716 Dr. Gr. C. Simpson on 



earth connected. The problem therefore consists in rinding 

 a method for keeping an insulated plate placed on the ground 

 at zero potential, and for measuring the change of charge 

 which this entails. 



The solution ohtained was the following : — Water was 

 made to flow from an insulated vessel in fine jets, and the 

 points at which the jets broke into drops were surrounded 

 by an earth-connected cylinder. The vessel was connected 

 by a wire to a large plate placed in the open as near to the 

 ground as was consistent with efficient insulation. By the 

 well-known " collector " action the jets prevented the potential 

 of the insulated system from varying appreciably from zero, 

 and the drops as they formed carried with them the charge 

 necessary to effect this. In other words, the drops carried 

 away all the charge set free on the insulated plate which 

 remained at zero potential. The drops as they fell were 

 caught in an insulated vessel connected to an electrometer, 

 and the change in the deflexion of the electrometer recorded 

 the charge carried away by the drops. 



The details of the apparatus as used in Simla are shown 

 diagrammatically in fig. 1. The nozzle from which the 

 water issued is shown at A, surrounded by the earth- 

 connected cylinder B. In order to get a large discharging 

 capacity with a small expense of water, it was necessary to 

 have the water issuing from the nozzle in fine jets which 

 broke up into very small drops. For this reason, the nozzle 

 was attached to the lower end of a glass tube 150 cms. long 

 which was usually kept full of water, so that the jets issued 

 under a considerable pressure. There were three jets each 

 having a diameter of about *2 mm. To the top of the glass 

 tube a vessel C was attached into which a regulated supply 

 of water dropped from the funnel D. The point at which 

 the water detached itself from the funnel D was well within 

 the surrounding cylinder E, so that the drops were entirely 

 free from charge when they fell into the vessel C. 



At first considerable trouble was experienced on account 

 of the fine holes in the nozzle becoming choked up. To 

 prevent this filter-papers were introduced into the vessel C 

 and also just above the nozzle itself ; but it was found that 

 owing to some form of impurity in the water, the papers 

 became impervious to water after being in use for a day or 

 so. After trying several methods of filtering the water 

 without success, the following satisfactory method was hit 

 upon. A piece of sponge was placed in the funnel D to 

 remove all large impurities, and then the water was caused 

 to pass through the trap F in the vessel C before it reached 



