2j ; 
232 Mr. Lane on Electric Conduction in Metals. * 
rable poster on the needle. Let AD (fig. 1) bea conduetor Nine 
up of a.single wire AC,* and several ns Fig. 1. 
CD. "The battery circuit being eom- - : 
pleted, let the points C and.D be/con-. (^ P 
nected by one of the wires of the ciábteioriótotà 'TThe needle 
will be powerfully deflected. Now let E and F be connected by 
the other wire of the galvanometer, but so that the current aris- 
in may act in opposition to the former, and let the space EF" be 
increased till the actions on the needle balance each other. Next, 
let the points A and B be connected by the same' wire as C and 
D before, and AB increased till the needle is stationary. "'l'hen 
AB and CD will have equal differences of tension, for though 
the galvanometer wire did draw out a small fractional part of the - 
battery current, still it was the same quantity in both cases. EF 
might be taken instead of AB, if we could be sure the wires of 
the galvanometer were exactly equal in their actions, and íhis- 
adjustment might be made without much difficulty. 
5. A better method, and the one I adopted, is the following. 
It requires only a galvanometer of the ordinary construction. . 
Let A/D'/ be a uniform conducting wire placed. alongside of AD, 
and well connected with it at the extremities. 1f, while a cur- 
rent is passing, any point of AD be connected through the gal- 
vanometer with a point of A'/D', of different tension, the needle . 
will be deflected, but a point may always be found in A/D'; where 
no deflection will take place. Let A and A' be points of equal 
tension ascertained in this manner, as also B and B/, € and C, 
and D and D', and let »» represent the number of the wires AC, 
and » the number of CD.  'Then, if A/B/ be made equal to C/D', 
AB and CD will have equal d ifotcdicda of tension, and mes 
to the first proposition we shall have 
n.AB-m.CD. 
Or if A/B' and C'D' be not equal, we may reduce to equal lengths 
by substituting for the ratio of AB to CD, that of me C'D' to 
CD. A'B', and we shall have 
n. AB.C'D'—m.CD. A'B'. 
* If this method be taken, AC must either be a single wire, or, if several, they 
must be well joined near as may be to DB, A being at the same time taken near 
the connected extremities; and the same caution must be applied to the common 
measure EF, unless it be taken on a single wire, introduced for the purpose, so as 
to convey the whole current. C and D also must be near the. connected extremi- 
ties of the wires. 
