Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/448

Now the resistance of the connector $Q$ is as small as we can make it. If it were zero this equation would be reduced to

${\frac {F}{C}}={\frac {H}{A}},$

and the ratio of the resistances of the conductors to be compared would be that of $C$ to $A$, as in Wheatstone's Bridge in the ordinary form.

In the present case the value of $Q$ is small compared with $P$ or with $R$, so that if we assume the points $V,\,V'$ so that the ratio of $R$ to $C$ is nearly equal to that of $P$ to $A$, the last term of the equation will vanish, and we shall have

$F:H::C:A.$

The success of this method depends in some degree on the perfection of the contact between the wires and the tested conductors at $SS',\,T'$ and $T$. In the following method, employed by Messrs. Matthiessen and Hockin^[1], this condition is dispensed with.

Fig. 35.

352.] The conductors to be tested are arranged in the manner already described, with the connexions as well made as possible, and it is required to compare the resistance between the marks $SS'$ on the first conductor with the resistance between the marks $T'T$ on the second.

Two conducting points or sharp edges are fixed in a piece of insulating material so that the distance between them can be accurately measured. This apparatus is laid on the conductor to be tested, and the points of contact with the conductor are then at a known distance $SS'$. Each of these contact pieces is connected

1. Laboratory. Matthiessen and Hockin on Alloys.