Page:Elementary Text-book of Physics (Anthony, 1897).djvu/206

193 the air-thermometer is taken as the standard instrument for scientific purposes. Its use, however, involves several careful observations and tedious computations. It is therefore mainly employed as a standard with which to compare other instruments. If we make such a comparison, and construct a table of corrections, we may reduce the readings of any thermometer to the corresponding readings of the air-thermometer.

163. Limits in the Range of the Mercurial Thermometer.—The range of temperature for which the mercurial thermometer may be employed is limited by the freezing of the mercury on the one hand, and its boiling on the other. For temperatures below the freezing-point of mercury alcohol thermometers may be used. For the measurement of high temperatures several different methods have been employed. One depends upon the expansion of a bar of platinum, another upon the variation in the electric resistance of platinum wire, another upon the strength of the electric current generated by a thermo-electric pair, another on the density of mercury vapor.

The special devices used in applying these methods need not be considered here.

164. Unit of Heat.—It is evident that more heat is required to raise the- temperature of a large quantity of a substance through a given number of degrees than to raise the temperature of a small quantity of the same substance through the same number of degrees. It is further evident that the successive repetition of any operation by which heat is produced will generate more heat than a single operation. Heat is therefore a quantity the magnitude of which may be expressed in terms of some unit. The unit of heat generally adopted is the heat required to raise the temperature of one kilogram of water from zero to one degree. It is called a calorie.

It is sometimes convenient to employ a smaller unit, namely, the quantity of heat necessary to raise one gram of water from zero to one degree. This unit is designated as the lesser calorie or the