CONSTRUCTION OF A NEW ELECTRICAL BALANCE. 
423 
direction of the needle m n. It consists of pieces of light reeds of straw inserted one 
on each side in thin spring tubes ; these are fixed on the ends of a short wire of brass 
about 2 inches in length, accurately passed through the rod o u, about inches 
below the needle, so as to project from it for about \ an inch or more. A perfectly 
straight line may be obtained in this way by constructing each arm of two or more 
pieces of straw reed, fitted at their extremities one within the other, so as to present 
a sort of tapering appearance, the last piece being cut to a point, after the manner 
of a common writing-pen. Immediately over this index there is a sort of flattened 
stage of sheet brass, fitted to the rod by a spring tube, for receiving small circular 
weights, by which the reactive force imparted to the threads may be occasionally 
increased (6.). The weight of the needle, with its index, &c., is about 480 grains. 
The lower extremity of the index-rod at u has a conical hole drilled up it, which 
allows the whole to play freely about a central pivot fixed in the end of a cylindrical 
rod u z. This rod passes through a spring socket immediately in the centre of the 
base of the cage, so as to be easily elevated or depressed by means of a milled head 
beneath, in which it terminates. 
S-. A little on one side of the central point 2 , but very near it, two other brass rods, 
e e', pass through the base in a similar way to the former ; one of these, e, carries a 
bent rod y w, fixed on it by a small spring socket, and terminating in a sort of 
fork, w, about o of an inch in width. One of the arms of the index v v’ may, by 
elevating or depressing the rod, be caused to pass through the fork, and thus the 
oscillations of the needle become completely checked in any point, or confined within 
a very limited number of degrees ; the forked portions of the lever w are covered with 
some soft substance, so as to prevent, as much as possible, the index from rebounding. 
In this way by turning the rod e from beneath the base, the repulsive force acting on 
the needle is completely restrained, and gradually eased off to the point at which it 
becomes balanced by the reactive force of the threads. The opposite rod e' also carries 
a bent rod e'w', by which we may at any time act on the other arm of the index, and 
thus obtain a very complete command over the oscillations of the needle. These two 
bent levers w w' may be changed to either side, or one of them only may be employed, 
according as it suits the convenience of the experimentalist. 
i. Two other rods, v n, fig. 3, pass with friction through the base of the cage, in a line 
perpendicular to that of the rods e' e ; these carry two small stages temporarily fixed 
on the extremities of the rods by spring sockets, to which they are attached. The 
milled heads of these rods are seen projecting under the base. The stages on ele- 
vating the rods support and bear up the needle m n, at the time of suspending it, or 
when the instrument is not in use, or otherwise when it is required to be moved about 
from one place to another. When the rods are elevated the connecting joint w! n' of 
the needle is received between the perpendicular sides of the stages, as at pp, fig. 11 ; 
and thus it is at once supported and protected from damage, and its weight taken off 
the threads of suspension. The stages being temporarily fixed on the rods may be 
