A moving coil instrument has a resistance of 75 ohms and gives full scale deflection of 100 scale divisions for a current of 1mA. This instrument is connected in parallel with shunt resistance 25 ohms.The combination is then in series with load and a supply. What is the current through the load when the instrument gives an indication of 80 scale divisions?

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  1. instrument current ... (80/100) * 1 ma

    shunt is 1/3 the resistance of the instrument
    ... so, three times the current

    the two parallel currents add to equal the load current

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  2. I1 = (80/100)*1ma = 0.8 ma. = current thru instrument.
    I2 = (75/25)*0.8ma = 2.4 ma = current thru 25 ohm resistor.

    I1+I2 = 0.8+2.4 = 3.2 ma = current thru load.

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