1860.] on Atmospheric Electricity. 279 



(9) A very light glass mirror, about three-quarters of an inch 

 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 influ- 

 ences, 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 (gene- 

 rally brass). Its outer diameter is about 4 inches, its inner diameter 2i ; 

 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 yL to -^ 

 of an inch broad, vacant between them. They are placed with their 

 cx)mmon 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 coin- 

 cide 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 projecting 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 potential 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 instru- 

 ment 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 

 constantly in communication with the charging electrode (8). 



The electrometer (II.) is to be ordinarily used for that purpose 

 in the Kew apparatus. Failing any such gauge 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 this electrometer, the two bodies of which the difference 

 of potentials is to be tested are connected, one of them, which is gene- 



u2 



