Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . others are known, by balancing the po-tential differences in the divided circuit by moving the slidingcontact 4 to the point of balance. From the proportion, we get Res, a-3 _ Res. 3-bRes. c-4 Res. 4-d Res. 3-b = Res- a-3 X Res. 4-dRes. c-4. When balance is obtained the values of the three resistances,a-35 c-4 and 4-d are substituted in the above formula andthe unknown resistance, 3-b, isreadily calculated. 248. Lamp Chart Analogyof Wheatstone Bridge.— The balancing of potentials may
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . others are known, by balancing the po-tential differences in the divided circuit by moving the slidingcontact 4 to the point of balance. From the proportion, we get Res, a-3 _ Res. 3-bRes. c-4 Res. 4-d Res. 3-b = Res- a-3 X Res. 4-dRes. c-4. When balance is obtained the values of the three resistances,a-35 c-4 and 4-d are substituted in the above formula andthe unknown resistance, 3-b, isreadily calculated. 248. Lamp Chart Analogyof Wheatstone Bridge.— The balancing of potentials may bepractically illustrated by a number of16-C. P. 50-volt lamps (50 ohms hot),arranged as shown in Fig. 226 andconnected to a dynamo circuit. Twolamps are connected in series at A,making the total resistance 100 ohms,which is connected in series withtwo lamps connected in parallel at C,the joint resistance of which is 25ohms, Formula (43). The total resist-ance of A and C in series is, therefore,125 ohms. If 125 volts are maintainedacross points 1 and 2, the circuit, AC,will receive one ampere, the drop. Fig. 226.—Lamp Chart Analogyof the Wheatstone Bridge. 254 PRACTICAL ELECTRICITY. across the lamps at A will be 100 volts and across the lamps at C, 25volts. The A lamps burn at normal candle power, but the C lamps,dimly since they get only % ampere each. Consider now the lower half of the circuit. At B, 4 lamps are con-nected in series so that resistance of B = 200 ohms, Formula (36)plus 1 lamp in series with them at D = 250 ohms for this lowercircuit. The current therefore through B and D is one-half ampere,Formula (28), and all the lamps burn dimly since the drop acrosseach lamp is only 125 h- 5 = 25 volts. The drop across B is therefore100 volts, the same as it was across A, and 25 volts across D, or thesame as across C. Now since the drop across 1-3 is exactly the sameas that across 1-4, 100 volts, if points 3 and 4 are connected b3r a wireno current will flow through i
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