# NEET Physics Thermometry, Calorimetry & Thermal Expansion Thermometry

## Thermometry

Category : NEET

A branch of science which deals with the measurement of temperature of a substance is known as thermometry.

(1) The linear variation in some physical properties of a substance with change of temperature is the basic principle of thermometry and these properties are defined as thermometric property $(x)$ of the substance.

(2) Thermometric properties $(x)$ may be as follows

(i) Length of liquid in capillary

(ii) Pressure of gas at constant volume.

(iii) Volume of gas at constant pressure.

(iv) Resistance of a given platinum wire.

(3) In old thermometry, freezing point $({{0}^{o}}C)$ and steam point $({{100}^{o}}C)$ are taken to define the temperature scale. So if the thermometric property at temperature ${{0}^{o}}C,\,{{100}^{o}}C$ and ${{t}^{o}}C$ are ${{x}_{0}},\,{{x}_{100}}$ and $x$ respectively then

$\frac{t-0}{100-0}=\frac{x-{{x}_{0}}}{{{x}_{100}}-{{x}_{0}}}$ Þ $t{}^\circ C=\frac{x-{{x}_{0}}}{{{x}_{100}}-{{x}_{0}}}\times 100{}^\circ C$

(4) In modern thermometry instead of two fixed points only one reference point is chosen (triple point of water 273.16 K) the other is itself 0 K where the value of thermometric property is assumed to be zero.

So if the value of thermometric property at 0 K, 273.16 K and TK are $0,\,\,{{x}_{Tr}}$ and $x$ respectively then

$\frac{T}{273.16}=\frac{x}{{{x}_{Tr}}}$  $\Rightarrow$ $T=273.16\,\left[ \frac{x}{{{x}_{Tr}}} \right]\,K$

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