. The practical telephone handbook and guide to the telephonic exchange . sentthrough a known high resistance, usually 100,000 ohms, thegalvanometer being shunted so as to get a convenient de-flection on the scale. If this cannot be done with the 10100shunt (which is usually the case), a part only of the battery,such as Y^-th of the whole, should be used. The deflec-tion being noted, it is multiplied by the shunt number (10, 456 PRACTICAL TELEPHONE HANDBOOK 100, or 1000). This will give the deflection which we mayimagine would be obtained if it were possible to do withoutthe shunts. This numbe
. The practical telephone handbook and guide to the telephonic exchange . sentthrough a known high resistance, usually 100,000 ohms, thegalvanometer being shunted so as to get a convenient de-flection on the scale. If this cannot be done with the 10100shunt (which is usually the case), a part only of the battery,such as Y^-th of the whole, should be used. The deflec-tion being noted, it is multiplied by the shunt number (10, 456 PRACTICAL TELEPHONE HANDBOOK 100, or 1000). This will give the deflection which we mayimagine would be obtained if it were possible to do withoutthe shunts. This number is now multiplied by the resistance (as100,000), and again by 10 if ^th of the testing battery has beenused. The constant so obtained (say, 50,000,000,000 ohms,or 50,000 megohms) will represent the resistance through whichthe testing battery will give a deflection of 1°. Dividing thisnumber by any deflection obtained (allowing for the shunt),the quotient gives the resistance in the path of the very accurate results are required, the resistance of ^^^. Fig. 432.—Connections for Galvanometer Constant the galvanometer may be neglected, especially when the shuntsare used. Figure of Merit.—Dividing the constant as above by thevoltage of the battery used gives a number which is called thefigure of merit of the galvanometer, and represents theresistance through which 1 volt will give a deflection of 1degree. The sensibility of galvanometers is in proportion totheir figures of merit. Fig. 433 shows the connections for making an insulationtest of a cable wire. The resistance-box is short-circuitedby a brass bar, and the wire to be tested put on at L in placeof earth, the full battery being, of course, used. The oppositeend of the wire must be carefully insulated. If the wire is ina cable the outer covering of the latter at the far end shouldbe stripped off for about 4 inches, and the wires opened outfrom each other. ELECTRICAL MEASUREMENTS 457 After the battery is
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Keywords: ., bookcentury1900, bookdecade1900, booksubjecttelephone, bookyear19
