. American telephone practice . unt because it represents the number by which the current through the galvanometer must be multiplied, in order to give the value of the current being measured. Shunt boxes are usually provided for a given galvanometer with a number of coils specially arranged to give such convenient values of the multiplying powers, as 10, 100, and 1000. For this purpose TESTING. 867 the various coils of the shunt box have resistances of £, *V, and mof the resistance of the galvanometer. To better show this relation, assume that a multiplying powerof iooo is desired, then i?s +
. American telephone practice . unt because it represents the number by which the current through the galvanometer must be multiplied, in order to give the value of the current being measured. Shunt boxes are usually provided for a given galvanometer with a number of coils specially arranged to give such convenient values of the multiplying powers, as 10, 100, and 1000. For this purpose TESTING. 867 the various coils of the shunt box have resistances of £, *V, and mof the resistance of the galvanometer. To better show this relation, assume that a multiplying powerof iooo is desired, then i?s + R iooo — Rs • oooRs — Rs = xs R, A iooo - i 999 A commercial form of shunt box is shown in Fig. 634, the variousmultiplying values of the shunt being obtained by plugging the blockcorresponding to the multiplying power desired. For moderate deflections, the current traversing the coils of areflecting galvanometer may, without sensible error, be taken asproportional to the deflection of the spot of light on the scale, or. FIG. 634.—SHUNT BOX. to the deflection read through the telescope. The current is, ofcourse, inversely proportional to the total resistance of the circuit,and from this it follows that the deflections are inversely proportionalto the resistance. This fact enables the galvanometer to be used formeasuring unknown resistances by comparing the deflection obtainedwhen a given E. M. F. acts through a known resistance with thatobtained when the same E. M. F. acts through an unknown resist-ance. The general method of measuring resistances by the use of agalvanometer is to note the deflection obtained with a given bat-tery and a known resistance in the circuit, and from this to com-pute what is called the working constant. This working constantmay be defined as the number of scale divisions deflection that wouldbe obtained by causing the current from the given battery to pass 868 AMERICAN TELEPHONE PRACTICE. through the galvanometer and a resistance of one meg
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