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The dc current flowing in a circuit is measured by two ammeters, one PMMC and another electrodynamometer type, connected in series. The PMMC meter contains 100 turns in the coil, the flux density in the air gap is 0.2 Wb/m², and the area of the coil is 80 mm². The electrodynamometer ammeter has a change in mutual inductance with respect to deflection of 0.5 mH/deg. The spring constants of both the meters are equal. The value of current, at which the diflections of the two meters are same, is _______.
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- 32
- 0.32
- 0.032
- 3.2
Correct Option: D
Given pmmc and electro dynamometer type meters are connected in series.
Both meters are carrying same current and both are having same spring constants.
Both are reflecting same readings. i.e. we should equate the reflecting torques.
For pmmc, T def = BAN.I.
Electrodynometer, (Tdef) = I2. I² . dM/dθ
BAN.I = I⊃. dm/dθ
(0.2) × (80 × 10–6) × 100 × I = I² × 0.5 × 10–3
∴ I = 3.2
