hif tlw Motion of Positive Ions in a Gas. 



mi 



with ilirt'oront Llistiinccs botwoou the phites, the electric force 

 and the pressure being- unaltered, then the charges acquired 

 by one of the plates will be given by formula 1, in which the 

 quantities a and ^ are constant. 



o. To illustrate the theory I may quote ex[)eriments made 

 with air at a millimetre pressure, the electric force being 

 i)5() volts per centimetre in each experiment. 



The distance a between the plates in centimetres is given 

 in the first line of the following table ; the second line gives 

 the charge q acquired by one of the plates determined ex- 

 perimentally "^ ; the third line gives the values of e"-'^ where 



^ = 5'25 ; and the fourth line oives the values of ~ rr — 



where a = o"25 and /3 = *0141. 





0. 



'■ 



4. 



6. 



8. 



xo. 



11. 





u 





2-86 



8-3 



24-2 



81 



373 



2250 





t*« 



1 



2-8(3 



8-2 



23-4 



66-5 



190 



322 





. (a-/3)€(«-^>« 



1 



2-87 



8-3 



24-6 



80 



380 



2150 



1 a-)3e(— ^)« 



The numbers in the third line agree only with the experi- 

 ments at the shorter distances. The numbers in the fourth 

 line give the conductivities which would be produced if both 

 j)ositive and negative ions generate others by collisions, and 

 it is evident that these numbers are in ao-reemeut wdth the 

 experiments at different distances up to eleven millimetres. 



A comparison between the numbers given by the two 

 formulae shows that for the larger distances positive ions give 

 rise to a large increase in the conductivity, although the 

 actual number of ions which they generate is in this case 

 very small. 



The formula can thus be easily tested by experiments on 

 the conductivity produced by ultra-violet light between plates 

 at different distances apart, and a number of values of a and 

 /? can be obtained for different forces and pressures. 



4:. The form of apparatus which was used for finding the 

 conductivities is shown in fig. 1. The conductivity takes 

 place in the gas between the two plates A and B. The 



* [The values of (j are multiplied by a factor so that their relativi 

 values may be compared with the numbers given by the formulae.] 



