344 Prof. J. J. Thomson on the 



electrometer. The object of connecting the funnel to the vessel 

 in which the drops are received is to eliminate the effect of 

 any electrification that may be produced by the flow of the 

 liquid through the funnel, or any electrical separation taking- 

 place when the drops fall from the spout of the funnel. It 

 was found, however, by connecting the funnel by itself to the 

 electrometer, that any effect of this kind was extremely small 

 compared with those observed when both the funnel and 

 receiver were connected with the electrometer. To prevent 

 the electricity which goes into the air from charging the 

 receiver, and thus neutralizing the opposite electrification 

 given to the receiver from the drops, different methods were 

 employed according as the drops fell through air, when the 

 experiments coald be made in the open, or through other 

 gases, when the apparatus had to be enclosed in an air-tight 

 vessel. In the first case the separation of the two electricities 

 was effected by blowing a current of air across the plate on 

 which the drops fell ; the electrification in the air was by this 

 means blown against a metal plate connected with the earth, 

 and so discharged. The current of air was produced by a fan 

 worked by a water-motor. This method could not be used 

 when the apparatus had to be closed up so as to allow of gases 

 other than air being used. In this case the electrification in 

 the air was discharged by placing over the plate a piece of 

 wire gauze connected with the earth : a hole was cut through 

 the gauze to allow of the drops falling through without touch- 

 ing the gauze ; the gauze was separated from the plate by a 

 distance only just great enough to allow the drops to fall 

 into the receiver after rebounding from the plate without 

 striking against the gauze. Plates of various kinds were 

 tried, but I found, as Lenard observes, that the material of 

 which the plate is made exerts no influence provided it is 

 thoroughly wetted by the liquid which falls upon it. Irregu- 

 larities occur if the plate is greasy, when the drops, instead 

 of spreading over the plate, roll off it, still retaining their 

 globular form. 



With either of these pieces of apparatus the effect dis- 

 covered by Lenard, that after striking against the plate drops of 

 distilled water had a positive charge, could easily be observed. 



The first variation of the experiment which I tried was to 

 make the drops fall through water-vapour instead of air. For 

 this experiment water was boiled for several hours in a vessel 

 like that represented in fig. 2 until the steam had completely 

 expelled the air : the water used for the drops had been well 

 boiled so as to thoroughly expel the air from it. In this case no 

 electrical separation whatever was observed, though when air 

 was blown into the vessel the normal effect at once reappeared. 



