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# Meters - DIRECT CURRENT MEASUREMENTS In this laboratory use...

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DIRECT CURRENT MEASUREMENTS In this laboratory, use the white and black milliammeter Ammeter The moving coil galvanometer, from which the milliammeter of this experiment is derived, contains a pivoted coil of fine wire through which a current passes. The magnetic interaction between the magnetic field set up by this current and the magnetic field of the permanent magnet housed in the meter casing results in movement of the coil against a hair-spring. The resulting displacement is shown by the meter needle against a scale. The sensitivity of a particular meter movement is the current required for full- scale deflection. Very sensitive meters deflect full-scale for currents in the order of 10 to 100 µA. Meter movements are usually rated by current and resistance. A typical rating is 1 mA, 50 Ω. For a given meter, the deflection is proportional to the coil current. Since this current is proportional to the coil voltage, the deflection is also proportional to this voltage. For an illustration, consider a meter that deflects full-scale for a coil current of 1.0 mA, and that has a coil resistance of 20 Ω. The corresponding coil voltage is V T = R m I mf = 20(0.001) = 20 mV In which R m is the movement resistance, and I mf is the current required for full-scale deflection. The range of an ammeter is the current that causes full-scale deflection. Normally, a scale is not calibrated beyond the region for linear deflection nor for a current that would damage the fine wire in the coil movement. The range can be extended and at the same time the equivalent resistance reduced by connecting a resistor R sh of low resistance in parallel with the meter terminals, as is shown in Fig. 3.1. Resistor R sh is known as the shunt resistor. Incidentally, the expression “in shunt with” is sometimes used to mean “in parallel with.” Fig. 3.1 V T I A V T I R sh I sh I mf R m + +

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An expression will now be derived for extending the range of an ammeter for the measurement of a current I greater than I mf . With reference to Fig. 3.1,
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Meters - DIRECT CURRENT MEASUREMENTS In this laboratory use...

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