7 IS Dr. G. C. Simpson on 



Usually the water-supply was regulated so that it just 

 provided the flow through the nozzle when the glass tube 

 was full; if, however, the supply fell short of this amount, 

 the head of water sank in the tube until the flow from the 

 nozzle just carried away the supply ; if the supply was too 

 great, the vessel C gradually filled with water until the surplus 

 overflowed from the pipe Gr. The water which escaped here 

 carried away no charge as the drops detached themselves 

 within the cylinder H. 



The water which left the nozzle A fell into the insulated 

 funnel I and left this free of charge by dropping from the 

 end of the small pipe K well within the cylinder L. Thus 

 all the electricity which the drops from the nozzle carried 

 into the funnel I remained in the funnel and its connexions. 

 The capacity of this funnel was made as small as possible in 

 order to obtain a large deflexion of the electrometer to which 

 it was connected. 



As the drops of water from the nozzle entered the re- 

 ceiver I with considerable velocity, it was deemed necessary 

 to investigate whether any error was produced by the "Lenard 

 effect." A careful investigation proved that no error of this 

 kind was present, and this agreed with previous experiments, 

 which had shown that, owing to some impurity in the Simla 

 tap-water, it is incapable of developing the Lenard effect. 

 If this work were repeated elsewhere it would be necessary 

 to test for this source of error, and if found it would no 

 doubt be possible to remove it by adding a little salt to the 

 water. 



The electrometer used was one of the pattern designed by 

 Benndorf for automatically recording the potential gradient. 

 In this instrument an electrical circuit containing a magnet 

 is closed every two minutes, and a light boom attached to the 

 needle of the electrometer is thereby pressed into contact 

 with the paper taking the record. Thus the deflexion of the 

 freely swinging electrometer-needle is automatically recorded 

 at the end of each two minutes. A slight change in the 

 construction of the electrometer made it possible to auto- 

 matically earth the receiver as soon as its potential had been 

 recorded. Each deflexion, therefore, was proportional to 

 the charge obtained by the receiver in the previous two 

 minutes. 



The potential gradient was recorded by a second Benndorf 

 electrometer, and the recording mechanisms of both electro- 

 meters were actuated by the same electrical current. Thus 

 simultaneous records were made on the two instruments. 



It was desirable that the exposed plate, should be placed 



