Thermometer measures temperature in all the three states. Solid, liquid and gas. Celsius, Fahrenheit and Kelvin are the most common units of measurement.
Celsius scale is widely used by almost all the countries in the world. Physicists and scientists uses Kelvin for more accuracy and only few countries like United States uses Fahrenheit.
Liquid and Electronic thermometers are the commonly used ones.
"Liquid expands when exposed to heat" is the theory behind the functioning of this thermometers.
Mercury is the most common liquid material in thermometers. Kerosene or Ethanol may also be used.
When the thermometer is heated, the liquid is pushed up the glass tube. Temperature is measured with accurate markings on the tube.
The drawback in liquid thermometers is when the temperature lowers, the liquid cannot fall back easily.
Electronic thermometer is built with an instrument called thermistor.
Thermistor(thermal+resistor) changes its resistance based on the temperature. A computer in turn measures the thermistor's resistance and converts it into temperature reading. The accurate temperature is displayed on the digital screen.
Pyrometers: To measure very high temperature. Ex. Steel industry.
Cyrometers: To measure very low temperature. Ex. Space.
Pill thermometers: Athletes intake to prevent themselves from heat related illnesses like heat cramps, heat stroke etc.,
Stem thermometers:To monitor food safety temperatures.
Bi-metallic thermometers: To maintain cooking and baking temperature in the oven.
Nanothermometers: To measure temperature variations in each living cell.