Definition: The instruments which use the heating effect of the current for knowing their magnitude such type of instrument is known as the hot wire instrument. It works on the principle that the length of the wire increases because of the heating effect of the current flow through it. The hot wire instrument is used for both the AC and DC current.
Construction of the Hot Wire Instrument
The construction of the hot-wire instruments is shown in figure below. The current whose magnitude is used to be determined is passed through the platinum-iridium wire. The hot wire instrument uses two-wire.
The one wire is bonded between the two terminals. And the second terminal is bound between the 1st wire and the 3rd terminal shown in the figure above. The thread passes over the pulley and attaches to the spring. This spring contract the platinum-iridium wire.
Hot Wire Instruments Working
When the current passes through the platinum-iridium wire, it gets heated and expands. The heat increases the sag of the wire. The wire regains their original position with the help of the spring. Expansion and the contraction of the thread rotate the pulley due to which the pointer deflects. The expansion of the thread is equal to the square of the RMS value of the current.
Advantages of the Hot Wire Instrument
Following are the advantage of the hot wire instrument.
- The instrument is used for both AC & DC measurement.
- It is a transfer-type instrument i.e, the calibration is the same for both the AC and DC measurement.
- The hot wire instrument is free from the stray magnetic field.
- Their construction is very simple and cheap.
Disadvantages of Hot Wire Instrument
Following are the disadvantages of the hot-wire instrument.
- The Hot Wire instrument gives a slow response.
- The instability occurs in the instruments because of the stretching of the wire.
- The instrument consumes more power.
- The instrument is not able to withstand overload and mechanical shock.
Because of the above mention disadvantage, the instruments are replaced by Thermo-electric instruments.