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By S. M. Geddes (auth.)

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Between A and D = I 2 R 3 . e. d. d. between A and D I2R3. (1) I 1R 2 = I 2 R 4 • (2) I1R1 = Similarly, for R 2 and R 4 , ELECTRICITY AND MAGNETISM 41 Dividing (1) by (2), Rl R2 R3 R4 (3) This equation holds only if no current flows through CD, and in practical forms of the circuit a sensitive galvanometer is connected between C and D. If the resistances are correctly adjusted so that the galvanometer shows no deflection, then the bridge is said to be balanced. The metre bridge Figure 20 shows this practical form of the Wheatstone bridge circuit, commonly used in school laboratories to measure an unknown resistance.

On the atomic scale, this is because its atoms vibrate more vigorously in their positions in the lattice. This will make it more difficult for the electrons to pass in between them when an electric current flows, so the resistance of the conductor increases. In general, the resistance R, of a conductor at temperature t°C changes according to the equation R, = R 0 (1 + rxt + Pt 2 ), where rx and pare constants for the particular metal, called the temperature coefficients of resistance. Over small ranges of temperature, this equation may be approximated to R, = R 0 (1 since + rxt) p is very small compared to rx.

What is the value of the resistance across the right-hand gap? 7 Q. Where is the balance-point expected to be? The potentiometer This circuit is not a form of the Wheatstone bridge in spite of a superficial resemblance. As its name implies, it measures potential differences (voltages). The simplest form of potentiometer consists of a uniform length of resistance wire AB, 1 m long, mounted on a board alongside a millimetre scale. A current from a low-voltage source Vis passed through the wire (Fig.

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