Several methods have been developed for measuring heat energy. We know that measure as temperature. The common measures of Celsius and Fahrenheit scales are useful in most applications in normal day-to-day lives. For some understandings, however, the measurement needs to have only positive values. In that case we reference the heat energy with respect to when there is zero heat energy, also known as absolute zero. Absolute zero is measured as zero degrees Rankine or 0°R. It was named after the Glasgow University engineer and physicist Macquorn Rankine.

The divisions of the temperature scales are somewhat easy. Using the Celsius method, the temperature when fresh water freezes is defined as 0°C. The value when fresh water boils is 100°C. Corresponding values for the Fahrenheit scale are 32°F and 212°F. So, there are 100 divisions between freezing and boiling in Celsius. At one point, the scale was known as Centigrade due to that simple set of endpoints. In the Fahrenheit scale, there are 180 divisions between freezing and boiling. In effect, then, the ratio of the units of Celsius to Fahrenheit is 5 to 9. That ratio or fraction is recognized in formulas converting from one scale to another.

Using Rankine for the measuring unit, we note that absolute zero is 0°R, and is also noted as −459.67 °F. Rankine uses the same smaller divisions of temperature as does the Fahrenheit scale. The important advantage is that all temperatures measured in Rankine are positive numbers.

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