2.3 tons of coal will drive a heavy passenger engine and loaded train weighing more than 350 tons from New York to Philadelphia, a distance of 90 miles.

Questions

1. What is "lost work" in a machine?

2. What is meant by the term "mechanical equivalent of heat”? 3. What is the effect of raising the temperature upon the expansive power of steam?

4. What is the cause of the sparks that fly from the driving wheels of an engine when the brakes are put on?

5. Which is greater at the same temperature, the pressure of water vapor, or the pressure of ether vapor? Why?

6. Explain why water boils at a lower temperature under reduced pressure.

7. Why is the water in a steam boiler at a higher temperature than 100° C.?

8. The successful use of automobiles and aëroplanes depends upon the gasoline engine. Why?

Problems

1. How many foot pounds of work must be done on 5 lb. of water to raise its temperature 15° C.?

2. 56,000 foot pounds of work were expended on 4 lb. of water. What was its rise of temperature in Fahrenheit degrees?

3. 31,120 foot pounds of work were expended in raising the temperature of a certain quantity of water 8°C. How much water was there? 4. Suppose the 500-lb. head of a pile driver falls 25 ft., and that 20 per cent of the kinetic energy on striking is converted into heat. What change of temperature, in Fahrenheit degrees, would be given. to 1 lb. of water by the heat generated?

5. Suppose the average pressure of the steam that is used in an engine is 60 lb. per square inch, the piston head is 10 in. in diameter, and the stroke of the piston is 18 in. How many foot pounds of work are done at every stroke of the piston? How many at every rotation of the flywheel? How many per minute if the speed is 106 revolutions per minute? What is the horse power of the engine?

CHAPTER VIII

MAGNETISM

316. Natural Magnets. ore have the property of attracting iron; and when suspended so as to swing freely, they will come to rest in a nearly northand-south direction. They are called natural magnets. The ore is called magnetite, from Magnesia in Asia Minor near which it was first found.

Pieces of a certain kind of iron

317. Artificial Magnets. Demonstration. Select a sewing needle that has no attraction for iron. Rub it gently from the middle toward the point across one end of a natural magnet (a bar magnet will serve as well); then rub the other half across the other end of the magnet. Hold one end of the needle near some iron filings. They will be found to cling to the needle.

A piece of iron or steel that has the property of attracting iron as in the above demonstration is called an artificial magnet. This name is not very appropriate, and is used simply to distinguish magnets composed of manufactured iron or steel from natural magnets.

Artificial magnets are made in various forms. A bar magnet is a straight bar; a horseshoe magnet is a bar bent into U shape.

318. Permanent and Temporary Magnets.-Demonstration. -Repeat the above demonstration, using a bar of soft iron instead of the steel needle. Does the bar become a magnet? Now hold one end of a soft iron bar on or near the end of a magnet, and bring the other end of the bar near some iron filings. What is the result?

Pieces of soft iron can be made to act as temporary magnets while near another magnet, but they retain hardly any magnetism after the other magnet is removed. Pieces of steel retain their magnetism to a great degree and hence can be made into permanent magnets.

tion.

319. Magnetic and Nonmagnetic Substances.-Demonstra- Place on a table a collection of a dozen different materials, nails, screws, pins, coins, pencils, wire, paper, etc., and try to pick them up with a magnet. Part of them will be picked up, but upon others the magnet will have no effect whatever.

Those substances that are attracted by a magnet are called magnetic substances, while those that are not attracted by it are called nonmagnetic substances. Iron and steel are magnetic; also, to a less degree, nickel and cobalt. Some ores of iron are magnetic, while others are not. Nonmagnetic iron ores can be made magnetic by being strongly heated, as in the flame of a blowpipe.

320. Polarity.-Demonstrations. -Lay a bar magnet upon a table covered with small nails, and then lift it by the middle. It will be found that the nails

cling to the magnet, the greater number being near the ends, as in Fig. 272. The two points near the ends where the most nails cling indicate the position of the poles of the magnet.

Suspend the bar magnet by a thread at

FIG. 272

tached to its middle point, and after swinging back and forth for a time it will finally come to rest in a direction that is nearly north and south.

or

It has been agreed to call the end which points to the magnetic north the + or north pole and the other the south pole. The strictly correct names would be north-seeking pole and south-seeking pole. A body with such poles is said to be polarized, and to possess polarity.

321. The Magnetic Needle is a small bar magnet suspended by a thread or balanced upon a pivot (Fig. 273). It is used in many instruments, as in the compass. If we should continuously follow the direction in which it points, our path would be a magnetic meridian; and that spot in the

FIG. 273

Arctic regions where all magnetic merid

ians meet is called the north magnetic pole.

Demonstrations. Magnetize a sewing needle by drawing it across the ends of a bar magnet. Begin the stroke at the middle of the needle and end it at the point, drawing it across the + end of the magnet. Do this ten or twelve times. Reverse the needle and draw it across the end of the magnet, beginning at the middle

of the needle and ending at the eye. Unravel a fine silk thread, and tie a single strand around the middle of the needle. When you have balanced the needle so that it will hang horizontally, fasten

the thread in place by a bit of beeswax or sealing wax, and you have a magnetic needle that will serve for many experiments.

Suspend the needle and find the magnetic meridian. Observe which end of the needle points north, and notice whether or not it is the end that was drawn over the north end of the magnet.

Make a small bar magnet 'out of a good-sized knitting needle. In stroking it with the magnet, lay it upon a piece of board and stroke it with the end from the middle to one end of the needle. Reverse the needle and magnet and repeat. In order to make a strong magnet, the steel

must be stroked a great many times. Test it with iron filings and nails.

322. Mutual Action of Magnets. Demonstration. - Bring

the

+

+

FIG. 275

end of a bar magnet near the

Χ

+

FIG. 275 a

end of a magnetic needle and

note the result. Bring the end of the magnet near the the needle and observe. Make the same experiment on the of the needle.

The results of the above demonstration may be stated in the following terms: Like poles repel and unlike poles attract each other. This law is a fundamental one in mag

netism and should be made very familiar.

FIG. 276

The force of the repulsion or the attraction varies directly as the magnetic strength of the poles and inversely as the square of the distance between them.

Demonstrations. — Lay a horseshoe magnet upon a table. Pass

a light thread through the eye of a magnetized sewing needle, and