DE. EVEEETT ON ATMOSPHERIC ELECTRICITY. 
353 
its marked position. The square root of the number of degrees of torsion required to 
effect this measures the potential of the Leyden charge. This result is called the 
reduced earth-reading. When the atmosphere inside the jar is kept sufficiently dry, this 
charge is retained from day to day with little loss, not more, often, than 1 per cent, in 
twenty-four hours. 
In using the instrument the charging electrode (9) of the jar is left untouched, with 
the aperture through which it projects closed over it by the metal cap referred to above. 
The electrode (10) of the cage, when an observation is to be made, is connected with 
the conductor to be tested, and the needle is brought by torsion to its marked position.. 
The square root of the number of degrees of torsion now required measures the differ- 
ence of potentials between the conductor tested and the interior coating of the Leyden 
jar. The excess, positive or negative, of this result above the reduced air-reading, 
measures the excess of the potential, positive or negative, of the conductor tested above 
that of the earth ; or simply the potential of the conductor tested, if we regard that of 
the earth as zero. 
The mode of employing this instrument at Kew was to keep its Leyden phial (1) 
always connected with the Leyden phial of the self-recording electrometer by means of 
a wire protected by an air-tight tube, and to keep the cage-electrode (10) always con- 
nected with the earth. 
Leadings of the gauge-electrometer were taken daily at about 10 h 30 m a.m. and entered 
in a book. Whenever the charge of the Leyden jars as thus tested was found to have 
fallen too low, a fresh charge was given. The earth-readings of the gauge-electrometer 
were thus always kept between 245° as a lower, and 255° as an upper limit, or, to 
speak more strictly, in the few instances in which these limits were exceeded, the obser- 
vations were rejected in the reductions. These readings, however, require to be cor- 
rected by subtracting the index-error, which was carefully ascertained from time to time, 
and never varied much from 230°. The corrected readings were therefore contained 
between the limits 15° and 25°; and as the square roots of these numbers are as 1:1-3, 
the weakest and strongest charges must have differed by about 30 per cent. This dif- 
ference, however, only affects the comparison of one day with another. The loss of 
charge in twenty-four hours was from 1 to 3 per cent., and it is this loss only which 
affects the diurnal curve. As the fresh charge was always given at about 10 h 30 m a.m., 
the disturbing effect is to be looked for in a sudden rise of the curve about this time, a 
consideration to which we shall hereafter recur. 
Since the erection of the Kew instruments the divided-ring electrometer has under- 
gone considerable modification at the hands of its inventor. The flat ring (fig. 4) divided 
into two segments is changed for a hollow box (figs. 5, 6) divided into four segments, of 
which one opposite pair are connected with earth, and the other pair with the conductor 
to be tested. This box encloses the needle, which is represented by. the dotted outline 
in fig. 5, and projects symmetrically on both sides of the suspending fibre, thus obviating 
the necessity for a counterpoise. 
