The principle of operation of hot-wire instruments is:

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SSC JE EE Previous Paper 10 (Held on: 10 Dec 2020)
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  1. electrodynamic effect
  2. thermal effect
  3. magnetic effect
  4. chemical effect

Answer (Detailed Solution Below)

Option 2 : thermal effect
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  • The instruments which use the heating or thermal effect of the current for knowing their magnitude such type of instrument is known as the hot wire instrument.
  • The hot wire instrument is used for both the AC and DC current.
  • Hotwire instrument works on the principle of the thermal effect, that the length of the wire increases because of the heating effect of the current flow through it.
  • When the current is passed through the fine platinum-iridium wire it gets heated up and expands.
  • The sag of the wire is magnified, and the expansion is taken up by the spring.
  • This expansion causes the pointer to deflect, indicating the value of the current.
  • This expansion is directly proportional to the heating effect of the current and hence directly proportional to the square of the RMS value of the current.
  • Therefore, the meter may be calibrated to read the rms value of the current.​
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