A repulsion type ammeter when used on A.C circuit reads

Question

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Option B: R.M.S value of the current

Explanation & Concepts:
A repulsion-type moving iron ammeter operates on the principle of magnetic repulsion between two soft-iron vanes placed within a current-carrying coil. Because the deflecting torque is proportional to the square of the current ($T_d \propto I^2$), the instrument responds to the mean square of the instantaneous current, thereby measuring the Root Mean Square (R.M.S.) value regardless of the current's waveform.

Working Principle: When current passes through the fixed coil, it magnetizes both the fixed and moving iron vanes in the same polarity. This creates a repulsive force proportional to the product of the pole strengths, which are both proportional to the instantaneous current $i$. Thus, the instantaneous torque is $T_i = k \cdot i^2$. The inertia of the moving system causes it to respond to the average torque, which is $T_{avg} = k \cdot \frac{1}{T} \int_{0}^{T} i^2 dt = k \cdot I_{rms}^2$.

Key Formulas: $T_d = I^2 \frac{dM}{d	heta}$ — Deflecting torque for moving iron instruments; $I_{rms} = \sqrt{\frac{1}{T} \int_{0}^{T} i(t)^2 dt}$ — Definition of R.M.S value

Key Points: Moving iron instruments are inherently R.M.S responding.; They can be used for both A.C. and D.C. measurements.; The scale of a repulsion type ammeter is non-uniform (cramped at the beginning).; It is unaffected by the waveform of the alternating current.

Advantages: Low cost and robust construction; Can measure both A.C. and D.C. currents

Disadvantages: Non-linear (cramped) scale; Errors due to hysteresis and frequency variations

Applications: Industrial power frequency ammeters; Low-cost laboratory measurement devices

Additional Info: The repulsion type is specifically used for ammeters and voltmeters.. Option A (Peak value) is measured by Peak-reading voltmeters. Option C (Mean value) is measured by Rectifier-type instruments calibrated to read the R.M.S of a sine wave.

Answer: B is correct — Repulsion type moving iron instruments measure the R.M.S value of current because their deflection is proportional to the square of the current.

Exam Tip: Always remember that moving iron (MI) meters read R.M.S, whereas moving coil (PMMC) meters read the average (mean) value of the current.

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Option A: Peak value of current

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Option C: Mean value of current

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Option D: Equivalent D.C value of the current

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