Hot Wire Instruments.—Instruments of this class depend for their operation on the expansion and contraction of a fine wire carrying either the current to be measured or a definite proportion of that current.

The expansion or contraction of the wire is caused by temperature changes, which in turn are due to the heating effect of the current flowing through the wire.

Since the variations in the length of the wire are extremely small, considerable magnification is necessary. Pulleys or levers are sometimes used to multiply the motion, and sometimes the double sag arrangement shown in fig. 2,504.

As shown here, A is the active wire carrying the current to be measured and stretched between the terminals T and T´. It is pulled taut at its middle point by another wire C, which carries no current, and is, in its turn, kept tight by a thread passing round the pulley D attached to the pointer spindle, the whole system being kept in tension by the spring E.

Hot wire instruments are equally accurate with alternating or direct current, but have cramped scales (since the deflection is proportional to the square of the current), and are liable to creep owing to unequal expansion of the parts. There is also the danger that they may be burnt out with even comparatively small overloads. They are not affected by magnetic fields but consume more current than the other types, these readings are inaccurate near either end of the scale.

Fig. 2,504.—Diagram illustrating the principle of hot wire instruments. The essential parts are the active wire A, stretched between terminals T and T´, tension wire C, thread E, and pulley D to which is attached the pointer.

Induction Instruments.—These were invented by Ferraris, and are sometimes called after him. They are for alternating current only, and there are two forms: