A thermometer is a device that is used for measuring the degree of hotness or coldness of the given object. It consists of a glass tube inside which there is a liquid known as mercury. When the bulb of the thermometer is dipped in the given solution or object, the mercury starts to expand. This expansion of mercury is read on the temperature. Depending on the reading, the temperature of the given object or solution can be determined. Therefore, the working principle of thermometer depends on the degree of hotness or coldness of the given solution or object.