Electricity at the Kew Observatory. 97 



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 investiga- 

 tion 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 Uni- 

 versity 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 water- 

 dropper 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 

 ^n 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. 



* ' Phil. Trans.' for 1868, p. 347. 

 t ' B. A. Keport,' vol. 51, p. 443. 



.J (July 28.) Some alterations have been effected since the above was written 



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