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Thermometers.-There are two fixed points on all thermom. eters, viz., the freezing and the boiling. On the Fahrenheit thermometer, the distance between the two fixed points is divided into 180 equal parts, or degrees. The freezing point is marked 32° and the boiling point 2120; 32 parts are marked off from the freezing point downwards, and the last one is marked 0°, or zero. The graduations are carried above the boiling point and below the zero point as far as desired.
In graduating a centigrade thermometer, the freezing point is marked 0°, and the boiling point 100°; the distance between the freezing and boiling points is divided into 100 equal parts; these equal divisions are carried as far below the freezing point and above the boiling point as desired.
In Russia and a few other countries another thermometer is used, called the Réaumur; the freezing point is marked 0°, or zero, and the boiling point 80°, the space between these two points being divided into 80 equal parts.
The abbreviations for these scales are: Fahrenheit, F.; centigrade, C; and Réaumur, R. The reading on one scale may be converted into a corresponding reading on either of the other scales by the following formula:
= R + 32
C + 32
Absolute Zero.-It has been found by experiment that all perfect gases will expand of their volume when heated from zero to 10 above it. It is inferred, therefore, that the ultimate limit of contraction will be found at 460° below zero on the Fahrenheit scale, and that at this point all motion of the molecules ceases. This point is called the absolute zero, and temperatures measured therefrom are called absolute temperatures.
The temperature that is indicated by the thermometer may be converted into absolute temperature by adding it to 460°. Thus, a temperature of 85° by the thermometer
corresponds to the absolute temperature of 85 + 460 = 545°. On the centigrade scale the absolute zero is 2731° below the zero point. On the Réaumur scale it is 219° below zero. When the thermometer indicates temperatures below the zero point of its graduation, the indicated temperature must be subtracted from 460, 2731, or 219, respectively, to find the absolute temperature.
TRANSMISSION OF HEAT.
Heat is transmitted by radiation, by conduction, and by convection.
The term radiation is commonly used to signify the transmission of heat through space unoccupied by tangible forms of matter, or through matter unaffected by such transmission of heat. For example, heat may be transmitted, by radiation, through ice without causing it to melt, none of the heat being absorbed until it reaches some heat-absorbing surface.
When the transmission of heat through any substance takes place without causing appreciable motion; i. e. circulation of the affected particles of the heated body, the manner of transmission is called conduction, which term, as a rule, is applied to the transmission of heat through solids.
The diffusion of heat through a liquid or gas by motion, or circulation, of its constituent particles is termed convection. Convection currents caused by the application of heat at the bottom of a vessel containing a liquid, such as water, are due to the expansion and consequent decrease in density of the lower particles which become lighter when heated and ascend because of their buoyancy, the rising particles being replaced by descending currents of colder particles. If the heat is applied to the surface of the liquid, little convection will occur; being confined to the upper portion of the liquid, the heat will be conducted downwards through the liquid, without motion, in the same manner as though the liquid were a solid substance. The diffusion of heat throughout a liquid may be greatly facilitated by convection if the heat is applied to the lower part of the mass. When in motion each heated particle comes into successive contact
with a great number of colder particles, to which its heat is conducted by actual contact. Air and other gases must be heated mainly by convection.
The transmission of heat through a body may be divided into three phases: (1) the absorption of the heat at the receiving surface; (2) the conduction through the substance of the body; (3) the emission from the radiating surface.
All metals conduct heat much faster than they can either absorbit at, or emit it from, their surfaces, and hence a knowledge of their actual conducting power is not so valuable or essential in the arts of heating and ventilation as a knowledge of their transmitting power.
EXPANSION OF BODIES BY HEAT.
If a body absorbs heat, its volume will be changed correspondingly. Nearly all bodies expand when heated: a few substances, however, contract, but these exceptions are of no practical importance.
Air and all other gases expand uniformly for each degree of rise in temperature above zero. Air, at zero F., will expand z of its volume for each degree of rise in temperature. Thus, air at 70° will have a volume equal to 1 + 7%, or 8 of its volume at zero, if its tension remains unchanged. By tension is meant the pressure that a gas exerts on the vessel that confines it.
If metallic bodies are heated above a certain temperature, varying for different metals, and the heat is continued for any considerable length of time, the metal will become permanently elongated, and upon cooling will not contract to its original dimensions. The metal is then said to be swelled. Thus, grate bars in a furnace, or pipes which are exposed to intense heat, will increase considerably in length during long use. The strength of the material deteriorates at the same time. Thus, plates or other parts of furnaces which are unduly heated will swell permanently, and bulge or crack the adjoining parts.
The linear expansion, or extension, of metals for 1° rise of temperature is given in the following table:
COEFFICIENT OF EXPANSION FOR A NUMBER OF
Name of Substance.
EXAMPLE.-A wrought-iron bar 22 ft. long is heated from
70° to 300°. How much will it lengthen?
22 X (300-70) × .00000686 = .0347116 ft. ==
HEAT CONTAINED IN AIR.
If a current of hot air, of a given volume of flow per minute, is cooled, the quantity of heat given off in the process may be computed by the following rule:
Rule.-Multiply together the given volume of the air, the number of degrees through which it is cooled, and the amount of heat contained in 1 cu. ft. of air at the original temperature, as shown in the accompanying table. If this product be divided by the original temperature, the quotient will be the amount of heat given off, in heat units.
EXAMPLE.-A current of hot air having a temperature of 150° and a volume of 400 cu. ft. per min. is cooled, in passing through a room, to 65°; what amount of heat is given off per minute?
From the table, the heat contained in 1 cu. ft. at 150° is 2.3196 B. T. U. Applying the rule just given, 400 X (15065) X 2.3196 150
=525.78 B. T. U. Ans.
PROPERTIES OF AIR AT PRESSURE OF 14.7 LB. PER
B. T. U. Given Up by 1 Cu. Ft. of Air in Cooling to
EXAMPLE.-How much heat will be required to warm 400 cu. ft. of air to a temperature of 150°, the temperature out-ofdoors being 10°?
SOLUTION. The weight of 1 cu. ft. at 10° is .08451 lb. Hence, the number of B. T. U. required to heat 400 cu. ft. from 10° to 150° is
U = .23751 X .08451 (150-10) X 400 1,124 B. T. U. Ans.