A// tJte Motion of Podt'irc Ions In a Gas. (UlT) 



Quantities of eloctricity can be thus measured over a lar<;e 

 range since the capacities c and the eh'ctroniotive forces E 

 can be easily ahered in hirge proportions. In the following 

 experiments 4 volts (E) and a capacity of 60 electrostatic 

 units (ci) were used to determine the smaller currents. 



A battery of small Leclanche cells was used to establish 

 the electric force X between the plates A and B. The elec- 

 tromotive force of these cells remained very constant as the 

 largest currents which were used were small compared with 

 the currents which would polarize the cells. 



The conductivity between the plates was started by the 

 action of ultra-violet hght on the zinc plate. A s])ark-gap 

 8 (fig. 1) in a circuit through which a leyden-jar discharge 

 takes place was used as the source of ultra-violet light, the 

 spark passing between aluminimn electrodes. The leyden- 

 jar was charged from the secondary of a Ruhmkorff' coil. 



The apparatus used for generating the light was separated 

 by a metallic screen from the rest of the apparatus in order 

 to prevent induction effects on the insulated conductors. 



In making the experiments I was assisted by Mr. Bennett 

 as it requires two observers to make all the necessary obser- 

 vations in the course of an experiment. 



5. The results of the experiments are given in the following- 

 tables. 



The })ressure of the gas at w4iicli experiments were made 

 is given at the head of each table. The distance a between 

 the plates in millimetres is given in the first line of the 

 tables. 



The electric force X is given in volts per centimetre. 



The quantity of electricity (j which passed between the plates 



under the action of the force X while the lioht acted for 



1... . . - 



ten seconds is given in arbitrary units. (The absolute value 



of y in electrostatic units may be found approximately by 



multiplying the numbers in the tables by '03.) 



The ratios of the quantities q are independent of the in- 

 tensities of the light, the same relative values being obtained 

 when the light falling on B was reduced so as to give one- 

 twentieth of the usual conductivity. 



The values of n calculated by the formula 



n — ;iu 7 TT — 



in which /^)=1 are given in the line below the numbers 7, the 

 quantities a and ^ being chosen so that the values of it should 

 as nearly as possible be equal to the values of 7. 



