The thermometer is an instrument used to measure the temperature of objects accurately and express it in numbers. A thermometer usually consists of a hollow glass tube containing a liquid substance (alcohol or mercury). The top of the thermometer is the fluid that fills the vacuum.
The thermometer is based on the principle that the volume of liquid will change when heated or cooled. The volume of liquid will increase when heated, while the volume of liquid when cooled will be reduced. Rise or fall in the liquid used as a reference for determining the temperature of an object.
There are two kinds of liquids that are commonly used as material for filling the thermometer. The substance is alcohol and mercury. (image source : http://arifansyah.wordpress.com/)