another in a practically constant ratio, a function only of their geometrical coordinates.
If once this ratio were determined, one could deduce the potential at either point from that observed at the other. Regarding the spot where the water jet breaks up as one of these points, and selecting for the other a spot sufficiently distant from the building, one could deduce the potential gradient in the open, i.e., the increase in voltage per unit of height above the ground. This point of view was apparently acted upon by Exner,* and by Elster and Geitel.f In both instances the existence of corroborative evidence is referred to, but I am not aware that particulars have been published. It would also appear that Exner and Elster and Geitel directed their attention mainly, if not exclusively, to clear quiet days.
There being no limitation to the use of the Kew electrograph, it appeared advisable not to restrict the investigations to days of a special kind, or to a particular season of the year. Electricity at the Kew Observatory.
Selection of Stations.
§ 4. It appeared desirable to compare the potential at more than two stations, so as to ensure a sufficient variety in the surroundings. I shall distinguish the stations selected by the letters A, B, C, D, E, F. Of these A is the flat top of a stone pillar, 3f feet high, in the Observatory garden, about 56 yards from the Observatory; it is surrounded by a frequently mown grass lawn. B is the top of a temporary wooden stand, 6 | feet high, and only 3^ feet from the west wall of the Observatory. C is the centre of a flat plank supported 3f feet above the ridge of a wooden building, situated about 100 feet to the south-west of the Observatory ; it is 18 feet above the ground. D is on the south side of a stone parapet, 2-g- feet high, encircling the flat roof of the Observatory; it is 37 feet from the ground. E is the top of a camera stand, 5£ feet above the Observatory roof, and 17 feet to the east of the central dome. F is the top of a stand on the roof— used for testing anemometers—level with the cups of the standard anemometer, from which it is distant about 17 feet to the north; it is 57 feet above the ground.
The observations were taken with the portable electrometer, and, as the burning end of the fuse was at a height of some 12 to 16 inches above the base of the electrometer, an addition of, say, l j feet requires to be made to the altitudes of the several stations to get the height from the ground of the spot whose potential was measured.
A was the only station that could be regarded as practically uninfluenced by the neighbourhood of a building, and even in its case we
- ‘ Wien. Sitz.,’ vol. 98, 1889.
t ‘ Wien. Sitz.,’ vol. 101, p. 703, 1892.
