hotwire instrument1

Hotwire instrument

Hotwire instrument is one of the types of electrothermal instrument, another one is thermocouple instrument.

Both the hotwire instrument and thermocouple instrument use the heating effect of current flowing through resistance to cause meter deflection. Since the operation of both of the instruments depends only on the heating effect of current flow, they may be used to measure both direct and alternating current.

What is hotwire instrument?

An instrument that shows the magnitude of the measuring current because of the heating effect of the current flowing through a wire is known as hotwire instrument.


The current passes through the wire which is made up of platinum or iridium. There is a thread attached to the wire and spring on the other end. The thread is also connected to a pulley which is connected to a pointer.

Working of hotwire instrument

In this instrument, deflection in the pointer depends upon the expansion of the wire because of the heating effect of the current.

When current passed through high resistance wire, it gets heated, and due to which it expands. This expansion causes a sag in the wire. the thread connected to the wire deflects the pulley, which deflects the pointer connected to it. And the spring connected to the other end helps the wire to regain its original position.

After the current stops flowing through the wire, it contracts itself or regains its original form. The spring releases the thread and the thread that is connected to the pulley deflects the pointer to regain null position or its original position.

This is how the hotwire instrument works by the expansion and contraction of the wire because of the heating effect of the current.

Advantages of hotwire

  • It can be used for AC as well as DC i.e. the deflection depends upon the RMS value of the current flowing through the wire irrespective of its waveform and frequency
  • It is very cheap
  • The construction of this instrument is very simple
  • Hotwire instrument is a transfer type instrument because the calibration is the same for AC as well as DC.
  • It is free from the stray magnetic field because no magnetic field is present during the operation

Disadvantages of hotwire

  • Power consumption by this instrument is high
  • This instrument has a very  slow response because of the time taken in heating the wire
  • It has a non-uniform scale.

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