324 
MR. WHEATSTONE ON NEW INSTRUMENTS AMD PROCESSES FOR 
vanometer wire, and if the four wires he of equal length and thickness, and of the same 
material, perfect equilibrium is established, so that a rheomotor however powerful 
will not produce the least deviation of the needle of the galvanometer from zero. 
The circuits Z b a C Z, and Z a b C Z, are in this case precisely equal, but as both 
currents tend to pass in opposite directions through the galvanometer, which is a 
common part of both circuits, no effect is produced on the needle. Currents are 
however established in Z b C Z, and Za CZ, which would exist were the galvano- 
meter entirely removed. But if a resistance be interposed in either of the four wires, 
the equilibrium of the galvanometer will be disturbed ; if the resistance be interposed 
in Z b or C a, the current ZabCZ will acquire a preponderance ; if it be inserted 
either in Z a or C b, the opposite current, Z b a C Z, will become the most energetic. 
If the resistance interposed in the wire be infinite, or which is the same thing, if the 
wire (which we will suppose to be C b ) be removed, the energy of the current 
passing through the galvanometer will be that of a partial current Z b a passing 
through one of the wires plus the galvanometer wire ; the path of the diverted portion 
of the current being Z a. According to this disposition, the force of the original 
E 
current = ^ and that of the partial current acting on the galvanometer 
E v 
— r (3 ~r~+~g) + 9 r 2 + r]j ’ ^ being the resistance of the rheomotor, r that of a single 
wire, and g that of the galvanometer. 
The equilibrium having been disturbed by the introduction of a resistance in one 
of the wires, it may be restored by placing an equal resistance in either of the adja- 
cent wires. For the purpose of interposing the measuring resistance and the resist- 
ance to be measured, the wires Z b and C b are interrupted, and binding screws, c, d 
and e,/', are fixed for the reception of the ends of the wires. The equilibrium when 
once established is not in any degree affected by fluctuations in the energy of the 
rheomotor. 
Fig. 6 represents a different and, in some respects, a more convenient arrangement 
of the wires to produce the same result ; the same reference letters are employed, 
and the preceding observations apply to it equally. 
Slight differences in the lengths, and even in the tensions of the wires, are sufficient 
to disturb the equilibrium ; it is therefore necessary to have an adjustment, by means 
of which, when two exactly equal wires are placed in C a and Z a, the equilibrium may 
be perfectly established. For this purpose, in the instrument, fig. 6, a piece of metal 
n, connected with the binding-screw b , is inlaid in the board, and another piece of 
metal m moves round n as a centre, whilst its free extremity always rests on the wire. 
According as the moveable piece of metal makes a greater angle with the fixed piece, 
the resistance of the path Z b is diminished ; if, however, the equilibrium is dis- 
turbed because the resistance in C b is too great, the moveable piece of metal must 
be placed on the opposite side of the fixed piece. 
