The accompanying engraving shows a form of Thomson's double bridge, as
modified by Kirchhoff and Hausemann. The chief advantage claimed for
this instrument consists in the fact that all resistances of defective
contact between the piece to be measured and the battery are entirely
eliminated--an object of prime importance in measuring very small
resistances. By the use of this instrument resistances can be measured
accurately down to one-millionth of a Siemens unit.
The general arrangement of the instrument is shown in Fig. 1; Fig. 2
being a diagram of the electrical connections.
[Illustration: FIG. 1.--KIRCHHOFF AND HANSEMANN'S BRIDGE FOR MEASURING
SMALL RESISTANCES.]
The piece of metal to be measured, M, is placed in the measuring forks,
gg, in such a manner that the movable fork is removed as far as possible
from the stationary one; if the weight of the piece be insufficient to
secure a good connection, additional weights may be placed upon it. The
main circuit includes the battery, B (Fig. 2), consisting of from two to
four Bunsen cells, the key, T, the German silver measuring wire, N, and
the piece of metal resting on the forks, all being joined in series. The
German silver wire, N, is traversed by two movable knife-edge contacts,
cc, as shown. Connections are made between these contacts, cc, the
resistance box, the prongs, k and l, of the forks, gg, and the
reflecting galvanometer, as shown in Fig. 2. A resistance of ten units
is inserted at o and n, while at m and p twenty units or one thousand
units are inserted. The positions of cc are then varied until the
galvanometer shows no deflection when the key, T, is depressed.
[Illustration: FIG. 2.--DIAGRAM SHOWING ELECTTRICAL CONNECTIONS OF
BRIDGE.]
When such is the case, the ratio of resistances n/m is equal to o/p;
letting M equal the resistance of the metal bar between the points, h
and i, and N equal to the resistance between the points, cc, on the
measuring wire, N, then we shall have