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Three scales of temperature, each of which permits a precise measurement, are in current use: the Fahrenheit, Celsius, and Kelvin scales. These three different temperature scales were each developed by different people and have come to be used in different situations.

The scale that is most widely used by the general public in the United States is the Fahrenheit scale. In 1714, Daniel Gabriel Fahrenheit, a German physicist who was living in Holland and operating an instrument business, developed a thermometer and the temperature scale that still carries his name. His original scale had two fixed points: 0º was the lowest temperature and 96º was what he believed was the normal temperature of the human body. Based on this scale, he calculated that the freezing point of water was 32º; in later studies, it was determined that the boiling point of water was 212º. The Fahrenheit scale came to be accepted as the standard measure of temperature in a number of countries. Today, however, the United States is the only major country in the world that still uses the Fahrenheit scale.

The scale that is in use in many other countries is the Celsius scale. Anders Celsius, a Swedish astronomer, developed a thermometer in 1741 that based temperatures on the freezing and boiling temperatures of water. On the thermometer that Celsius developed, however, 0º was used to indicate the boiling temperature of water, and 100º was used to indicate the freezing temperature of water. After his death, the scale was reversed by a friend, the biologist Carl von Linne. On the new scale after the reversal by von Linne, 0º indicated the freezing temperature of water, and 100º indicated the boiling temperature of water. At around the same time, a similar thermometer was being developed in France. After the French Revolution, the scale developed in France was adopted as part of the metric system in that country under the name centigrade, which means "a hundred units," and from there it spread worldwide. In 1948, an international agreement was made to rename the centigrade scale the Celsius scale in honor of the scientist who was first known to use a 100-degree scale, though it should be remembered that the scale that Celsius actually used himself was the reverse of today's scale.

A third scale, the Kelvin scale, is generally used today for scientific purposes. This scale was first suggested in 1854 by two English physicists: William Thomson, Lord Kelvin and James Prescott Joule. The Kelvin scale defines 0º as absolute zero, the temperature at which all atomic and molecular motion theoretically stops, and 100º separates the freezing point and boiling point of water, just as it does on the Celsius scale. On the Kelvin scale, with 0º equal to absolute zero, water freezes at 273º, and water boils at a temperature 100º higher. The Kelvin scale is well suited to some areas of scientific study because it does not have any negative values, yet it still maintains the 100º difference between the freezing point and boiling point of water that the Celsius scale has and can thus easily be converted to the Celsius scale by merely subtracting 273º from the temperature on the Kelvin scale.

(Adapted from TOEFL Reading Practice by Deborah Philips)

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