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The potential divider The potentiometer principle is used in the potential divider for the purpose of providing a variable voltage supply. For example, if the terminals A and B of the resistor in Fig. 35.2 on page 406 are connected to a 12 volt battery, then any voltage from zero to 12 volts may be tapped off from A […]

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To compare the e.m.f.s of two cells by using a potentiometer In the circuit diagram of Fig. 40.7 the points D and F may be considered as the two terminals of a voltmeter. D is connected directly to ·A, and F is connected to a sliding contact on the potentiometer wire via a sensitive centre-zero galvanometer. As […]

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The potentiometer The potentiometer is a piece of apparatus for comparing potential differences. The simple potentiometer used in elementary work consists of a length of one or more metres of uniform resistance wire mounted on a board between thick brass or copper strips of negligible resistance. A millimetre scale is fixed to the board for measuring the distance from […]

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To measure the resistivity of constantan A metre-bridge circuit, in which the unknown resistance consists of a length of constantan wire, is connected up as in Fig. 40.6 (b). It is convenient to use from 0.75 to 1 m of wire for this purpose, and the experimenter must use his own ingenuity to ensure that the length of […]

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Resistivity By measuring the resistance of: (a) various lengths of wire of the same thickness and made of a given material; (b) several wires of equal length but different areas of cross-section also of the same material; we may verify the relations (1) and (2) above. From this expression we see that the resistivity of a […]

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Factors affecting the resistance of a wire at constant temperature When resistors are being made, short lengths of thick wire are used for the low resistances and long lengths of thin wire for the high resistances. Besides length and thickness the material of which the wire is composed is another important factor to be considered when deciding the […]

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The metre bridge The metre bridge consists of a straight uniform resistance wire AB, 1 m long, stretched over a boxwood scale graduated in millimetres and mounted on a board (Fig. 40.6 (b)). The ends of the wire are soldered or clamped to two stout copper or brass strips a and c, and a third and longer […]

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Electricity and magnetism found it more convenient to replace P and Q by a uniform resistance wire which could be divided into two parts by a movable contact. He used this principle in the metre bridge form of the Wheatstone bridge.

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The Wheatstone bridge One of the most accurate methods of measuring resistance was devised in 1843 by Charles Wheatstone, the first Professor of Physics at King’s College, London. It is now known as the Wheatstone-bridge method. Suppose that four resistors, R, S, P, and Q in ohms are connected so as to form the four arms ab, be, […]

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Resistance of an electric lamp and other conductors The ammeter-voltmeter method is an excellent one for measuring the resistance of an electric lamp when run at different voltages. The circuit shown in Fig. 40.4 (a) is used in which the single cell is replaced by any convenient 6 V d.c. source and the resistor R changed for a […]