Ionization by Spraying. 393 



enough to join with me in this part of the work, and he took 

 measurements with his direct-reading electrometer*. The 

 instrument is particularly well suited for this purpose, because 

 a constant difference of potential is maintained between the 

 inner and the outer cylinder of the testing vessel. To make 

 certain that an insulation leak was not vitiating the results, 

 a guard ring was used. 



A testing vessel, as shown in fig. 8, was first employed. 



-cu — 



Fig. 8. 



-c>— 



V 



-C3- 



/o cm 



The outer cylinder was insulated and kept at a potential of 

 550 volts. The electrodes are denoted by Z, II, III in the 

 figure. If I m and II were put to earth and III to the elec- 

 trometer, and air with water spray passed rapidly through 

 the testing vessel, no current was detected by the electro- 

 meter. Hence all the ions were extracted from the air by 

 / and II. If I was joined to the electrometer, the 

 current, for constant spraying, reached a maximum of 170 

 scale-divisions. 



If I was put to earth and II connected to the elec- 

 trometer, the current was measured by 16 scale-divisions. 

 Hence about 10 per cent, of the ions were driven past the 

 first electrode without capture, but all ions were removed by 

 the two electrodes together. 



In these experiments the air-current was not introduced 

 for a definite time, as with the electroscope, but the current 

 was continued until a steady reading of the electrometer was 

 obtained. The maximum was generally reached after one or 

 two minutes. For this reason we must not expect the value 

 obtained with the electrometer to coincide with those found 

 by the electroscope method. When the air-current was 

 abruptly stopped the needle returned steadily towards its 

 new position, reaching it in one-half to one minute. After 

 that period less than one per cent, of the maximum value 

 remained. 



When I and II were connected with the electrometer and 



«^r 



* Rutherford's ' Radioactivity,' 2nd edition, p. 104. 



