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In hot wire instrument, the sensing wire is made of
Copper
Silver
Platinum-iridium
Copper-Nickel
Platinum-iridium
Quick Summary: 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.
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.
H=I2Rt тАФ Heat generated in the wire
╬ФL=L╬▒╬ФT тАФ Linear expansion of the wire
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 (I2R), 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.
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.
Independent of frequency (suitable for high frequencies)
Not affected by stray magnetic fields
High power consumption compared to other types
Susceptible to ambient temperature changes
Slow response time
Radio frequency current measurement
Used as a standard in some high-frequency applications
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.
C is correct тАФ 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.
Always remember that hot-wire instruments follow a square-law scale because deflection is proportional to the heat generated, which follows I2R.