Temperature

In 1714, the German physicist Daniel Gabriel Fahrenheit (1686-1736) made a better choice by selecting liquid mercury. Mercury has a uniform volume change with temperature, a lower freezing point and higher boiling point than water, and does not wet glass. Mercury thermometers made possible the development of reproducible temperature scales and quantitative temperature measurement. Fahrenheit first chose the name " degree" (Grad, in German) for his unit of temperature. Then, to fix the size of a degree (°), he decided that it should be of such size that there are exactly 180° between the temperature at which water freezes and the temperature at which water boils. (180 is a "good" number because it is divisible by one and by 16 other whole numbers. That is why 360, or 2 × 180, which is even better, was originally chosen as the number of "degrees" into which to divide a circle.) Fahrenheit now had a size for his degree of temperature, but no standard reference values. Should the freezing and boiling points of water be called zero and 180? Or 180 and 360, or something else? In other words, where shall 0° fall on the scale? He eventually decided to fix zero at the coldest temperature that he could make in his laboratory by mixing ice with various salts that make it colder. (Salts, when mixed with cold ice, lower the melting point of ice, so that when it is melting it is at a lower temperature than usual.) When he set his zero at that point, the normal freezing point of water turned out to be 32° higher. Adding 180 to 32 gave 212° for the normal boiling point of water. Thus, freezing water falls at 32° and boiling water falls at 212° on the Fahrenheit scale. And the normal temperature of a human being turns out to be about 99°.