Which instrument has a necessarily the “square law” type scale?
Right Answer is:
Hotwire instruments have a necessarily the “square law” type scale.
Working Of Hot Wire Instrument
The working of the hot-wire instrument is based on the heating effect of electric currents. When the current to be measured passes through the hot wire, the wire gets heated and then expands. Since the wire is fixed between two points, it sags due to expansion which is magnified by the phosphor-bronze wire and silk thread. This expansion is taken up by the spring and the silk thread, which causes the pulley to rotate, and the pointer is deflected.
The deflection of the pointer of the hot-wire instrument is proportional to the extension of the hot wire, which is itself proportional to the square of the current.
Deflecting Torque. Td ∝ I2
If spring control is used, then
Controlling torque Tc ∝ θ (deflection)
For balanced condition
TC = Td
∴ θ ∝ I2
Thus, these instruments have a square-law type scale. They read the R.M.S value of current and are independent of its frequency.