Correct Answer - Option 1 : moving iron (attraction type) ammeter
Operating torque:
In moving iron instruments, the deflecting torque is given by
\({T_d} = \frac{1}{2}{I^2}\frac{{dL}}{{d\theta }}\)
I is the operating current in Ampere
L is the inductance in Henry
θ is the deflection angle in radians
As the operating torque is directly proportional to the square if the operating current. Hence, this instrument possesses high operating torque.
Applicable to both DC and AC circuits:
- Whatever the direction of the current in the coil of the instrument, the iron vanes are so magnetized that there is always a force of attraction in the attraction type and repulsion in the repulsion type of instruments.
- Thus, moving iron instruments are unpolarised instruments i.e. they are independent of the direction in which the current passes. Therefore, the instruments can be used on both ac and dc.
Power consumption in these instruments is higher for low voltage range.
