. Electrical world. voltmeter. Insuch a case, if D, d^ and rf, are deflections corresponding to thereadings £, F^ and F„, and G is the total resistance in series withthe instrument, we have as before: G (D — d^ — dj (3) May 21, 1904. ELECTRICAL WORLD and ENGINEER. 967 A„ = G {D — d^ — dj (4) If two or more electric lamps are connected in series, their re-sistances, while carrying current, can be determined by means ofthree readings, as above. g (£ - KJ If X„ ^ K, I\ = o, and A = , which is the ordi- F, nary expression used in measuring a resistance with a voltmeter byreading the voltmeter with
. Electrical world. voltmeter. Insuch a case, if D, d^ and rf, are deflections corresponding to thereadings £, F^ and F„, and G is the total resistance in series withthe instrument, we have as before: G (D — d^ — dj (3) May 21, 1904. ELECTRICAL WORLD and ENGINEER. 967 A„ = G {D — d^ — dj (4) If two or more electric lamps are connected in series, their re-sistances, while carrying current, can be determined by means ofthree readings, as above. g (£ - KJ If X„ ^ K, I\ = o, and A = , which is the ordi- F, nary expression used in measuring a resistance with a voltmeter byreading the voltmeter with the resistance in series with it and againwith the resistance cut out. II.—GALV.^NOMETER METHOD. This method may be used when greater accuracy is required orwhen the insulation resistance to earth, of at least one side of theline, is over a megohm. The wiring system is represented in I of Fig. 2, and 2 of Fig. 2gives equivalent circuits. The method consists in connecting across the bus-bars a mod-. FIG. 2.—GALVANOMETER METHOD. erately high resistance and finding on this resistance a point, p,where the potential due to the generator is the same as that of theearth, and then with the aid of a sensitive galvanometer and anexternal source of , measuring the resistances, r^ and r„, toearth in the following manner: fe is a key and 5 an Ayrton uni-versal shunt. This latter may be omitted if the source of canbe varied in a known manner. \ It is evident from Fig. 2 that a balance will be had when — = —, b r„ the key, k, being in its upper position. If k is now depressed, theresistance, R, encountered by the current generated by the source,e, will be R = g^ + I I + 6 -f r„ o -|- r^ where g^ is the resistance of the galvanometer; but in comparisonwith fj and r„, g^, a and b can be neglected, and -I + -2r^ aBy construction, — = — = iV, a known ratio. From the lastr„ b two relations we deduce R (N + 1) r^ = R (N + 1)Taking d as the deflect
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectelectri, bookyear1883
