured and compared with this one and their masses deduced. 

 In the case of the lighter particles, the hydrogen atom and the 

 hydrogen molecule are taken as the standards ; in the case of 

 heavier particles, the mercury atom is particularly useful as a 

 standard, as it is almost always present and for some reason at 

 present unexplained gives a very bright curve. In order that 



