The change in temperature causes either expansion or contraction of a substance, and hence a thermometer works on this principal. A liquid-in-glass thermometer is the most frequently used model today. 

The end of this kind of thermometer comes with a bulb that is filled with a liquid like mercury or alcohol. Liquid in a thermometer flows up a narrow tube when the thermometer is put into a hot space. The more you will increase the temperature of the liquid it rises more. Placing of the thermometer in a cold environment causes contraction of the liquid which moves down the tube.

• Digital thermometers constitute another category of thermometers. Currently, digital thermometers include various types of sensors for measuring the temperatures such as thermocouples, RTD’s and thermistor. Thermocouples, on the other hand, produce voltage when heated; while RTDs and thermistors tend to change their resistances upon heating. Thermometer’s electronics translate the sensor’s signal into a digital temperature reading to be output onto a screen.

Here are some examples of different types of thermometers and how they work:

• Liquid-in-glass thermometers: Thermometers of this kind have liquids ranging from mercury to alcohol that expand of contract upon heating and cooling.? Inside this bulb, there are some amount of liquids which expand or contract according to the temperature hence rising or falling with the help of a slim tube. A measurement of temperature in the body fluid is taken by reading the temperature scale which is located on the side of the tube. 
• Digital thermometers: Many varied types of sensor based thermometers exist in the market such as the thermocouples, RTDs, and thermistors ones. There is the case of thermocouple which produces voltage upon heating while RTDs and thermistors only resist with temperature. Thermometer’s electronic will generate an output voltage proportional to the sensor’ s signal and display it on an LCD screen as a temperature.
• Infrared thermometers: The thermometers used in this case detect the infrared radiations coming out of an object, hence measuring the temperature of that object. Infrared radiations are some forms of electromagnetic radiations emitted by all objects having temperatures above absolute zero. Electronics of the thermometer converts the infrared radiation into a temperature value for display on a screen.