422 
MR. HARRIS ON THE ELEMENTARY LAWS OF ELECTRICITY. 
y. The needle m n, with its index, &c., is constructed in the following way : two light 
arms of glass, m n, ri m, figs. 3 and 14, are inserted into the opposite ends of a small 
connecting joint, rri ri, about two inches or more in length, and ”2 of an inch in dia- 
meter, the whole forming one straight line. The connecting joint rri ri consists of 
two pieces of light brass tube attached to opposite points of a small solid brass cylin- 
der by short projecting studs : the central portion of the cylinder receives the ex- 
tremity of the vertical rod o u. There are two sliders of brass on the connecting 
cylindrical tubes, which travel on them with a steady friction, and carry the hooks b V 
for the attachment of the suspension silks, which may be thus placed at any distance 
apart. 
c>. A small circular area, n, of gold plate or light wood gilded, about "4 of an inch 
in diameter and 05 of an inch thick, with smoothed edges, is fixed to one extremity 
of the needle ; and a small coated circle, m, of very thin varnished glass, or otherwise 
a similar disc, to the other. The coatings of the glass circle consist of circular pieces 
of gilt paper of about '4 of an inch in diameter. This little element, when charged 
in the usual way with either electricity, maintains a very steady electrical state, not 
being liable to the variations which frequently arise in a simple insulated conductor ; 
a property of great consequence in refined experiments, since in examining, from time 
to time, an insulated and similarly charged disc, p, figs. 3 and 14, or q and q’, figs. 7 
and 8, introduced within the cage, we thus obtain a more accurate measure of its 
electrical conditions than is usually arrived at by the electrization of a simple con- 
ductor. The small circular discs just mentioned are attached to the extremities of 
the needle m n by a cement of shell lac, a portion of the glass rod near the end being 
ground away or drawn out in the flame of a lamp for a short distance so as to receive 
them ; and thus the plane of the disc coincides with the axis of the rod. When the 
disc is of gilded wood, a small hole is drilled into its edge, or a corresponding flattened 
groove cut out of it for about two thirds of the radius, by which a similar result is 
obtained. The glass arms of the needle sustaining the discs are each covered with 
a thin coating of shell lac or good sealing-wax, the glass being previously cleaned, 
and heated sufficiently to melt the wax. 
s. The vertical brass rod o u, carrying the index v v, is about inches long, 
and - 25 of an inch in diameter ; its upper end terminates in a short shoulder ; this 
shoulder passes through the centre of the connecting joint rri ri, and has a screw 
cut on it: there is a nut fitted to this screw, by which the two become completely 
united, and accurately set at right angles to each other. 
The needle m n, with its index, See., is suspended from the light frame a x a!, fig. 3 : 
the suspension silks are connected by loops to the hooks b b', and after passing 
through fine holes in the moveable plate rr, and other holes in the fixed bar a a 
above it, are secured round the moveable pins a a! for the convenience of adjust- 
ment. 
ri. The index v ri is fixed horizontally to the rod o u, exactly at right angles to the 
