SOLUTION

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.

Deflecting Torque

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.

Hence

Deflecting Torque. T_d ∝ I²

If spring control is used, then

Controlling torque T_c ∝ θ (deflection)

For balanced condition

T_C = T_d

∴ 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.

Advantages of Hot Wire Instrument

Following are the advantages of hot-wire instruments:

• Since the deflection of the pointer depends on the r.m.s value of the alternating current, it can be used both for a.c. and d.c.
• These instruments are free from wave-forms and frequency errors.
• As the instruments do not depend upon any magnetic effect for their operation, they are free from external stray magnetic field errors.

Disadvantages of Hot Wire Instrument

• The very slow response of the circuits, as the wires take time to heat up.
• High power consumption as compared to moving coil instruments.
• Their zero position needs frequent adjustments due to changes in room temperature.
• Inability to withstand overload because the hot wire is so fine that it may melt before the fuse.
• Inability to withstand mechanical shocks because the wire Is very fragile.

Hot-wire instruments are now obsolete and have been replaced by the more sensitive, more accurate and better-compensated combination of the thermoelectric heating element and PMMC movement.