File:Electrical instruments and telephones of the U.S. Signal corps (1911) (14733586476).jpg

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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are are requisite to obtain accurateresults. In measuring resistances of a telegraph line be certain all line bat-teries are disconnected befoi-e making the measurement. The line isconnected with one of the A binding posts, the ground to the other.(See fig. 170.) ELECTRICAL INSTRUMENTS U. S. SIGNAL CORPS. 257 Many modifications of this form of bridge are in use. Some arearranged so the plugs are inserted at the points where the introduc-tion of resistance marked is desired. One of the familiar forms in which the bridge is made for labora-tory use is shown in figure 177. In this the resistances are introducedby taking out plugs. The further row of strips are for the A and Barms, the other rows constituting the R arm. The keys are for intro-ducing battery and galvanometer into circuit. The following graphical demonstration of the stated proportion-ality of the resistances in the four arms of the Wheatstone bridgewhen balance is obtained is of interest in connection with the fore-going :
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O^mS Lay off A B to represent the resistance in the upper branch of thebridge (fig. 178) and .1 F to represent resistances in the lower. LetA C represent the difference in potential between the two ends of thebridge, and draw lines C B and C F. These are the fall of poten-tial lines along the upper and lower branches respectively. A horizontal line H D touches points H and D at the same poten-tial. These projected on lower lines represent the points of galva-nometer connection. The resistances passed over in lower and upperbranches to reach these points of equal potential, measured on theloAver line, are A G and A E. Similar triangles give these proportions: CD :D B : :AE :EB G H : // F : : .1 G :GF CD .DB : :C H H F AE :EB: :A G :GF AE .AG : :E B :GF 60240°—11- -17 258 ELECTRICAL INSTRUMENTS U. S. SIGNAL CORPS. F and B being at the same point in the conventional diagram of thebridge (fig. 174). the hist proportion will be seen to be identical withthat accompanying that figure. The

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