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Hot Wire Instrument
The instruments which use the heating of the current for knowing their magnitude such type of instrument is known as the hot wire instrument. It work 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 AC & DC current.
Construction of Hot Wire Instrument
The constructional features of a hot wire type instrument are shown in fig. The current to be measured is passed through a fine platinum iridium wire. The wire is stretched between two terminals.
A second wire is attached to the fine wire at one end & to a terminal at the other end. A thread is attached to the second wire. This thread passes over a pulley & is fixed to a spring.
Working of Hot Wire Instrument
When the current is passes through the fine wire is gets heated up & expands. The sag of the wire is magnified & the expansion is taken up by the spring. This causes the pulley to rotate & the pointer to deflect, indicating the value of the current. The expansion is proportional to the heating effect of the current & hence to square of the RMS value of the current. Therefore, the meter may be calibrated to read the RMS value of the current.
Advantages of Hot Wire Instrument
- The instrument is used for both AC & DC measurement.
- Their construction is very simple & cheap.
- The hot wire instrument is free from the stray magnetic field.
- It is a transfer-type instrument i.e., the calibration is same for both the AC & DC measurement.
Disadvantages of Hot Wire Instrument
- The hot wire instrument gives the slow response.
- The instrument consumes more power.
- The instability due to stretching the wire.
- Non uniform scale.
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