424 
MR. HARRIS ON THE ELEMENTARY LAWS OF ELECTRICITY. 
removed, when required, at the time of using- the instrument, and the ends of the 
rods depressed below the level of the base. 
y.. The upper part or roof d' l of the cage d' d is, as already stated (a.), a wood frame, 
inclosing a circular opening of about a foot in diameter ; this opening is just covered 
by the circle A t I, which is about 13 inches in diameter, and *3 of an inch thick. 
The circumference of this circle is about an inch wide, and is divided upon its upper 
surface in each quadrant from 0 to 90°. It has two cross pieces or arms, 1 £, A I, 
figs. 3 and 14, at right angles to each other; through the intersection of these there 
is a large round hole, M, fig. 14, for receiving the hollow vertical pivot, P, figs. 3 and 
15, by which the circle may be accurately turned about its centre. The openings 
between the cross anus are covered in by thin glass plates, secured to the brass 
underneath by the heads of small screws. These screws project over the edges of 
the plates, the glass being defended by a thin leather collar. The under surface of 
the circle A 1 1 is ground flat, and is smeared with a little grease of some kind, where 
it bears on the framework beneath, in order to allow of motion about the central 
pivot with an easy friction : the angular quantity by which this circle is at any time 
turned, is shown by a small bent index, I, outside the circumference, fixed either at 
1 or A*. 
X. One of the cross bars A I carries the insulated disc p, or an insulated coated 
disc of glass g, fig'. 5. These discs are similar to those of the needle already de- 
scribed. Each disc is insulated on a slender varnished glass rod, moveable in a 
spring socket, fixed in a plug of wood, as represented in fig. 5, so as to be readily ad- 
justed to a given length. The plugs A or I, figs. 3 and 14, fit accurately in corre- 
sponding holes passing through the arms of the brass circle ; they are placed in two 
similar and opposite points of the diameter A I, their centres being 10 inches apart, 
or a distance just equal to the length of the needle. By this arrangement either of 
the discs, or any other body, may be introduced within the cage of the instrument at 
pleasure, and the action on the needle observed. 
tjj. There is a vertical index-rod, £ y, figs. 3 and 14, in the diameter t £ 90° distant 
from the holes A I, pointing immediately over zero of the graduated circle v d when 
the needle is in its position of rest, and the two discs n p just touch. This index 
is sustained in a spring tube fixed in a neat wood plug, h, fig. 6, so as to be readily 
adjusted to any length, and can be applied at the opposite side t, if required. 
v. Beside the discs n, p, m, above mentioned, we may, by raising one of the glass 
faces, occasionally place within the cage any other electrified body whose condition we 
wish to examine, or otherwise small insulated proof planes, such as q q', figs. 7 and 8. 
These are insulated on slender rods of varnished glass, q h, lif, fig. 7, set by an in- 
tervening ball of wood at right angles to each other. The horizontal rods vary from 
2 to 5 inches in length ; the vertical insulators are from 4 to 5 inches in length ; 
* The circle A 1 1 may he of wood, having a graduated circle of card-board fixed on it ; the remaining gra- 
duated circles may be also of stout card-board, where economy is an object. 
