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261. Ventilation of houses. In order to secure satisfactory ventilation it is estimated that a room should be supplied with 2000 cubic feet of fresh air per hour for each occupant (a gas burner is equivalent in oxygen consumption to four persons). A
current of air moving with a speed great enough to be just perceptible has a velocity of about 3 feet per second. Hence the area of opening required for each person when fresh air is entering at this speed is about 25 or 30 square inches. The manner of supplying this requisite
amount of fresh air in dwelling houses depends upon the
particular method of heating employed.
If a house is heated by stoves or fireplaces, no special provision for ventilation is needed. The foul air is drawn up the chimney with the smoke, and the fresh air which replaces it finds entrance through cracks about the doors and windows and through the walls.
262. Hot-air heating. In houses heated by hot-air furnaces an air duct ought always to be supplied for the entrance of fresh cold air, in the manner shown in Fig. 201 (see "cold-air inlet "). This cold air from out of doors is heated by passing in a circuitous way, as shown by the arrows, over the outer jacket of iron which covers the fire box. It is then delivered to the
FIG. 202. Principle of hot-water heating
rooms. Here a part of it escapes through windows and doors, and the rest returns through the cold-air register to be reheated, after being mixed with a fresh supply from out of doors.
When the fire is first started, in order to gain a strong draft the damper C is opened so that the smoke may pass directly up the chimney. After the fire is under way the damper C is closed so that the smoke and hot
gases from the furnace must pass, as indicated by the dotted arrows, over a roundabout path, in the course of which they give up the major part of their heat to the steel walls of the jacket, which in turn pass it on to the air which is on its way to the living rooms.
263. Hot-water heating. To illustrate the principle of hot-water heating let the arrangement shown in Fig. 202 be set up, the upper vessel being filled with colored water, and then let a flame be applied to the lower vessel. The colored water will show that the current moves in the direction of the arrows.
This same principle is involved in the gas heating coil used in connection with the kitchen boiler (Fig. 203). Heat from the flame passes through the copper coil to the water, and convection begins as indicated by the arrows. When hot water is drawn from the top of the boiler, cold water enters near the bottom so as not to mingle with the hot water that is being used. The principle is still further illustrated by the cooling systems used for keeping automobile engines from becoming overheated. Heat passes from the engine into the water, which loses heat in circulating through the coils of the radiator.
The actual arrangement of boiler and radiators in one system of hotwater heating is shown in Fig. 204. The water heated in the furnace rises directly through the pipe A to a radiator R, and returns again to the bottom of the furnace through the pipes B and D. The circulation is maintained because the column of water in A is hotter and therefore lighter than the water in the return pipe B.
By eliminating the expansion tank and partly filling the boiler with water the system could be converted into a steam-heating plant.
QUESTIONS AND PROBLEMS
1. If we attempt to start a fire in the kitchen range when the chimney is cold and damp, the range "smokes." Explain.
2. Why is a hollow wall filled with sawdust a better nonconductor of heat than the same wall filled with air alone?
3. In a system of hot-water heating why does the return pipe always connect at the bottom of the boiler, while the outgoing pipe connects with the top?
4. Which is thermally more efficient, a cook stove or a grate? Why? 5. When a room is heated by a fireplace, which of the three methods of heat transference plays the most important rôle?
6. Why do you blow on your hands to warm them in winter and fan yourself for coolness in summer?
7. If you open a door between a warm and a cold room, in what direction will a candle flame be blown which is placed at the top of the door? Explain.
8. Why is felt a better conductor of heat when it is very firmly packed than when loosely packed?
9. If 2 metric tons of coal are burned per month in your house, and if your furnace allows one third of the heat to go up the chimney, how many calories remain to be used per day? (Take 1 g. as yielding 6000 calories. A metric ton 1000 kg.)
GENERAL PROPERTIES OF MAGNETS
264. Magnets. It has been known for many centuries that some specimens of the ore known as magnetite (Fe,O) have the property of attracting small bits of iron and steel. This ore probably received its name from the fact that it was first observed in the province of Magnesia, in Thessaly. Pieces of this ore which exhibit this attractive property are known as natural magnets.
It was also known to the ancients that artificial magnets may be made by stroking pieces of steel with natural magnets, but it was not until about the twelfth century that the discovery was made that a suspended magnet will assume a northand-south position. Because of this latter property natural magnets became known as lodestones (leading stones), and magnets, either artificial or natural, began to be used for determining directions. The first mention of the use of the compass in Europe is in 1190. It is thought to have been introduced from China. (See opposite p. 223 for the gyrocompass.)
Magnets are now made either by stroking bars of steel in one direction with a magnet, or by passing electric currents about the bars in a manner to be described later. The form shown in Fig. 205 is called a bar magnet, that shown in Fig. 206 a horseshoe magnet. The latter form is the more common, and is the better form for lifting.
*This chapter should be either accompanied or preceded by laboratory experiments on magnetic fields and on the molecular nature of magnetism. See, for example, Experiments 25 and 26 of the authors' Manual.
If a magnet is dipped into iron filings, the filings will be seen to cling in tufts near the ends but scarcely at all near the middle (Fig. 207). These places near the ends of a magnet at which its strength seems to be concentrated are
FIG. 205. A bar magnet
called the poles of the magnet. The end of a freely swinging magnet which points to the north is designated as the north-seeking pole, or simply the north pole (N); and the other end as the south-seeking pole, or the south pole (S). The direction in which a compass needle points is called the magnetic meridian.
FIG. 206. A horseshoe magnet
265. The laws of magnetic attraction and repulsion. In the experiment with the iron filings no particular difference was observed between the action of the
two poles. That there is a difference,
however, may be shown by experi- FIG. 207. Iron filings clingmenting with two magnets, either
of which may be suspended (see
ing to a bar magnet
Fig. 208). If two N poles are brought near one another, they are found to repel each other. The S poles likewise are found to repel each other. But the N pole of one magnet is found to be attracted by the S pole of another. The results of these experiments may be summarized in a general law: Magnet poles of like kind repel each other, while poles of unlike kind attract.
The force which any two poles exert upon each other in air is equal to the product of the pole strengths divided by the square of the distance between them.
FIG. 208. Magnetic attractions and repulsions
A unit pole is defined as a pole which, when placed at a distance of 1 centimeter from an exactly similar pole, in air, repels it with a force of 1 dyne.