A moving coil ammeter having a resistance of 10 ohms gives full-scale deflection when a current of 5 mA is passed through it. The instrument can be used for the measurement of voltage upto 5 V by

A moving coil ammeter having a resistance of 10 ohms gives full-scale deflection when a current of 5 mA is passed through it. The instrument can be used for the measurement of voltage upto 5 V by

Right Answer is:

Connecting a resistance of 990 ohms series with the instrument

SOLUTION

The resistance of instrument Rm = 10 ohms

External resistance R =?

Full scale deflection current Im = 5mA = 0.005A

Voltage across ammeter Vm = ImRm = 10 × 0.005 = 0.05A

Voltage across external resistance R =  ImR = 0.005R

Total voltage = ImRm + ImR

5 = 0.05 +  0.005R

R = 990 Ω

Note:- Direct formula

R = V/I −Rm

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