322 
MR. WHEATSTONE ON NEW INSTRUMENTS AND PROCESSES FOR 
tract, by means of the rheostat and coils, a resistance which shall make the force again 
E — s E s 
equal to F ; the expression will then become j, - = whence x — \ ^ R. 
Therefore the resistance x of the liquid is equal to the resistance \ taken out of the 
circuit by the rheostat, minus the total resistance of the original circuit multiplied 
by the ratio 
§ 15 . 
When a galvanometer is employed to measure the force of a current, its wire is 
usually interposed in the circuit. But it is impossible, in this way, to make use of the 
same galvanometer to measure the force of the current in circuits of different kinds. 
A galvanometer with numerous coils of thin wire adds a very considerable resistance 
to a circuit in which the electro-motive force is great and the resistance small ; while, 
on the other hand, a galvanometer with a short thick wire will give scarcely any in- 
dication in a circuit in which the resistance is very great, though the electro-motive 
force may be considerable. Besides, a delicate galvanometer is incapable of indi- 
cating energetic forces. 
But by the following simple means the same delicate galvanometer may be em- 
ployed to measure forces of every degree of energy, and in all kinds of circuits, with- 
out introducing any inconvenient resistance into them. 
If the current be caused to pass simultaneously through two paths, one being the 
wire of the galvanometer, and the other another wire connected with its two ends, 
the current will be divided in the inverse proportion of the resistances of the two 
paths. The action upon the needle of the galvanometer may hereby, by employing 
different wires to divert a portion of the current, be reduced to any degree. If the 
proportionate forces are known for the galvanometer without the reducing wire, 
they will remain equally proportionate whatever the resistance of the latter may be ; 
but measures made with the same instrument, with different reducing wires applied, 
will not be comparable unless the changed resistance of the galvanometer thus mo- 
dified be taken into account. 
But strictly comparable measures may be obtained, if the precaution be taken of 
adding, to the principal portion of the circuit, a resistance which will compensate for 
the diminution of resistance occasioned by placing the reducing wire. Let g be the 
reduced length of the galvanometer wire, and ng that of the reducing wire. The 
force of the current in the principal portion of the circuit will be to that in the gal- 
71 
vanometer wire as 1 : -- y . The resistance to be added to the principal portion of 
the circuit, in order to maintain the current the same as when no reducing wire is 
added, is^-p- 
When the measures of energetic currents are required to be determined by means 
of a delicate galvanometer, it is sufficient to attach its two ends to two points of the 
