For accurate readings, thermometer must be much smaller than the system, so that the energy the thermometer gains or loses does not significantly alter the energy content of the system.
Thermometer can be calibrated by placing them in thermal contact with environments that remain at constant temperature. Eg. Pure melting ice(0°C) and pure boiling water (100°C) at 1 ATM.
Problem with centigrade thermometers
- Unable to give accurate readings.
- Assumption that all physical properties vary linearly with temperature is not always true. E.g. mercury and alcohol have different thermal expansion properties which cause them to display different readings in the same temperature.
Solution: Temperature scale independent of the physical properties of any substances $\rightarrow$ Thermodynamic scale (absolute temperature scale) $\rightarrow$ Gas thermometer (constant-volume thermometer)
Temperature readings are nearly independent of the type of the gas used when gas pressure is low and temperature is high.
Pressure of gas varies linearly with temperature.
Pressure extrapolates to zero when temperature is -273.15°C.(Absolute zero) $\rightarrow$ all particle motion cease (minimum internal energy).