In hot wire instrument, the sensing wire is made of

Question

Examoogle · Accepted Answer

Platinum-iridium

Explanation: In a hot-wire instrument, the sensing element is typically made of a platinum-iridium alloy. This material is chosen because it combines high electrical resistivity with excellent resistance to oxidation and thermal degradation at the high temperatures required for operation.

Working Principle: The instrument operates on the thermal effect of electric current, where the current to be measured passes through a thin wire, causing it to heat up and expand. The expansion of the wire is proportional to the square of the RMS current ($I^2R$), and this linear expansion is translated into a mechanical deflection of the pointer on a calibrated scale. Platinum-iridium is used to ensure the wire can withstand repeated heating and cooling cycles without significant changes in its mechanical or electrical properties over time.

Key Formulas: $H = I^2 R t$ — Heat generated in the wire; $\Delta L = L \alpha \Delta T$ — Linear expansion of the wire

Key Points: Hot wire instruments are suitable for measuring both AC and DC.; The scale of a hot wire instrument is non-linear (square-law scale).; Platinum-iridium has high tensile strength and anti-corrosive properties.; These instruments are generally sluggish compared to moving-coil types.

Advantages: Independent of frequency (suitable for high frequencies); Not affected by stray magnetic fields

Disadvantages: High power consumption compared to other types; Susceptible to ambient temperature changes; Slow response time

Applications: Radio frequency current measurement; Used as a standard in some high-frequency applications

Additional Info: The wire is often kept under tension by a spring to ensure it remains taut during expansion.. Option A (Copper) has low resistivity and poor oxidation resistance. Option D (Copper-Nickel, or Constantan) is often used for shunts, but platinum-iridium is preferred for the sensing element due to superior thermal stability.

Answer: C is correct — Platinum-iridium is the standard alloy used for the sensing wire in hot-wire instruments due to its superior thermal stability, oxidation resistance, and mechanical robustness.

Exam Tip: Always remember that hot-wire instruments follow a square-law scale because deflection is proportional to the heat generated, which follows $I^2R$.

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