Experiments on Delta Rays. 537 



another about 1^ mm. ; the results given below apply 

 equally to both series. 



In the former experiments measurements were never made 

 with a potential difference less than that required to saturate 

 the delta ray current when the vessel was completely 

 exhausted. Accordingly the variations of current noted can 

 never have been due to any effect of the field in stopping the 

 delta rays from one electrode from reaching the other. It 

 appeared that it might be possible to obtain some information 

 as to the delta rays liberated in the gas, and not at the 

 electrodes, by measurements with much smaller potential 

 differences. 



Let us suppose that the delta rays are liberated uniformly 

 throughout the gas by the alpha rays, that they are all pro- 

 jected from the atoms with the same velocity of V volts, 

 but that their velocities are equally distributed in all direc- 

 tions ; let us suppose further that the pressure of the gas is 

 so low that very few encounters occur between the delta 

 rays and the molecules, and that the positive ions have initial 

 velocities so low that they all travel to the negative electrode 

 under the smallest potential differences with which we ar« 

 concerned. Then a simple calculation will show that, if 

 N delta rays and N positive ions are liberated per second in 

 the gas, the relation between Y and the current i will be 



if V is less than V , 



if V is greater than V . 

 It is to be noted that the tangents to the two portions of the 

 curve are coincident at the point where they cut, so that the 

 curve has no " knee/' but if the relation between i and V 

 turned out to be that predicted an estimate of the value of 

 V could be obtained. 



13. If collisions between the delta rays and the molecules 

 occurred, this theory would not be applicable. It was thought 

 that, if the current for all P.D/s was found to be proportional 

 to the pressure of the gas (and therefore to N) for all pressures 

 under a certain limit, it might be assumed that the effects of 

 such collisions were negligible. However, in the experiments 

 on air it was found that the current for all P.D.'s between 

 1 and 15 volts was accurately proportional to the pressure, 

 until a pressure of 4 mm. was reached with the smaller 



