~ 
932  Mr.\Lane on Electric Conduction in Metals. 
rable power on the needle. Let AD (fig. 1) be a conductor made 
up of a single wire AC,* and several Fig. 1. . 
CD. The battery cireuit being com- F = 
pleted, let the points C and D be con- : 
nected by one of the wires of the galvanometer. The 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- 
ing may act in opposition to the former, and let the space EF be 
*inereased till the actions on the needle balance each other. Next, 
let the points A and B be connected by the same wire as © and 
D before, and AB increased till the needle is stationary. Then 
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 this 
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. If, 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’, C and C’, 
and D and D’, and let m represent the number of the wires AC, 
and n the number of CD. ‘Then, if A’B’ be made equal to C’D’, 
AB and CD will have equal differences of tension, and according 
to the first proposition we shall have ; . 
-AB=m.OD. > 
Or if A’B’ and C’D’ be not equal, we may redi ce 
by substituting for the ratio of AB to ¢ 
CD. A’B’, and we shall have Pity 
n. AB.C'D/=m.CD. A’/B’. 
ties of the wires. 
