A moving coil has a resistance of 10 ohms and gives full scale deflection at 0.5 Volts potential difference across it. How can it be adopted to measure a current up to 100 A
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The only way to have the single meter movement you mentioned work to measure both voltage and current is to add a two-position function selector switch to the instrument, that configures it for measuring voltage in one position, and current in the other. For the sake of explanation, let’s designate the two positions of this switch “A” (when the meter is configured to measure voltage) and “B” (when it is configured to measure current).
When the switch is in position “A”. the meter movement is connected in series with a resistance Ra that limits the current through the movement to its full-scale value (50 mA) when the input voltage is 750V. Thus the value of this series resistance would be equal to 750/0.050 = 15kΩ.
When the switch is in position B, the series resistance Ra is bypassed, and the meter movement is connected in parallel with a shunt resistance Rb that shunts all but 50 mA of the current being measured, around the meter movement and through the shunt. For your specific example, this would mean that 50mA would flow through the meter movement (causing full-scale deflection), while the remainder (1000 A - 0.050 A = 999.050 A) would flow through the shunt resistance Rb. The value of this shunt resistance would thus be equal to 10 (0.050/999.050) = 5.05μΩ (that’s 0.0000505 ohms — good luck with producing a shunt resistance constructed to this level of precision).
Note that in position A (voltage mode), the shunt resistance Rb must also be removed from the circuit, while in position B (current mode), the series resistance Ra must also be bypassed. The function selector switch would of course have to be designed to accomplish this additional switching. If you need assistance visualizing/designing the associated schematic, feel free to reply to this answer.