350 
DE. EVERETT ON ATMOSPHERIC ELECTRICITY. 
(9) A very light glass mirror, about three-quarters of an inch in diameter, attached 
by its back to the wire (4), and therefore rigidly connected with the aluminium needle. 
(10) A circular aperture in the case, shut by a convex lens, and a long horizontal slit, 
shut by plate glass, with its centre immediately above or below that of the lens, one of 
them above, and the other equally below the level of the centre of the mirror. 
(11) A large aperture in the wide metal tube (7), on a level with the mirror (9), to 
allow light from a lamp outside the case, entering through the lens, to fall upon the 
mirror, and be reflected out through the plate-glass window ; and three or four fine 
metal wires stretched across this aperture to screen the mirror from irregular electric 
influences, without sensibly diminishing the amount of light falling on and reflected 
off it. 
The divided ring (1) is cut out of thick strong sheet metal (generally brass). Its outer 
diameter is about 4 inches, its inner diameter 2 \ ; and it is divided into two equal parts 
by cutting it along a diameter with a saw. The two halves are fixed horizontally ; one 
of them on a firm metal support, and the other on glass, so as to retain as nearly as 
may be their original relative position, with just the saw cut, from rg- to of an inch 
broad, vacant between them. They are placed with their common centre as nearly as 
may be in the axis of the case (5), which is cylindrical, and placed vertically. The 
Leyden jar (3) and the tube (7), carried by its inside coating, have their common axis 
fixed to coincide as nearly as may be with that of the case and divided ring. The glass 
fibre hangs down from above in the direction of this axis, and supports the needle about 
an inch above the level of the divided ring. The stiff wire (4) attached to the needle 
hangs down as nearly as may be along the axis of the tube (7). 
Before using the instrument, the Leyden phial (3) is charged by means of its pro- 
jecting electrode (8). When an electrical machine is not available, this is very easily 
done by the aid of a stick of vulcanite, rubbed by a piece of chamois leather. The po- 
tential of the charge thus communicated to the phial, is to be kept as nearly constant 
as is required for the accuracy of the investigation for which the instrument is used. 
Two or three rubs of the stick of vulcanite once a day, or twice a day, are sufficient 
when the phial is of good glass, well kept dry. The most convenient test for the charge 
of the phial is a proper electrometer or electroscope, of any convenient kind, kept con- 
stantly in communication with the charging electrode (8). The gauge-electrometer 
described below was used for that purpose at Kew. Failing any such electrometer or 
electroscope, a zinc-copper-water battery of ten, twenty, or more small cells, may be very 
conveniently used to test directly the sensibility of the reflecting electrometer, which is 
to be brought to its proper degree by charging its Leyden phial as much as is required. 
In the use of the divided-ring electrometer, the two bodies of which the difference of 
potentials is to be tested, are connected one of them with the metal case of the instru- 
ment, and the other with the insulated half-ring. In the Kew observations of atmo- 
spheric electricity these two bodies were the earth and the water-dropping collector. 
The needle being, let us suppose, negatively electrified, will move towards or from the 
