hy M. Ampere in Ms Electro-Magnetic researches. 411 
mg equal and opposite to that of the portion AB of the con- 
ductor, there remains in this apparatus only the action of the 
projections in planes perpendicular to the axis of the tube ; and 
as those of the arcs M, N, O, &c. are very small, we shall have 
only the actions of the rings E, F, G, &c. remaining. 
In order to measure the attractive or repulsive action of two 
electric currents, as modified by their distance, and by the angles 
which determine their respective position, M. Ampere employed 
the apparatus shewn in Fig. 6. 
The support KFG has three different motions. The first 
is produced by the screw M, and the two others by fixing the 
support to a piece of wood N, which can slide both in a hori- 
zontal and vertical direction on another piece of wood O, fixed 
to the foot of the instrument. In one of these is formed a 
horizontal groove, and in the other a vertical one, and at their 
intersection is a screw Q, which fixes the moveable piece in the 
required position. The rotatory motion of the graduated circle 
for inclining the portion of the conductor attached to it, is pro- 
duced by two pullies P and P'. In order that the earth may 
not affect the moveable conductor, which is balanced by small 
counter weights it is composed of two equal and opposite 
parts ABC d, ah c DE, shaped as in the figure, in order that 
its two extremities may communicate with those of the pile. 
It is interrupted at the angle A, by which it is suspended to a 
wire HH', ‘whose torsion ought to balance the attraction or re- 
pulsion of the two currents. The branch BA is prolonged be- 
yond A, and the branch DE beyond E, and they terminate by 
the points E, L, which are plunged in two small cups full of 
mercury j without reaching the bottom. The foot which carries 
these two small cups, may be advanced or brought back by 
means of the screw which fixes it in the groove ef. These 
cups may be either of iron or platinum. One of them commu- 
nicates with one of the extremities of the pile by the conductor 
XU, enclosed in a glass tube, round which is twined a spiral 
with high coils : The conductor YVT is terminated by a sort of 
copper spring, which presses by its point at T upon the cir- 
cumference of the graduated circle, where it is in contact with a 
circular brass-wire, communicating with the branch S' S of the 
conductor, of which the part SR is destined to act as the 
