DETERMINING THE CONSTANTS OF A VOLTAIC CIRCUIT. 
325 
No fixed dimensions can be assigned to these instruments. The boards of those 
I employ are fourteen inches long and four inches wide, and the wire is copper 
of an inch in diameter. A single voltaic element of large surface will produce a 
more considerable effect than a battery of small elements*. A thermo-electric ar- 
rangement, or a magneto-electric machine may be substituted for the voltaic element 
or battery ; and a voltameter or any other description of rheometer may in some 
cases supply the place of the galvanometer. It is scarcely necessary to state that 
these instruments are not adapted to measure the resistances of substances capable 
of undergoing chemical changes from the action of an electric current, on account of 
the contrary electro- motive forces which arise under such circumstances'!'. 
§ 17 . 
Another differential arrangement, which will be found useful in some circum- 
stances, may be worth mentioning ; it is much more sensible than the preceding, but 
as the equilibrium indicated is that between two currents generated by independent 
rheomotors, instead of diverted portions of the same current as in the instruments 
previously described, the state of equilibrium will be disturbed by every fluctuation, 
whether of the electro-motive force, or resistance of either of the rheomotors ; it can 
therefore only be safely employed when these are perfectly constant, or when the 
object is not to measure resistances, but to observe the comparative changes in two 
rheomotors. 
Fig. 7 represents a circular board on which are fixed ten binding screws ; the wires 
proceeding from one of the rheomotors are to be attached to C 1 and Z 1 , those from 
the other to C 2 , Z 2 , and the ends of the galvanometer wire are to be fixed to a and b. 
The two currents, C 1 a b Z 1 and Z 2 a b C 2 , tend to pass through the galvanometer wire 
in opposite directions. When two equal wires are interposed between e /‘and e'f', if 
the opposing currents be equal, perfect equilibrium is established in the galvanometer 
wire, and the needle remains at zero. But if the force of the current in either of the 
rheomotors varies, or, if while the force of the two rheomotors remains constant, the 
slightest difference is occasioned in the resistance of either of the wires interposed 
between ef or e'f, the equilibrium in the galvanometer wire is disturbed and the 
needle is deflected. 
* When a single element of Daniell’s battery, 6 inches high and 3^ inches diameter, is employed, and two 
copper wires two feet long and ^jth of an inch diameter are interposed in the instrument, an augmentation 
of the tenth of an inch in one occasions a deviation of 2° in the galvanometer needle. This will suffice to 
show the accuracy with which resistances may be measured by this instrument. 
t Mr. Christie, in his “Experimental determination of the laws of magneto- electric induction” printed in 
the Philosophical Transactions for 1833, has described a differential arrangement of which the principle is the 
same as that on which the instruments described in this section have been devised. To Mr. Christie must, 
therefore, be attributed the first idea of this useful and accurate method of measuring resistances. Another 
differential arrangement, proposed also in the same memoir, is analogous to that which forms the subject of the 
following section. 
2 u 2 
