412 Description and Use of the Apparatus employed 
moveabie conductor, and of which the branch R' R communi- 
cates with a second circular brass-wire, on which there presses 
■Bit Z a spring Z I similar to the first, and which communicates 
on the side I with the other extremity of the pile. By turning 
the graduated circle round its horizontal axis, the part SR of 
the conductor will move in a vertical plane^ forming every pos- 
sible angle with the part BC of the moveable conductor, on 
which it acts through the glass cage. 
In order to place SR at a given distance from BC, the verti- 
cal axis H' H is turned, till BC points to the zero of the scale 
^hich is done by placing it immediately above the mark on the 
bevelled extremity of the piece of copper m. An index np, fixed 
at n to the support of the graduated circle, marks on the scale the 
distance of BC and SR. When a communication is established 
between the two extremities of the circuit and those of the pile, 
BC is carried forward or backward according as it is attracted 
or repelled by SR, but it is brought back into its former posi- 
tion by turning the axis of suspension H' H, and the number of 
turns and portions of a turn marked by the index r on the 
divided circle, gives the value of the attraction or repulsion, as 
measured by the torsion of the suspending wire. 
By the addition of another moveable conductor similarly 
suspended, and composed of equal and opposite parts, the pre- 
ceding instrument can measure the momentum of the forces 
which tend to turn a conductor by the action of anbther con- 
ductor, which forms with it different angles corresponding to 
different momenta. This moveable conductor ABOCDEF is 
shewn in Fig. 10. It is suspended between the points A^ F, 
where it is interrupted, and where the two extremities of the 
conductor carry two steel points M, N, situated in the same 
vertical line^ and plunged in the mercury of the two small cups 
in Fig. 6.) without reaching the bottom. In order to measure 
the momentum of rotation produced by a rectilineal conductor, 
one of them is placed under the glass cage, and very near the 
lower side CD, Fig. 10. of the moveable conductor, so as to be 
opposite to its middle. This last will then turn by the action 
of the fixed conductor, without being influenced by that of the 
earth, because there is a compensation between the action exer- 
