

Electricity at the Kew Observatory. 99 



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 bhe 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.j 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. 



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, 3 j 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, 6f feet high, and only 3 feet from the west 

 wall of the Observatory. C is the centre of a flat plank supported 

 3J 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 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-g- 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; ii is 

 57 ft. et 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, 1 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 unin- 

 fluenced by the neighbourhood of a building, and even in its case we 



* ' Wien. Sitz.,' vol. 98, 1889. 



f ' Wien. Sitz.,' vol. 101, p. 703, 1892. 



