Identifier: electricalinstru00unitrich (find matches)
Title: Electrical instruments and telephones of the U.S. Signal corps
Year: 1911 (1910s)
Authors: United States. Army. Signal Corps
Subjects: Military telegraph -- United States Military telephone -- United States
Publisher: Washington, Govt. print. off.
Contributing Library: University of California Libraries
Digitizing Sponsor: MSN

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Text Appearing Before Image:
The relation of parts in the conventional diagram of the AVheat-stone bridge (fig. 175) will now be aj^parent. If the resistance in thecoils of ^1 and B are equal or bear any other simple numerical rela-tion, then the same numerical relation exists between R and A, and ifR be a box of known resistance coils, X, the unknown resistance,becomes known from the above-stated relation A : B : : R : X.
Text Appearing After Image:
Fig. 175 If we straighten out .1 and B and bend up R into compact form,put keys into the galvanometer and battery circuits, we shall have thediagram of the ordinary or post-office form of the bridge (fig.177). The resistance in the balance arms A and B^ and in A areshort-circuited by inserting the plugs, and they are introduced bywithdrawing the plugs. The galvanometer now most usually em-ployed is some sensitive form of the suspended-coil type. The simplest measurement is made with A and B equal. Start outwith, say, .1 and B 100 ohms each. Then connect up the terminals 256 ELECTRICAL INSTRUMENTS U. S. SIGNAL CORPS. of the unknown resistance A (Z^?ie and ground-), and closingthe battery key, tap the galvanometer key. There being no resistanceunplugged in R. the galvanometer needle will be deflected to the sideindicating too small for R. Xow unplug in R and test until theright amount is unplugged in R to get a balance or no deflection.

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