Discussion
Assertion (A) : A hot wire ammeter has a cramped scale.
Reason (R) : The heat is proportional to square of current.
Answer: A
The answer is both (A) and (R) are true and (R) is the correct explanation of (A).
A hot wire ammeter measures the expansion of a wire heated by the current. The heating power is \(I^{2}R\), so the deflection is proportional to the square of the current. Equal steps of current therefore produce small deflections near zero and large ones near full scale: the scale divisions are crowded at the low end, which is what a cramped (non-uniform) scale means.
The square law that R states is the direct cause of the cramped scale, so R explains A. The same square law is why the instrument reads true rms values and works on both ac and dc.
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