Principle of thermometer:
The temperature of a substance can be measured by measuring
(1) increase in volume of a gas on heating at constant pressure
(2) increase in pressure of a gas on heating at constant volume
(3) increase in resistance of a conductor on heating.
Gas thermometer, platinum resistance, thermometer, vapour pressure thermometers are not ideal for measuring changing temperatures.
Thermocouple Thermometer:
A thermocouple thermometer is formed by joining two dissimilar metals and kept at different temperatures and is ideal for measuring rapidly changing temperatures.