1912-13.] Atmospheric Electric Potential Results. 
317 
XXVIII. — Atmospheric Electric Potential Results at Edinburgh 
during 1912. By G. A. Carse, M.A., D.Sc., and G. Shearer, M.A., B.Sc. 
(MS. received June 2, 1913. Read November 3, 1913.) 
§ 1. In this paper are given the results of calculations made from con- 
tinuous records of atmospheric electric potential obtained during the year 
1912 at the Physical Laboratory of the University of Edinburgh. 
During the summer of 1909 intermittent observations of the atmospheric 
electric potential and earth-air current were made at various points in 
Edinburgh, the results of which have already been communicated to this 
Society,* and since October 1911 there has been an electrograph in operation 
at the laboratory. 
The apparatus used for recording the electric potential of the atmosphere 
consisted of a water-dropper of the well-known Kelvin type, which was 
connected to an electrometer of the Dolezalek pattern. As is usual in 
work of this kind, the water-dropper was connected to the needle of the 
electrometer, the quadrants being kept at a constant difference of potential 
by means of cells, the potential of the cells being so adjusted that the 
instrument could record potential gradients from +400 v/m to —200 vjm. 
The tank containing the water was supported on sulphur blocks, the 
insulating power of which was tested from time to time and was found 
satisfactory. 
A daily record was made by the trace produced by a beam of light which 
was reflected from the mirror of the electrometer needle on to a drum 
carrying bromide paper and revolving at such a rate that one hour was 
represented on the record by L34 cms. approximately. A clock was 
installed which earthed the tank for five minutes every hour, the clock 
being adjusted so that it unearthed exactly at the hour. The object of this 
was to obtain a series of points which gave the reading of the electrometer 
corresponding to zero potential on the tank, and it also had the additional 
advantage of dividing the record into hour intervals, thus saving a con- 
siderable amount of time in the measurement of the curves. The position 
of these points showed at a glance whether or not the electrometer was 
maintaining a constant zero. 
At first a little difficulty was experienced in obtaining the zero points in 
* Carse and MacOwan, Proc. Boy. Soc. Edin ., vol. xxx. pt. vi. p. 460. 
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