How a thermometer measures temperature

The principle of measuring temperature is different for each kind of thermometer. First look at what kind of thermometer it is and then analyze it in detail. General principle of classification is as follows: 1) the principle of heat bilges cold shrink, water thermometer, bimetallic thermometer etc. 2) the principle of resistance with the temperature change of the object, such as sensors is thermal resistance, thermal resistor type of thermometer, such as digital thermometer measure body temperature, refrigeration thermometer etc. 3) using the principle of different objects of different electric potential with the temperature change, such as sensor is thermocouple thermometer, like molten steel measuring thermometer, etc.