Hot Wire Ammeter

Hot wire ammeter is based on the principle that when a current flows through a wire it gets heated. As a result the length of the wire increases.

The construction of hot-wire ammeter. A platinum-iridium wire AB is fixed tightly between two fixed ends A and B. A spring is fixed at one end C and is permanently connected to a thin wire at the other end. The thin wire is wound several times over a cylinder D and the end is connected to the middle point of AB. The cylinder can rotate about its axis. A pointer P connected to the cylinder moves along a graduated scale when the cylinder rotates. A small resistance r is connected in parallel to the wire AB as a shunt. This makes the total resistance of the ammeter small so that it does not appreciably alter the current in the circuit. The points A and B are connected to the outer terminals T1 and T2.

The current to be measured is passes through the instrument via T1, T2. The wire AB gets heated due to the current. The rise in temperature is proportional to i2rms. The length of the wire increases and consequently its tension decreases. Because of the tension in the spring on the other sides, the cylinder rotates a little and the pointer deflects along the scale. When the ammeter is disconnected from the circuit, the wire contracts and the pointer returns to the zero position.

As the heating of a wire is directly proportional to the square of the current, the deflection is proportional to the square of the current. The scale is thus not uniformly divided. As the deflection is proportional to the square of the current, the ammeter can be used to measure direct as well as alternating currents.


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