278 



Mr. Not man Campbell on Delta Rays. 



3. The principles involved in the use o£ the electric field 

 are extremely simple. Let two bodies A and B be emitting 

 the rays, and let the circumstances be such that all of the 

 rays emitted by one body strike the other, when the bodies 

 are at the same potential. Let the current received by A be 

 measured when B is at different potentials, V, the potential of A 

 being always 0. Let this current, when V is 0, be represented 

 by : it will be made up of two parts, (1) a part independent 

 of V, due to the charge carried by the alpha rays them- 

 selves and similar causes, and (2) the difference between the 

 number of rays emitted by A and those emitted from B and 

 received by A. When V is positive, the number of rays 



Fig. 1. 



-oQ 



emitted by A will be unchanged, while the number of rays 

 received by A from B will be decreased by the number of 

 rays emitted from B which have a speed less than that cor- 

 responding to V. Accordingly PN, which is the difference 

 betweon the current for V = and that for V = ON, repre- 

 sents the number of rays emitted from B which have a speed 



