Describe with aid of a labelled diagram the structure and action of a hot wire ammeter.

Describe with aid of a labelled diagram the structure and action of a hot wire ammeter.

• The current flows through a fine resistance-wire XY, which it heats.
• The wire warms up to such a temperature that it loses heat-mainly by convection-at a rate equal to the average rate at which heat is developed in the wire.
• The rise in temperature of the wire makes it expand and sag; the sag is taken up by a second fine wire PQ, which is held taut by a spring.
• The wire PQ passes round a pulley attached to the pointer of the instrument, which rotates as the wire XY sags.
• The deflection of the pointer is roughly proportional to the average rate at which heat is developed in the wire XY; it is therefore roughly proportional to the average value of the square of the alternating current, and the scale is a square-law one.

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