A thermometer is an instrument that measures heat. The way we measure heat is by observing what heat does to certain materials. Heat causes certain materials to expand or contract.
Daniel Gabriel Fahrenheit is the German physicist who invented the mercury-in-glass thermometer. These thermometers rely on the principle that liquid changes its volume in accordance with the temperature. Liquids take up more space when they are hot and less space when they are cold. The reason mercury is used so commonly in thermometers is simply because mercury reacts quickly to a rise in temperature. It expands evenly and is easily seen.
Temperature is measured in degrees. On the Celsius scale the difference between the freezing and boiling points of water is 100 degrees. This is why it is also called the centigrade scale. On the Fahrenheit scale, the zero point represents the temperature of a mixture of ice and salt. This was the lowest temperature achievable when this scale was created in the early 1800s. The freezing point of water on this scale is 32 and the boiling point is 212.
The equation to convert Celsius to Fahrenheit is F = (9/5)C + 32. To convert Fahrenheit to Celsius, the equation is C = 5/9(F - 32).