The resistance of the platinum wire of a Platinum Resistance Thermometer at ice point is \( 6\,\Omega \) and at steam point is \( 6.43\,\Omega \). When the thermometer is inserted in a hot bath, the resistance of the platinum wire is \( 6.79\,\Omega \). Temperature of the hot bath is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( \sim 184^{\circ}\mathrm{C} \)
A platinum resistance thermometer uses the linear variation of resistance with temperature, \( R_t = R_0(1+\alpha t) \), so the temperature is found from the rise in resistance compared with the rise from ice point to steam point.
\( R_0 = 6\,\Omega \text{ (ice point, } 0^{\circ}\mathrm{C)} \)
\( R_{100} = 6.43\,\Omega \text{ (steam point, } 100^{\circ}\mathrm{C)} \)
\( R_t = 6.79\,\Omega \)
\( \displaystyle t = \frac{R_t-R_0}{R_{100}-R_0}\times100^{\circ}\mathrm{C} \)
\( \displaystyle t = \frac{6.79-6}{6.43-6}\times100 = \frac{0.79}{0.43}\times100 \)
\( t \approx 183.7^{\circ}\mathrm{C} \approx 184^{\circ}\mathrm{C} \)
Note: NTA picked this question from an NCERT (2026-27) example and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Temperature Dependence of Resistivity – …
