by Mike Wendling

"When you can measure what you are speaking about and express it in numbers, you know something about it."

Lord William Thomson Kelvin (1824-1907)

 Lord William Thomson Kelvin

There are two requirements for taking a measurement of something. The first is a tool for taking a measurement. The second is scale for making sense of the numbers of the measurement. For example, a ruler is often used to measure short lengths. It is the tool for measurement. On the ruler are one or more number scales with equally spaced numbers. These numbers can be compared with numbers from any other ruler that is accurately set to the same scale.

Measuring length is far simpler than measuring temperature. While there is evidence of tools for measuring length at various times in human history, tools and scales for measuring temperature do not appear until more recent human history.

Early thermometers, called thermoscopes, first appear in the 1500's. They were crude instruments that were not at all accurate. Most did not even have a number scale associated with them. This made them useless for most practical purposes.

 Early thermometers were crude instruments that were not at all accurate.

Gabriel Fahrenheit created the first accurate thermometer in 1714, and the Fahrenheit temperature scale followed it in 1724. The thermometer's accuracy was based on its use of mercury, a silver colored substance that remains liquid over a wide range of temperatures but expands or contracts in a standard, predictable way with changes in temperature.

To set the scale, Fahrenheit created the coldest temperature that he could. He mixed equal parts of ice, water, and salt, and then used this as the zero point, 0 degrees, of his scale.

He intended to make 30 degrees the freezing point of water and 90 degrees the temperature of the human body, but he had to later revise these temperatures to be 32 degrees and 96 degrees. In the final version of the scale, the temperature of the human body became 98.6 degrees.

In 1742, Anders Celsius recommended that the scale on the mercury thermometer be adjusted so that 100 degrees occurred at the freezing point of water and 0 degrees occurred at the boiling point of water. The range between the boiling and freezing points were divided into 100 equal parts. For this reason, the scale was first called the Centigrade scale ('centi' being the prefix for one hundredth). He also made the measurement of those key points more precise.

The scale did not become truly popular until after Celsius's death when the measurements for the freezing and boiling points were switched. 0 degrees was set at the freezing point of water, while 100 degrees became the boiling point of water.

 Anders Celsius

In 1948, the Centigrade scale became officially known as the Celsius scale in honor of its creator.

The Celsius scale is the most commonly used temperature scale in the world today, but it is not the best scale for every use. Certain formulas used to predict weather do not work if the number for temperature is a negative number. Since the temperature of the air can easily be lower than the freezing point of water, scientists need a different scale.

The scale they use is called the Kelvin scale, named for Lord William Thomson Kelvin who created it in the mid-1800's. Kelvin was able to calculate the coldest that anything in the universe could ever be. This became the zero point of his scale. 0 degrees Kelvin is -273 degrees on the Celsius scale. It is also the temperature at which molecules and atoms have the least possible motion. For this reason, 0 degrees Kelvin is also called "absolute zero." There can be nothing colder.