Physics Part 29

d. Iron tires are heated, slipped on to wagon wheels and then cooled, the contraction on cooling setting them tightly in place.

e. Metallic thermometers depend upon the movement due to the expansion of a coiled strip of metal which turns a pointer on the dial of the instrument. (See Fig. 129.)

f. The wires that are fused into gla.s.s in incandescent light bulbs must have the same coefficient of expansion as the gla.s.s. Platinum has therefore been used for this purpose. (See table above.)

[Ill.u.s.tration: FIG. 129.--Metallic thermometer.]

Important Topics

1. Expansion of Liquids; peculiarities. Anomalous expansion of water and its results.

2. Expansion of solids; peculiarities, applications.

3. Coefficient of linear expansion.

4. Coefficient of cubical expansion.

Exercises

1. The gas within a partly inflated balloon has a volume of 1000 cu. ft.

at a pressure of 74 cm., and a temperature of 15C. What will be the volume of the gas when its pressure is 37 cm. and the temperature is -17C.?

2. A man taking a full breath on the top of a mountain fourteen thousand feet high inhales 4 liters of air, the pressure being 40 cm. What volume would this same ma.s.s of air have in a place 600 ft. above sea-level when the barometer reads 75 cm. and the temperature is the same as on the mountain top?

3. If the coefficient of linear expansion of iron is 0.000012 per degree C., how much will an iron bridge 1000 ft. long change in length in warming from -20C. on a winter day to 30C. upon a summer day.

4. What are some of the results that would follow in freezing weather if water continually contracted on being cooled to zero instead of beginning to expand when cooled below 4C.?

5. Mention two instances that you have noticed of expansion occurring when a body is heated?

6. Compare the density of air at 30C. with that at 10C. at the same pressure. If both are present in a room, where will each be found? Why?

7. Compare the density of water at 40C. with that at 10C. If water at the two temperatures are in a tank, where will each be found? Why?

8. If water at 0C. and at 4C. are both in a tank, where will each be found? Why?

9. How much heat will be required to raise the temperature of a cubic foot of water 10F.?

10. How much heat will be required to raise the temperature of 4 liters of water 25C.?

11. How much longer would the cables of the Brooklyn suspension bridge be on a summer's day when the temperature is 30C. than in winter at -20C., the length of cable between the supports being about 1600 ft.

12. If 25 liters of air at -23C. is warmed to 77C. under constant pressure, what will be the resulting volume of air? Explain.

13. White pig iron melts at about 2000F. Express this temperature upon the centigrade and absolute scales.

14. If 200 ccm. of air at 76 cm. pressure and 27C. temperature be heated to 127C. at a pressure of 38 cm. what will be the resulting volume?

15. A balloon contains 10,000 cu. ft. of gas at 75.2 cm. pressure and 24C. It ascends until the pressure is 18 cm. and the temperature is -10C. What is the volume of gas it then contains.

16. A gas holder contains 50 "cu. ft." of gas at a pressure of one atmosphere and 62F. How much gas will it hold at 10 atmospheres and 32F.

17. One thousand "cubic feet" of illuminating gas has what volume with 75 lbs. pressure and temperature of 10C.

18. Define a "cubic foot" of illuminating gas.

=150. Methods of Transmitting Heat.=--One of the most practical benefits of the study of heat is clearer understanding of the different methods by which heat is transferred from one place to another and an intelligent idea of the means employed to prevent the transfer of heat.

It should be definitely understood at the beginning that _cold signifies the absence of heat_, just as darkness implies the absence of light, so when one speaks of cold getting into a house what is really meant is either the entrance of cold air by some opening or else the escape of the heat.

There are three distinct methods by which heat energy is transferred from one place to another, depending upon the medium or substance that transfers the heat.

a. A solid transmits heat by the method called _conduction_.

b. A fluid, either a liquid or a gas, transmits heat mainly by the method called _convection_.

c. s.p.a.ce transmits the energy of hot objects by the method called _radiation_.

[Ill.u.s.tration: FIG. 130.--Solids conduct heat.]

=151. Conduction.=--To ill.u.s.trate conduction, place in a gas flame the ends of same metal wires supported as in Fig. 130. In a short time the other ends of the wires become hot enough to burn one's hand. This may be explained as follows: The hot gas flame contains molecules in violent vibration and those striking the wire set its molecules rapidly vibrating. Since, in a solid, the molecules are held in the same relative positions, when one end of a wire is heated the rapidly vibrating molecules at the hot end set their neighbors vibrating and these the next in turn and so on until the whole wire is hot. It is a fortunate circ.u.mstance that different substances have different rates of conductivity for heat. To realize this, suppose that our clothing were as good a conductor as iron, clothing would then be very uncomfortable both in hot and in cold weather. The best conductors for heat are metals. It is interesting to note that, as a rule good conductors of heat are also good conductors of electricity, while poor conductors of heat are also poor electric conductors. Careful experiments in testing the rate that heat will be conducted through different substances show the following rates of conductivity.

