Question

Hot wire ammeters are used for measuring
A. A. C. only
B. D. C. only
C. both A. C. and D.C.
D. None of these

Answer 1

Hint: Hot wire ammeter, as the name suggests, measures the value of current in a device based on the amount of heating it does. The heat it produces is measured with the help of a wire; more the heat, more the current.

Complete step by step answer:
The cause for heating in an element, let's say a conducting wire is it's resistance. The resistance of the body creates a path of hindrance to the current i.e., it does not let the current pass easily. What the current does in turn to this obstruction is it heats the wire. This means that the electrons get struck (for a small time), they collide with atoms (ions) of the lattice and create heat. Therefore as we pass current, there is always some heat produced. This is the clear scenario for direct current.
For the case of alternating current, the supply consists of sinusoidal varying current. RMS is the root mean square average for this current which is used in place of a simple average (as it is zero for a sine/cosine wave). This (rms) is the net current that produces the heating when ac current is passed through the conducting wire. If the supply has a peak voltage $V_0$ and we have a circuit with our ammeter and Resistor R, it will measure current as:
$I_{rms} = \dfrac{V_0}{\sqrt{2} R}$
Whereas for DC, it will just be the current obtained by simple ohm's law (V=IR).
Therefore, the correct answer for this question should be (C). Both A.C. and D.C. as both supplies can cause heating.

So, the correct answer is “Option C”.

Note:
One might make a wrong assumption that the net current is zero as the positive and negative half cycle of a sinusoidal wave gives an average of zero. Remember, the concept of root mean square arises due to the fact that average over one cycle is zero. Therefore, think and recall and do not conclude that ac will not heat the wire.