M. Ampere m hk Electro-Magnetic researcJtis. 40^ 
Fig. 1. It is inclosed in a glass cage, and rests upon a pedes- 
tal m n. Without this cage are arranged four cups of box- 
wood, R, S, T, U, for holding the mercury, in which are plunged 
the brass wires which traverse the cage, and which are soldered 
to four supports M, N, P, Q, of which M, N, carry the fixed 
conductor AB, which may be placed nearer to or further from 
the other, by shifting the supports in the grooves I, J, where 
they can be fixed by screws under the pedestal m n. The other 
supports P, Q are terminated by steel cups X, Y, large enough 
to hold globules of mercury which are placed in them, and in 
which are plunged two steel points attached to copper boxes 
E, F, which receive the two extremities of a glass tube OZ, 
having at its middle another copper box, to which is soldered a 
copper tube V, in which the rod of the counter-weight H slides : 
This rod is bent, as shewn in the figure, in order to shift the 
centre of gravity of the moveable part of the apparatus, by turn- 
ing the bent rod about its own axis in the tube V. The sup- 
ports P, Q maybe placed at different distances, by sliding them 
in the groove K, L, and may be fixed by screws underneath. 
To E and F is soldered the brass wire ECDF, the part of 
which CD, immediately above AB, and parallel to it «, is called 
the moveable conductor. 
In using this apparatus, the conductors AB, CD are placed at 
the required distance, by putting the steel points at E and F 
into the cups X, Y, and adjusting the counter- weight H. In 
order to shew the attraction (^two currents when they take place 
in the same direction, a communication is then established be- 
tween the two opposite extremities of the conductors AB, CD, 
by the bent brass-wires immersed into the box-wood cups, such 
as R and U, or S and T, which again communicate with the two 
extremities of the pile by other two brass-wires. In order to 
shew the repulsion of the currents, it is necessary that the first 
brass-wire establish the communication between two cups, R 
and S, or T and U, corresponding to the extremities of the two 
conductors situated on the same side, while a communication is 
made between the extremities of the pile and the two cups 
placed on the opposite side. This arrangement of the box-wood 
cups occurs in most of the other apparatus which is to be de 
* I'he letters C, D are omitted in the figure, but should be placed at the wire, 
immediately above AB. 
