Temperature Measurement

An Informational Web Site

Last Updated: November 25, 1998

Why do we have so many different temperature scales?

Temperature conversion is a fact of life. The temperature scales used today came about from the work of scientists in the field of thermodynamics. The two main common scales are the Fahrenheit scale and the Celsius (Centigrade) scale. On this page we present the origins and reasons for the existance of the various temperature scales. Learn about the terms below.

Thermodynamics and Temperature
Fahrenheit
Rankine
Celsius
Kelvin
Reaumur
Converting Between Temperature Scales
Information on this Web Site

Thermodynamics and Temperature

Thermodynamics is the science of the ralationship between heat, work and the properties of thermodynamic systems. All things contain some heat because all molecules are in constant motion. The faster this molecular motion is, the hotter the object is. At absolute zero all molecular motion stops. This is the definition of absolute zero. There are two ways to quantify heat:

Heat is energy. The amount of heat energy contained in an object is measured in calories (metric) or british thermal units (english). We can also measure the heat in an object in units of mechanical energy such as jules (metric) or foot-pounds (english). The quantity of heat in an object cannot be measured directly. It must be calculated. The heat energy contained in an object will depend on its size and its capacity to hold heat energy.

Temperature is the measure of the heat intensity and has nothing to do with the size or heat capacity of an object. Temperature is measured in degrees (metric or english or any other system of measurement). The temperature of an object can be measured directly with a thermometer.

Fahrenheit

In 1714 Gabriel Fahrenheit built the first mercury thermometer. Mercury is what is used in most thermometers today but up until Fahrenheit most people had used alcohol or water, both of which had a limited range due to their boiling and freezing points. Mercury has a very wide range between boiling and freezing points. Mercury boils at about 674ºF and freezes at about -38ºF. Fahrenheit wanted to calibrate his thermometer scale according to two fixed points. He chose as one of the points the lowest temperature he could achieve in the lab, the temperature of a mixture of ice and salt. As the other point he chose his own body temperature. He had read from the works of Sir Isaac Newton that the body temperature of all well persons was the same.

Another suggestion he took from Newton was that the number of divisions on the scale between the two points should be 12 to conform with 12 inches to the foot. So Fahrenheit set zero as the temperature on an ice/salt mixture and 12 degrees (12º) as his own body temperature.

Fahrenheit soon discovered that 12 divisions between the two set points was not fine enought for good measurement so he doubled the divisions to 24. These divisions were also not fine enough so he doubled the divisions again to 48. Even this was not fine enough for precise measurement so he doubled them one last time to 96.

Those of us who grew up using the Fahrenheit scale are aware that the average body temperature is 98.6ºF (37ºC). It turns out that Gabriel Fahrenheit's normal body temperature was lower than the average person.

On the Fahrenheit temperature scale water freezes at 32º and boils at 212º.

Rankine

William John Macquorn Rankine is responsible for the Rankine thermodynamic temperature scale. This scale starts at absolute zero with 0ºR and uses Fahrenheit degrees for divisions on the scale. Water freezes at 491.69º Rankine.

The Rankine scale is virtually never used in scientific research today due to the world wide adoption of the SI system of units (i.e. metric). However, in engineering in the U.S. it is used widely because engineering units are still in the english system.

Celsius

The Swedish astronomer Anders Celsius conceived of the Celsius temperature scale, sometimes called the Centigrade scale. It has only recently been renamed the Celsius scale in his honor.

Celsius used two fixed reference temperatures for defining his temperature scale, the boiling and freezing temperatures of water. He then divided the scale between these two references into 100 equal parts. He originally set 0º to be the temperature of boiling water and 100º to be the freezing temperature of water. Luckily, some friends convinced him it should be the other way around so water now freezes at 0º Celsius and boils at 100º Celsius. Astronomers are really strange people. Want more information on Celsius? Try the Origin of Celsius.

Kelvin

William Thomson developed the concept of absolute zero. Absolute zero is defined as the temperature at which all molecular motion stops. From extrapolation he concluded that absolute zero was -273º Celsius. The modern figure for absolute zero is -273.18º Celsius. Thomson also proposed using a new temperature scale starting at zero at absolute zero and using Celsius degrees. He called the new scale the Kelvin temperature scale after his royal title, Lord Kelvin. The Kelvin scale is used today in virtually all thermodynamic research.

Reaumur

Rene Reaumur was a physicist who, in 1731, constructed a thermometer which was graduated so that the freezing point of water was 0º and the boiling point of water was 80º. The Reaumur scale, I am told, is still used in some parts of Europe.

Converting Between Temperature Scales

Since there are so many different temperature scales we are sometimes called on to convert the temperature from one scale to another. Converting between a common temperature scale and its corresponding thermodynamic temperature scale is just a matter of subtraction or addition. Converting Celsius to Kelvin, for example, is simply a matter of adding 273.18. Converting between common temperature scales, however is not that simple.

Converting from Celsius to Fahrenheit, for example, takes a bit of work. Between the freezing temperature of water and the boiling temperature of water there are 100 Celsius degrees. Between these temperatures there are 180 Fahrenheit degrees (212 - 32 = 180). To convert Celsius degrees to Fahrenheit degrees we must multiply by the ratio 180/100 or 9/5. This would be sufficient except for the fact that water freezes at 32º Fahrenheit instead of zero. So after we multiply by the ratio 9/5 we must add 32 to get the true Fahrenheit temperature. When converting the other way (from Fahrenheit to Celsius) we must remember to subtract 32 before multiplying by the ratio 5/9.

Simple Windows programs to do temperature conversions are available from Simple Software Solutions, P.O. Box 1658, Lawrenceville, Georgia 30046. Just send you name and address, $5.00 for copying, postage and handling, and state that you want the temperature conversion programs. Included with the temperature conversion programs is a Windows Help file containing the information on this web site.

Who wrote this web site anyway?

This site was written by David C. Swaim II, Ph.D., and is my first attempt at writing a web page. I hope you find the web page informative. To get even more details on the history of temperature measurement, check out www.unidata.ucar.edu.