352 
PE. EYEEETT ON ATMOSPHERIC ELECTRICITY. 
(5) A light aluminium needle (LL, figs. 1, 2, Plate XVIII.) attached across the lower 
end of the fibre (which is somewhat above the centre of the glass hell), and a stiff plati- 
num wire (F, figs. 1, 2, Plate XVIII.) attached to it at right angles and hanging down to 
near the bottom of the jar. 
(6) A very light platinum wire (G, fig. 1, Plate XVIII.), long enough to hang within 
one-eighth of an inch or so of the bottom of the jar and to dip into the sulphuric acid (H). 
(7) A metal ring attached to the inner coating of the jar, bearing two plates (M, M, 
figs. 1, 2, 3, Plate XVIII.) in proper positions for reflecting the two ends of the alumi- 
nium needle when similarly electrified, and proper stops (as O, fig. 3, Plate XVIII.) to 
limit the angular motion of the needle to within about 45° from these plates. 
(8) A cage (P P, figs. 1, 2, Plate XVIII.) of fine brass wire stretched on brass frame- 
work, supported from the main case above by two glass pillars (QQ) and partially 
inclosing the two ends of the needle and the repelling plates, from all of which it is 
separated by clear spaces of nowhere less than one-fourth of an inch of air. 
(9) A charging electrode (J, fig. 1, Plate XVIII.) attached to the ring (7) and pro- 
jecting over the mouth of the jar to the outside of the metal case (2), through a wide 
aperture, which is commonly kept closed by a metal cap (K), leaving at least one-quarter 
of an inch of air round the projecting end of the electrode. 
(10) An electrode (ST, figs. 1, 2, Plate XVIII.) attached to the cage (PP) and pro- 
jecting over the mouth of the jar to the outside of the metal case (2) through the centre 
of an aperture. In order to dry any air which may enter through this aperture, a 
hollow cylinder of pumice soaked in sulphuric acid is inserted in the leaden receptacle 
(U, fig. 1, Plate XVIII.), through the centre of which the wire (T) passes, and the leaden 
cover (V V) is pierced with a hole large enough for the wire to pass through without 
contact. This cover has a depression (W) to receive the droppings of acidulated water 
from the pumice. 
This instrument is adapted to measure differences of potential between two conducting 
systems, namely, as one, the aluminium needle (5), the repelling plates (7), and the inner 
coating of the jar, and, as the other, the insulated cage (8). This latter is commonly 
connected, by means of its projecting electrode (10), with the conductor to be tested. 
The two conducting systems, if connected through their projecting electrodes by a me- 
tallic wire, may be electrified to any degree, without causing the slightest sensible mo- 
tion in the needle. If, on the other hand, the two electrodes of these two systems are 
connected with two conductors, electrified to different potentials, the needle moves 
away from the repelling plates ; and if, by turning the torsion-head, it is brought back 
to one accurately marked position, the number of degrees of torsion required is propor- 
tional to the square of the difference of potentials thus tested. 
In the ordinary use of the instrument, the inner coating of the Leyden jar is charged 
negatively, by an external application of electricity through its projecting electrode (9). 
The degree of the charge thus communicated is determined by putting the cage in con- 
nexion with the earth through its electrode (10), and bringing the needle by torsion to 
