Gases by the Motion of Negatively charged Ions. 219 



plate in my experiments gives out more negative corpuscles 

 than the aluminium plate through which the Kontgen rays 

 pas?. 



The corpuscles emitted by the brass plate have an effect 

 upon the conductivity between the two plates which we shall 

 now proceed to investigate. 



Let C^ be the current when the lower plate is positive, and 

 C 2 the current with the same electromotive force when the 

 lower plate is negative. 



Let D be the number of negative corpuscles emitted from 

 the brass plate, and C the number of ions (positive or nega- 

 tive) produced between the plates in the volume of the gas. 



C 3 = C-D. 



Hence C = -L- — -, so that the correct value of C is ob- 

 tained by taking the arithmetic mean of the two currents. 

 The experiments at '171 millim. pressure give C 1 = 7*b > , and 

 C 2 = 4 , b' when the plates are one centimetre apart and differ 

 in potential by 10 volts. The corresponding numbers for 

 334 volts are 30 and 27*5. In the experiments at higher 

 pressures the ratio of D to C is smaller. 



The negative corpuscles D are probably emitted with 

 different velocities, so that when the pressure is increased, 

 some of them are stopped by the gas before they get to the 

 aluminium plate. It is evident from the experiments at '171 

 millim. pressure that an electromotive force of 334 volts has 

 little effect in stopping them, since the currents differ by the 

 same amount for high and low potentials. AVhen some of 

 the corpuscles which start with a small velocity are stopped 

 by the air at higher pressures, the equation C 2 = C — D does 

 not hold accurately, for although D corpuscles start from the 

 plate, some of them are stopped by the gas and driven back 

 to the plate by the electromotive force. In these cases the 

 mean current does not differ much from either of the currents 



C x or C 2 , so that the equation C = ' . ' 2 represents with 



sufficient accuracy the number of ions produced in the volume 

 of the air. 



At 4*13 millim. pressure Ci = 37*5 and C 2 = 41'5, when the 

 potential-difference between the plates was 20 volts. The 

 correct value of probably does not differ from 39'5 by as 

 much as 2 per cent. 



When the force is increased and larger conductivities are 

 obtained, the ratio of D to C diminishes. 



Q2 



