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 observations the fractions of the true potential picked up by the two instruments stood to one another in a fairly constant ratio. The presumption, 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 an appreciable diurnal variation.
§ 12. On the discovery of these defects it became not only justifiable 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 watei’-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 (involving 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 outside 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 electrofneter to test its scale value from time to tim e by comparison w ith an absolute electrometer or a large battery of constant cells. I t 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.
