Page:Proceedings of the Royal Society of London Vol 60.djvu/112

Rh cussed in 1868 by Professor Everett,* and the results obtained in 1880 were discussed in 1881 by my predecessor, Mr. Whipple.f Nearly two years ago, with the approval of the Kew Observatory Committee and the Meteorological Office, I commenced an investigation intended as preliminary to a consideration of the expediency of further publication of the electrograph records.

My first object was to find out whether definite quantitative measurements of potential could be derived from the electrograph curves. To aid in this investigation observations have been made at several spots near the Observatory with a portable electrometer, by White, of Glasgow, whose scale value was determined at University College by the kind assistance of Professor Carey Foster.

To render intelligible the bearing of these observations on the question, a brief description is required of the nature and position of the electrograph. J It consists essentially of a water-dropper and a quadrant electrometer. The water is held in a can, some 14 inches high and 15 inches in diameter, supported on three insulators of the Mascart pattern. From the can a tapering tube, resting on a fourth insulator, projects through a hole in a window facing the west. The end of the tube whence the water issues is 4^ feet from the west wall of the Observatory, and 10 feet above the ground. The stream of water is regulated by two taps in the long tube. From the waterdropper an insulated wire passes to the needle of the quadrant electrometer. One pair of quadrants are kept at a given positive potential, the other pair at an equal negative potential, by means of a battery of 60 cells in series whose centre is to earth. The needle suspension carries a mirror, and light reflected from it produces a curve on photographic paper which is wound round a cylinder driven by clock-work. The position of the base line answering to the earth’s potential—treated as zero—is obtained by putting the electrometer needle to earth, twice at least for each curve. Of late years the value of the curve ordinates, in volts, has been obtained from time to time by connecting the electrometer needle and one terminal of the portable electrometer, and varying their joint potential by means of an electrophorus. Simultaneous readings are taken of the curve ordinate and the portable electrometer.

If the ideal were attainable, the stream from the water-dropper should break up exactly at the end of the tube, and be always sufficiently copious to ensure the immediate picking up by the can and the electrometer needle of the potential existing in the air at the spot in question.

+ ‘ B. A. Report,’ vol. 51, p. 443. (July 28.) Some alterations have been effected since the above was written
 * ‘ Phil. Trana.’ for 1868, p. 347.