Page:International Library of Technology, Volume 93.djvu/35



The result, in each of the three cases, multiplied by $5⁄9$ will be the centigrade temperature; in each of the cases (b) and (c), the temperature will be below zero.

.—What will be the reading on a centigrade thermometer if a Fahrenheit thermometer indicates a temperature: (a) of 60° F.? (b) of 20° F.? (c) of &minus; 20° F.?

—Applying the rule,

16. High-Temperature Mercurial Thermometers. Until recently, instrument makers removed, as far as possible, the air from the thermometer tubes, and left the space above the mercury a nearly perfect vacuum. The boiling point of the mercury was lowered thereby, and, in consequence, mercurial thermometers could not be used for measuring temperatures much in excess of 500° F. Mercurial thermometers are now made, however, that are serviceable and accurate up to 900° F., although the ordinary boiling point of mercury is at 662°. This result is accomplished by filling the space above the mercury with gas under heavy pressure, and thereby raising the boiling point. Great care is necessary in using these instruments at high temperatures in order to avoid breakage.

17. Pyrometers.—Mercurial thermometers cannot be used for the measurement of extremely high temperatures, such as exist in furnaces. The instruments used for work of this character are called pyrometers. Their action usually depends on the known effects of heat on various substances, as, for example, the lengthening of metal rods, the expansion of gases, the contraction of clay blocks, and the change of color of some substances.

18. Heat may be transmitted from one body to another, or from one point to another in the same body, in three ways: by conduction, by convection, and by radiation.