Wednesday, May 30, 2007

Hot And Cold

Comparatively few people feel absolutely at home with both Fahrenheit and Celsius ('centigrade') temperature scales. Most of us, hearing a temperature expressed in the scale that is less familiar to us, will mentally convert the figure into the one we know better. But imagine how confusing things must have early in the 18th century - when at least 35 different measures of temperature were in use.

It was not until 1714, when the German-Dutch instrument maker Gabriel Daniel Fahrenheit made the first really efficient thermometer using mercury in a sealed tube, and created his own scale, that a single measure of temperature came into common use.

Fahrenheit started with the coldest thing he knew of - a mixture of ice and salt. This he marked on his thermometer as 0 degree. Next, he measured the temperature of the healthy human body. He had originally intended to divide his scale between this point and his zero into only 12 degree, but the mercury in his thermometer moved much further up the tube than he had expected. To avoid such large, unwieldy units he decided his scale needed eight times as many divisions. So, instead of assigning a value of 12 degree to body temperature, as he had first intended, Fahrenheit called body temperature 96 degrees (8 x 12). The precise figure is 98.6 degree on Fahrenheit's scale (37 degree C) - but small variations in the bore of the tube caused his thermometer to show a lower reading.

Fahrenheit next measured the freezing and boiling points of pure water, which came to 32 degree and 212 degree respectively. A scale based on the freezing and boiling temperatures of water had been proposed as long ago as the 2nd century AD, by Galen (Jaalinoos in Hindi), a Greek physician. Fahrenheit realised that these two temperatures are ideal reference points, because they are constant at a given pressure. His scale of temperature quickly became popular, particularly in English-speaking countries.

But it was soon followed by a rival. In 1742, the Swedish astronomer Anders Celsius proposed a scale on which water boiled at 0 degree and froze at 100 degree. (This system was reversed after his death in 1744.)

When, at the end of the 18th century, France introduced the decimal metric system of measures, Celsius's 'centigrade' scale found a natural home within it. It soon became the standard temperature scale for all scientific work, and is used in countries that have adopted the metric system. Fahrenheit, however, is still used in many English-speaking countries.

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