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pansion. One end of the spring is fixed, and the other end acts upon a pointer which marks off the temperature on a scale on the face of the instrument. Figure 226 shows a self-recording thermometer of this type. The time card is rotated once per day by clockwork, while the record is made by a pen at the end of the pointer.

255. The Clinical Thermometer is used in taking the temperature of the human body. The tube of this thermometer is pinched nearly together near the lower end. When the thermometer cools, the mercury column breaks at this narrowest part, and the column in the tube remains

FIG. 227. Clinical Thermometer

just as it was at the highest temperature during the time of its use; the reading, therefore, can be taken at any time. Before it is used again, the column is reduced by a jerking motion which forces some of the mercury past the constriction into the bulb.

256. Maximum and Minimum Thermometers are used by the stations of the Weather Bureau to show the highest and lowest temperatures during each day. One form of

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maximum instrument is made like the clinical thermometer. After being read it is set by swinging it around in a circle,

so that the centrifugal force drives the mercury back into the bulb, and the instrument is then ready for the next day. One form of minimum thermometer is an alcohol thermometer with a small glass spool or index moving loosely in the tube. This is drawn down by surface tension whenever the end of the liquid column touches the end of the spool as the liquid contracts; but when it expands, the liquid runs past the spool and leaves it at the point of lowest temperature. To set this thermometer it is merely necessary to tip the tube slightly, and the spool will run to the end of the liquid column and stop.

Questions

1. Suppose a basin (A) to be partly filled with ice water, another (B) with tepid water, and a third (C) with warm water. If the left hand is thrust into A, the right hand into C, and after a time both are thrust into B, how will the water in B feel to the left hand and how to the right hand? What does this teach?

2. What is the difference between quantity of heat and temperature?

3. Why is one end of a railroad bridge sometimes placed upon small rollers?

4. Which expands more with a rise of temperature, the glass bulb of a thermometer or the mercury that it contains? What proof have you?

5. Why is a wagon tire put on when hot?

6. What would be the effect of placing a football on a warm radiator?

7. What is the reading of the fixed points of a thermometer? Why are they called fixed points?

8. What is assumed when the length of a thermometer stem between 0° and 100° is divided into 100 equal parts?

9. Suppose that a compound bar is made of two metals, A on the right and B on the left. If A expands more than B, which will be on the outside of the curved bar when the temperature rises?

10. Why does the mercury column in a clinical thermometer break when it cools after being used?

11. Suppose you wish to take a temperature quickly. Would you use a thermometer with a large or a small bulb? Why?

12. Suppose you need to observe very slight changes of temperature. Would you use a thermometer with a large or a small bore? Why?

Problems

1. What change in Fahrenheit degrees corresponds to a change of 25 Centigrade degrees?

2. If the change in Fahrenheit degrees is 27, what is the equivalent change in Centigrade degrees?

3. What Fahrenheit reading corresponds to the reading 25° C.? 4. What Centigrade reading corresponds to the reading 27° F.? 5. For what temperature do the Centigrade and Fahrenheit thermometers give the same reading?

6. The normal temperature of the body is 98.4° F. What is it in the Centigrade scale?

7. Seventy-six degrees is called Summer Temperature on a Fahrenheit thermometer. What will be its reading on a Centigrade thermometer?

8. The boiling point of liquid air is 192.2° C. What is this temperature on the Fahrenheit scale?

9. The melting point of iron is 1520° C. What Fahrenheit reading indicates the same temperature?

10. The temperature of a room changed 17 Fahrenheit degrees. What was the change in Centigrade degrees?

II. PRODUCTION AND TRANSMISSION OF HEAT

257. Sources of Heat. The principal sources of heat are the sun; the interior of the earth; mechanical sources such as friction, impact, and compression, in which work is → changed into heat; chemical action, in which chemical energy is transformed into heat; and the heat caused by the passage of an electric current.

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That the sun is a source of heat needs no proof, while the experience of miners working in deep mines proves that there is internal heat, the temperature rising as the distance from the surface increases.

258. Friction. There are many familiar examples of the heating effects of friction. The train of sparks that fly from a sleigh runner as it passes over a stone, and the sparks that come from a car wheel when the brake is applied, both show that great heat is generated by the friction; for the sparks are burning iron. The hands are warmed by rubbing. A match is set on fire by friction, and a piece of wood in a turning lathe is charred when the corner of another piece is held against it.

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259. Impact. When two bodies meet in collision, the effect of the blow is to increase the rate of vibration of the molecules, and hence to raise the temperature of the bodies. An example of this is seen in the fact that one end of an iron bar can be heated by placing it on an anvil and striking it several vigorous blows.

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260. Compression. When a gas is suddenly compressed there is a corresponding sudden rise in its temperature. In steam air compressors of the double cylinder type, the heated air coming from the first compressor cylinder is cooled before going into the second by being passed through pipes surrounded by cold water.

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261. Chemical Action. Many chemical combinations give rise to heat. The most familiar of these is the combination of oxygen with carbon, as seen in combustion; for example, the burning of a match or of coal. If 1 c.c. of concentrated sulphuric acid is poured into 4 c.c. of water in a test

tube, both liquids being at the room temperature, it will be found that the mixture is several degrees hotter. Animal heat is due to oxidation within the body.

262. The Transmission of Heat. When two bodies are brought in contact with each other, each communicates a part of its molecular energy, or heat, to the other. If the two bodies are of the same temperature, each receives as much as it imparts, and there is no change in the temperature of either. If, however, the body A is of a higher temperature than the body B, it gives more heat than it receives, and its temperature is lowered by an amount which depends upon the difference between the two temperatures; while the body B has its temperature raised, and we say heat has been transmitted to it. Heat can be transmitted in three waysby conduction, by convection, and by radiation.

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263. Conduction; Conductivity of Solids. If one end of a copper wire 10 cm. long is held in the hand and the other end is placed in the flame of a Bunsen burner, the end in the flame will become red-hot, and in a short time the end in the hand will become uncomfortably warm. This method of transmission, by which the heat is transferred from molecule to molecule along the body, is called conduction. If the same experiment is made with a glass tube, the glass can be melted to within 4 or 5 cm. of the fingers without burning them, while a stick can be burned to the very fingers without harm.

These three examples illustrate bodies that are good, medium, and poor conductors of heat. A good conductor feels either warmer or colder than a poor conductor of the same temperature. This is due to the fact that it conveys its heat to the hand, or takes heat from the hand, more readily than the poor conductor.

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