148 Prof. J. Zeleny on the Ratio of the Velocities of the 



and for higher voltages it was connected to a source of 

 potential nearly equal to that of the place to be measured. 

 By means of the part E, sliding on the rod D, the wire can 

 be moved to any part of the field, and its position observed on 

 the scale M by means of the pointer F. 



In taking readings, the wire is simply allowed to remain 

 in the desired position until it assumes the potential of the 

 place due to the ions in the air, as is indicated by the elec- 

 trometer deflexion remaining stationary. 



In the following example the plates and wire were made of 

 aluminium, and the plates were 3 centim. apart and connected 

 to the opposite poles of two storage-cells the point between 

 which was to earth. 



The readings obtained are given in Table VI., together 

 with readings calculated for a uniform gradient such as exists 

 when the rays are not acting. 



Table VI. — Potential between Plates. 



Distance from 

 Negative Plate. 



Observed 

 Potential. 



Potential calculated 

 from a uniform gradient. 



mm. 







volts. 



-2 



volts. 

 -2 



1 



-1-26 



-1-87 



2 



—1-16 



-1-74 



4 



- -94 



-1-47 



6 



- -78 



-1-20 



Jl 



- -33 



- -53 



16 



+ -11 



4- -13 



21 



+ -GO 



4- -80 



26 



4-Pll 



+ 1-47 



28 



+1-31 



4-1-74 



29 



4-1-42 



4-1-87 



30 



+2 



42 



It will be seen that the deviations from the straight poten- 

 tial gradient are considerable, and that the fall of potential 

 is very rapid at the plates, being greater at the negative plate. 

 The reason for this is that the amount of positive electrifica- 

 tion at the negative plate is greater than that of negative at 

 the positive plate, due to the difference in velocity of the two 

 ions as has been shown in § 9. 



The results are represented by the curve in fig. 13. The 

 field near the centre is quite uniform, as there is little or no 



