EAIN AND ITS OEIGIN IN THUNDERSTORMS. 397 



on the air jet. The box was then connected to earth and a reading taken of the zero 

 of the electrometer ; after a convenient interval the earth connection was broken and 

 the drops counted as they fell. Readings were taken of the electrometer after each 

 100 drops had fallen until 500 had been counted, when the flow of water was 

 stopped. The following shows the result of a typical experiment in which distilled 

 water was used : 



TABLE VIII. 



Before going any further it will be as well to consider how much of the charge, 

 thus measured, could reasonably be ascribed to the breaking up of the drops on the 

 jet of air. 



Three possible sources of error have to be considered : 



(a) That the charging might be due to the blast of air alone. 



(b) That the drops might be electrified before being broken up. 



(c) That the effect might be due to the Lenard effect coming into play when the 



drops fell into the tray after being broken up on the air jet ; or in other words, 

 that the separation of electricity might not take place when the drops broke 

 on the jet, but when they splashed on the water in the bottom of the tray. 



(a) That the blast caused no charging could easily be tested by keeping it in 

 action without the drops falling. Frequent experiments were made and showed no 

 trace of charging. 



(6) The electrification of the drops was tested by placing a small cylindrical box 

 over the air nozzle (as shown by the dotted outline in the figure) to catch the drops 

 as they fell. A small hole was made in the side of this box about 2 cm. from the 

 bottom, so that a layer of water 2 cm. deep was always in the bottom, and into this 

 the drops fell. The Lenard effect would come into play when the drops splashed on 

 the water in the box, but as all the electricity separated remained within the tray 

 and its lid, this effect could cause no charging of the system. If, however, the drops 

 had fallen with a charge on them it would have been detected by a deflection of the 

 electrometer. No such effect could be observed. 



(c) In order to prevent the Lenard effect from coming into play when the drops 

 fell into the tray after breaking up on the air jet, use was made of the fact discovered 

 by LENARD that the splashing of salt water produces the opposite charge to that 



