Page:Handbook of Meteorology.djvu/190

 Three scales of degree measurements are more or less in use. In the Reaumur scale, now rarely used, the expansion is divided into 80 parts; in the centigrade, into 100 parts; in the Fahrenheit, into 180 parts. In the Reaumur and the centigrade scales the zero, 0°, of temperature is at the melting point of ice; in the Fahrenheit, 32 degrees below it. The centigrade scale is used chiefly in Continental Europe. The degree values are inconveniently large and, in many cases, fractional units of the degree must be expressed. Winter temperatures require the use both of positive and negative quantities, and this adds to the labor of computation and to the likelihood of error.

Absolute Temperature.—The fact that neither the centigrade nor the Fahrenheit scale per se expresses the relation of the volume of a gas to its temperature has led to the establishment of a theoretical absolute zero of temperature. The following demonstration and the accompanying cut explain the method of its determination. A glass tube about 50 inches long and closed at one end contains a free-moving piston of mercury, resting about 20 inches from the open end of the tube. The tube is first placed in a container filled with melting ice. When



The natural zero deduced from an investigation of the pressure of a gas at constant volume has the same scale value as the absolute zero. It is inferred, in consequence, that the absolute or natural zero is the temperature at which molecular motion ceases.