Electricity, at the Kew Observatory. 109 



and the same spot was attempted during the observations, as it 

 seemed undesirable to interrupt the continuity of the electrograph 

 records. All that 10 shows is that during any one series of observa- 

 tions the fractions of the true potential picked up by the two instru- 

 ments stood to one another in a fairly constant ratio. The presump- 

 tion, certainly, is that neither fraction altered much throughout the 

 few weeks covered by any one of the four series of observations. 



It is, however, I regret to say, perfectly certain, from the data on 

 which 10 is based, that one at least of the two instruments varied 

 very considerably in the course of a year and possessed a,n appre- 

 ciable diurnal variation. 



12. On the discovery of these defects it became not only justifi- 

 able but necessary to subject the water-dropper itself to direct 

 experiments. These have led to my proposing certain alterations 

 which are now in process of execution. They aim at bringing the 

 water-can and electrometer close together, and at maintaining a 

 more uniform water pressure than heretofore. 



It appears also desirable to check the working of the apparatus in 

 some way jinvolving the arrival at exact numerical results. The 

 following operations A, B, C will, it is hoped, prove sufficient. The 

 operation C need not be performed so frequently as A or B. 



A. Charge the quadrant electrometer needle to a high potential, 

 and observe the rate of leakage over a fixed range by timing the 

 motion of the spot of light across a scale with 



(1) the wire connexion to the water-can complete, but the jet not 

 flowing ; 



(2) the wire connexion broken at the can ; 



(3) the wire connexion broken at the electrometer. 



B. As a substitute, or as subsidiary to A. Connect a portable 

 electrometer to the water-can, and, with the jet flowing, observe the 

 potential recorded by the portable, when 



(1) the can is connected as usual to the quadrant electrometer; 



(2) the connexion is broken at the quadrant electrometer ; 



(3) the connexion is broken at the can. 



C. Take a sufficient number of observations at a fixed station out- 

 side with a portable electrometer, at or near two fixed hours a day, 

 so chosen that at one hour the can is almost full, whilst at the other 

 it is at least half empty. 



The use to be made of the results is obvious. 



I should also recommend any one using a portable electrometer to 

 test its scale value from time to time by comparison with an absolute 

 electrometer or a large battery of constant cells. It is well to lay in 

 a new stock of fuses before exhausting one's supply, and to compare 

 the old and new fuses by taking observations in rapid succession with 

 samples of the two at a fixed station. 



