Concept:Because the cycle average of AC is zero, standard moving-coil meters based on linear magnetic deflection or chemical electrolysis average to zero. Thermal heating depends on \(I^2\) and is unidirectional.
Formula / Reaction:$$P_{\text{avg}} = I_{\text{rms}}^2 R$$
Solution:- Hot-wire ammeters utilize the expansion of a metal wire heated by the current \(I^2 R\).
- Because heating \(H \propto I^2\), deflection is always in one direction regardless of AC alternating polarity, making it the standard method to measure RMS AC.
Why other options are incorrect:- Opt_A: Standard magnetic torque reverses every half cycle; a permanent-magnet moving-coil meter oscillates rapidly around zero and reads zero.
- Opt_C: Chemical electrolytic effects reverse every half cycle, resulting in zero net chemical deposition.
- Opt_D: Only thermal effect meters can directly measure AC without rectification.
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