Processes of' Ionization in Gases. 855 



source to fall on any of the three plates. The parallel 

 plates were mounted under a bell-jar, and the light from 

 the spark-gap was admitted through a quartz window in the 

 base of the apparatus ; the plates could be turned from outside 

 by an air-tight spindle without changing the pressure of the 

 gas in the be 11- jar. 



The distance between the top and bottom plates was 4 mm. 

 In this case, instead of comparing the values of a. for the 

 three plates by measuring the currents for different plate 

 distances, the more direct test was applied of comparing the 

 currents for the three plates for different voltages between 

 the plates, the distance being alwaj^s 4 mm. 



Any appreciable photo-electric effect of the gas radiation 

 would cause differences in the shape of the current E.M.F. 

 curves for the various plates ; but in no case, as will be seen 

 below, was there any appreciable variation. 



The main experiments were done with the copper and zinc 

 plates as negative electrodes, as of the three plates these two 

 had the most widely separated photo-electric effects. For 

 the light from a spark-gap with copper terminals the copper 

 was about 17 times as active as the zinc, while for zinc 

 terminals it was 25 times as active. It is therefore reason- 

 able to suppose that the radiation from molecules of the gas 

 between the plates would also produce very different results 

 on the two plates. 



The following figures were obtained for the current between 

 parallel plates 4 mm. apart, the initial ionization being due 

 to a copper spark-gap giving light for a definite time. The 

 top line gives the voltage V between the plates ; the second 

 gives the current in arbitrary units when the top plate — the 

 negative electrode — was copper, the third when the top plate 

 was zinc, and the last the ratio of the two currents. 



Table I. 



V 20 40 80 200 



Copper 90 9S'8 148 1480 



Zinc 5 5-7 87 83 



Eatio 18 17-3 17*0 ITS 



(Air pressure, 222 mm. of mercury ; distance between plates. '4 cm.) 



With a zinc spark-gap and the same air pressure, two very 

 different voltages were selected and the results are given in 

 Table IT. 



