in Gases by the Motion of Negatively Charged Ions. 633 



determine accurately. On the other hand, the bulbs which 

 give the strongest rays are as a rule less constant, so that 

 in order to arrive at the same degree of accuracy at the 

 lower pressures the observations had to be repeated several 

 times. 



Table I. — Currents in Hydrogen between plates 5'3 milli- 

 metres apart, 

 being given in volts per centimetre. 



The electric force X acting on the gas 







Pressures. 





x 











34 



23-5 



145 



95 



72 



19-1 



11 



625 



12 5 



215 









135 



287 









161 



358 









21-2 



430 









31-5 



502 









535 



574 





14 



157 



103 



665 











717 







39-7 



234 



789 



24-7 



23-5 



80-5 





843 







191 





897 



31 



4i 







951 



36 



58 









1. 



2. 



3. 



4. 



In the above experiments the current which is obtained 

 with a force of 72 volts per centimetre represents the number 

 of ions produced in the gas by the rays. This current 

 remains constant until new ions are produced by collisions. 

 The electric force required to produce an appreciable number 

 of ions diminishes as the pressure is lowered. For pressures 

 lower than 9*5 millimetres, the ions can be detected when the 

 difference of potential between the plates is as low as 38 volts. 

 This is shown by the results given in Table II. (p. 634). 



It will be seen from the last set of experiments that the 

 current reaches a constant value. When this stage is reached, 

 the force is large enough to produce new ions at every 

 collision. 



Phil. Mag. S. 6. Vol. 1. No. 6. June 1901. 



2T 