[Ill.u.s.tration: FIG. 131.--Water is a poor conductor of heat.]

These figures are averages taken mainly from the Smithsonian Physical Tables:

Silver 100 Copper 74 Aluminum 35 Bra.s.s 27 Zinc 26 Iron 15 Tin 14.7 German silver 8.4 Mercury 1.7 Granite 0.53 Limestone 0.52 Ice 0.5 Gla.s.s 0.2 Water 0.124 Pine, with grain 0.03 Pine, across grain 0.01 Felt 0.008 Air 0.005

To test the conductivity of _liquids_, take a test-tube nearly full of cold water, hold the lower end in the hand while the tube is inclined so that the upper end is heated by a gas flame until the water boils. The lower end will be found to remain cold. (See Fig. 131.) Careful measurements of the conductivity of water show that heat is transmitted through it only {1/800} as rapidly as in silver, while air conducts but {1/25} as rapidly as water.

[Ill.u.s.tration: FIG. 132.--Wall construction of a refrigerator. 1, Porcelain enamel lining lock joint; 2, inside wood lining; 3, 3-ply red rope waterproof paper; 4, wool felt deafening paper; 5, flaxlinum insulation; 6, dead air s.p.a.ce; 7, flaxlinum insulation; 8, wool felt deafening paper; 9, 3-ply red rope waterproof paper; 10, outside wood case.]

[Ill.u.s.tration: FIG. 133.--Sectional view of a Thermos bottle.]

=152. Non-conductors and Their Uses.=--Many solids, however, are poor conductors, as leather, fur, felt, and woolen cloth. These substances owe their non-conductivity mainly to the fact that they are porous. The air which fills the minute s.p.a.ces of these substances is one of the poorest conductors known and hinders the transfer of heat through these solids. For the same reason loosely packed snow is a protection to vegetation covered by it during a period of severe cold in winter. The efficiency of storm sash or double windows, and of the double and triple walls of ice-houses and refrigerators (see Fig. 132) in preventing the conduction of heat is also largely due to the poor conductivity of the air confined in the s.p.a.ces between the walls. To prevent the circulation of the air, sawdust, charcoal, and other porous material is often loosely packed into the s.p.a.ce between the walls of such structure.

Other ill.u.s.trations of effective non-conductors will occur to every one; such as _woolen_ clothing, _wooden_ handles for hot objects, and the _packing_ used in fireless cookers. A _Thermos_ bottle is effective as a non-conductor of heat because the s.p.a.ce between the double walls has the air exhausted from it (Figs. 133 and 134).

Of several objects in a cold room, some feel much colder to the touch than others, thus iron, marble, oil cloth, and earthenware will feel colder than woolen cloth, carpet, feathers, or paper. The first four objects feel cold because they are conductors, and conduct the heat away from the hand rapidly. The other substances named are non-conductors and hence remove heat from the hand less rapidly, and therefore do not feel so cold. In a similar way, if several hot objects are touched by the hand, the good conductors are the ones which will burn one most quickly by conducting heat rapidly to the hand. The non-conductors, however, will rarely burn one. Why are the handles of hot utensils often made of non-conducting materials such as wood, cloth, asbestos, etc.?

[Ill.u.s.tration: FIG. 134.--Cross-section of the vacuum flask in a Thermos bottle.]

=153. Radiation= is the method by which heat comes to us from the sun across s.p.a.ce containing no tangible matter. It is also the method by which heat gets to us when we stand near a fire. Everyone has noticed that this heat is cut off by holding an object between the person and the fire. This fact indicates that radiant heat travels in _straight_ lines.

_The radiation of heat_ is believed to be accomplished by means of waves in a medium called _ether_, which is invisible and yet pervades everything. Three of the most important characteristics of radiation are _first, heat is transferred by radiation with the speed of light_, or 186,000 miles per second. This fact is shown by the cutting off of both the sun's heat and light at the same instant during an eclipse of the sun. _Second, radiant heat[I] travels in straight lines_, while other modes of transferring heat may follow irregular paths. The straight line motion of radiant heat is shown by its being cut off where a screen is placed between the source of heat and the object sheltered. _Third, radiant heat may pa.s.s through an object without heating it._ This is shown by the coldness of the upper layers of the atmosphere and also by the fact that a pane of gla.s.s may not be heated appreciably by the heat and light from the sun which pa.s.ses through it.