What is Hot Wire Instrument? 
The hot wire instrument is a type of electrical measuring device that uses the heating effect of current to determine the magnitude of the current. It works on the principle that when current flows through a wire, the wire gets heated up and its length increases. This increase in length is directly proportional to the amount of current flowing through the wire.
The hot wire instrument consists of a wire made of a material that has a high resistance to electrical current. This wire is suspended between two fixed points and is connected to a source of electrical current. When the current flows through the wire, it gets heated up, and its length increases. The increase in length is directly proportional to the amount of current flowing through the wire.
The hot wire instrument can be used to measure both AC and DC current. The wire used in the hot wire instrument is made of a material that has a high resistance to electrical current, such as tungsten or nickel-chromium alloy. These materials have a high resistance to electrical current, so they do not get damaged easily even when a large amount of current flows through them.
Construction of the Hot Wire Instrument:
The hot wire instrument is designed to measure the current passing through a wire by measuring the heating effect of the current on the wire. The instrument is composed of a platinum-iridium wire that is passed through the current whose magnitude is to be determined.
The construction of the hot wire instrument is shown in the figure above. The current is passed through the platinum-iridium wire which is connected to two terminals. One terminal is connected to the wire and the second terminal is connected to a third terminal. The wire is passed over a pulley and attached to a spring. The spring applies tension to the platinum-iridium wire.
When the current flows through the wire, the wire begins to heat up due to the heating effect of the current. This causes the length of the wire to increase. The spring then exerts a force on the wire to keep it in a constant position. The magnitude of the current is determined by measuring the force exerted by the spring on the wire. This force is directly proportional to the heating effect of the current on the wire and thus the current's magnitude. The hot wire instrument can be used to measure both AC and DC currents.
Working:
When the current flows through the platinum-iridium wire, it causes the wire to heat up. As the wire heats up, it expands due to the increase in temperature. The expansion of the wire causes the wire to sag or lengthen. The spring attached to the wire helps to pull the wire back to its original position.
The movement of the wire, caused by the expansion and contraction, rotates the pulley. The rotation of the pulley causes the pointer on the instrument to deflect. The deflection of the pointer is proportional to the square of the RMS (Root Mean Square) value of the current flowing through the wire. This means that the deflection of the pointer is related to the amount of heat generated by the current, which is directly proportional to the square of the current. The RMS value of the current is used because it gives the equivalent heating effect of the current as compared to the direct current.
Deflection in hot wire instrument:
The deflection of the pointer in a hot wire instrument is directly proportional to the expansion of the wire caused by the heating effect of the current flowing through it. This expansion is in turn proportional to the square of the RMS (root mean square) value of the current.
A spring is used to control the expansion of the wire, and the tension in the spring (Tc) is directly proportional to the deflection of the pointer (θ). This means that the deflection of the pointer is proportional to the square of the RMS value of the current (θ ∝ I^2).
One of the advantages of hot wire instruments is that they can read the RMS value of the current, regardless of its waveform or frequency. This is because the heating effect of the current is dependent on the RMS value, rather than the peak value or the shape of the waveform. Therefore, the readings of hot wire instruments are independent of the form or frequency of the current being measured.
Damping in hot wire Instrument:
Damping is a mechanism that helps to control the movement of the pointer in the instrument. In a hot wire instrument, damping is provided by eddy currents produced in the aluminum disc. An eddy current is a circulating current that is produced in a conductor when it is placed in a changing magnetic field.
The aluminum disc is attached to the pulley in such a way that its edge moves between the poles of the permanent magnet M. When the pointer deflects, it causes the aluminum disc to rotate. As the edge of the disc moves between the poles of the magnet, it experiences a changing magnetic field. This induces eddy currents in the disc, which generate a magnetic field that opposes the motion of the pointer.
This damping effect helps to control the movement of the pointer, preventing it from overshooting or oscillating. It also helps to ensure that the pointer comes to a rest at the correct position, and that the readings are accurate and stable. The damping effect also helps to improve the overall performance and accuracy of the instrument, by reducing the effects of external vibrations or other disturbances.
Advantages of Hot Wire Instrument:
- Suitable for both AC and DC measurements.
- Readings are independent of waveform and frequency.
- Unaffected by stray magnetic fields.
- Simple and inexpensive construction.
- Transfer-type instrument, with the same calibration for both AC and DC measurements.
Disadvantages of Hot Wire Instrument:
- Slow response due to temperature dependent working.
- High power consumption.
- Instability due to stretching of the wire.
- Fragile and unable to withstand mechanical shocks or overloads.
- Slow response
- Consumes more power.