134 DE. C. CHREE: ATMOSPHERIC ELECTRICITY POTENTIAL GRADIENT 



Another reason requires fuller explanation. The Kew water-dropper the earliest 

 it is believed in regular operation -was erected in 1861 under Lord KELVIN'S personal 

 supervision. The original electrometer and batteries as they decayed were replaced 

 by others, but the instrument remained essentially unchanged in its original site until 

 1896. Of the records obtained prior to that date those of only three years had been 

 discussed, two years, 1862 to 1864, by Prof. J. D. EVERETT,* and one year, 1880, by 

 Mr. G. M WHIFFLE.! In both cases the results were expressed in what were really 

 arbitrary units. The relation between the voltage shown by the instrument and the 

 true potential gradient in the open was altogether unknown. 



In 1896, after a few years' experience, I recognised the expediency of avoiding a 

 variation in the water pressure which tended to affect the apparent diurnal variation, 

 and of eradicating a shrubbery in the immediate vicinity of the water jet, which 

 presumably influenced the annual variation to a small extent. 



2. The importance was also realised of securing that the curve measurements should 

 have a definite meaning. The discharge tube, from freezing of the water or natural 

 decay, required occasional repairs, and had at intervals to be replaced, so that sensible 

 variation in the position of the jet was at least a possibility. To provide for this, regular 

 observations were introduced with a Kelvin portable electrometer in the Observatory 

 garden, at some distance from the building. At first the electrometer was simply 

 placed on the top of a convenient stone pillar. Presently it was realised that the stone 

 pillar, whose diameter is considerable, largely reduced the potential in its immediate 

 neighbourhood, so that the electrometer had to be placed exactly on the same spot if 

 the results obtained on different days were to be comparable. Accordingly a special 

 stand was designed. A rod sliding inside a vertical tube, fixed in the ground, carried 

 at its upper end a small platform. This bore three short vertical pieces at equal 

 intervals round its perimeter, forming a stand just large enough to take the portable 

 electrometer. The sliding rod was designed to admit of observations being taken at 

 two different heights. It was supposed that the reduction of the field due to the 

 presence of the stand, electrometer and observer would be roughly the same at the 

 two heights, so that a fair approach to the true potential gradient in the open would 

 be obtained from the difference of the potentials observed. To reduce the disturbing 

 effect of the observer's presence, a device was introduced allowing manipulation of the 

 electrometer from a greater distance than previously. In practice, however, this 

 device proved troublesome to work. Also the difference J metre in the two levels 

 at which the potential was observed proved too small, in view of the variability of the 

 potential and the insensitiveness of the electrometer. Some experiments were supposed 

 to show that the disturbing effect due to the apparatus and observer was less than 

 I had anticipated. The outcome was that the special device was laid aside, and 

 observations were confined to one fixed height, about 1'465 metres above ground level. 



* ' Phil. Trans.,' vol. 158, 1868, p. 347. 



t ' British Association Report for 1881,' p. 443. 



