rent returns to the power house through the car track, and thus makes the circuit complete.

238. Exercises. · 1. What are several uses of electromagnets? What advantages have they over permanent magnets?

2. Write in the Morse code: "Buy one thousand shares Utah Copper at eighty-one."

3. Why do men use copper for trolley wires instead of iron?

4. What is the use of the telegraph in railroading? In the weather service? In business? In newspaper making?

5. Describe the changes of energy that take place when we use a waterfall on the Feather River to run a sewing machine in San Francisco.

6. Do you suppose there is a magnetic field around a dynamo? Why?

7. Why are glass or porcelain protectors used on telephone poles? 8. Can you suggest how men might distribute the energy of coal without shipping the coal away from the mine?

9. When you disconnect an electric flatiron, you will usually notice that a light which is on the same circuit burns more brightly. Why?

10. Where is the Gatun dam? The Assouan dam? The Keokuk dam? How are they used to generate electric currents?

11. Name all the uses you know of electric motors.

12. How can you light an electric lamp which is on the ceiling, by pushing a button on the wall?

13. Why are we told not to touch a bare electric wire carrying a current, if we are near some metallic fixtures or pipes at the same time?

14. If your electric doorbell will not ring, where will you look for the trouble. Where, if it will not stop ringing?

Summary. An electromagnet is a bar of soft iron placed in the magnetic field of a coil carrying a current. When the current is stopped, the iron ceases to be a magnet.

An electric bell rings because of the rapid making and breaking of its circuit by a spring attached to the hammer of the bell.

The telegraph is an instrument for making a sounder produce dots and dashes to correspond with the making and breaking of the circuit by means of the key.

In electroplating we use a solution containing a compound of the metal to be deposited. The metal wastes away at one pole and is deposited at the other.

When a current passes through a conductor, it meets with resistance. If the resistance is great enough, part of the current is changed into heat and light.

A dynamo is a machine in which energy of motion is changed into an electric current. It consists of coils of insulated wire, revolved rapidly in the fields of powerful magnets.

A motor is a machine for turning a current into mechanical motion.

CHAPTER XXVI

THE LIGHT OF DAY

239. Why Can We See Objects? As we look about this universe of ours, what is the most necessary thing in it? Is it not the light of the sun? How helpless mankind would be without it; how quickly all life would shrivel and die if it should be removed. No wonder early man worshiped the sun. We do not need to be sun-worshipers to appreciate the power of the sun and its benefits to us. The more we study about light, the more wonderful our great lamp, the sun, seems.

Why is it that we see the objects which surround us? Is it not, in the first place, because these objects send us light rays, and, second, because we have an instrument, the eye, which is affected by these rays? Think of some bodies besides the sun which produce within themselves the light they give off, and so can be called "sources of light. How about a burning stick, an electric bulb, a firefly, a flash of lightning? Name others. We call such bodies self-luminous. On the other hand, many bodies merely reflect light. We see them simply because another body shines upon them, and they reflect some of its light to our eyes. Such bodies are illuminated, or lighted, by the self-luminous bodies. How do we see a chair in daylight? By reflected sunlight, do we not? What hap

pens when the sun goes down, and the chair has no light to reflect? Why does the chair become visible again when we turn on a light in the room?

The moon, like the chair in daylight, reflects light from the sun. Objects which we see by moonlight reflect light that comes from the moon; but the moon, in turn, has reflected the light of the sun. In moonlight, then, we see an object by twice-reflected sunlight. How about an object seen by starlight? By the light of the planet Venus? By the light of a "shooting star"?

Can you name a substance through which you may look and see an object on the other side? Glass, you say. How about water and air? Such substances are transparent (see § 110). Other substances do not permit light to pass through them, so that an object on the other side of them cannot be seen. Name some of these. They are opaque bodies. There is also a third class called translucent substances; if objects are seen through them, the objects are indistinct. Oiled paper, horn, and thin china are translucent; how about fog? Ground glass is translucent because the light passing through it is scattered, or "diffused," in many directions. How is glass ground? See § 14. Very thin plates of all opaque bodies are probably translucent.

240. How Does Sunlight Travel? Notice how sunlight comes through a small opening into a dark room (Fig. 161); does it not come in a straight line, illuminating only those dust particles which are in that line? When some other force does not change the direction of light, it seems always to travel in straight lines, like matter itself (see § 105). But the velocity of light is much

greater than the speed of any body of matter we know of. A rate of 186,000 miles a second, which we have learned (see § 91) is that of light, is about 10,000 times that of the earth as it travels around the

sun.

How slow is the swiftest airplane compared with light, or even with the earth! Scientists have found that light comes to us in the form of light waves, which can travel enormous distances through space (see § 88). We call a single line of light a light ray; a number of parallel rays makes a beam. It is a beam that comes into the room: a "bundle" of light rays.

When we think of it, we shall see that if it were not for bodies that in some way turn the rays of sunlight from their straight course, we could get sunlight only when we are looking directly toward the sun. Yet we rarely look straight at the sun; its light is too intense and hurts our eyes. The objects around us: trees, grass, water, houses, stones, clouds, dust and water particles in the air, all these reflect sunlight to us, and we do not need to look at the sun itself in order to get its light.

The straight beam of sunlight is made visible by the dust particles which reflect its light to our eyes.

We must remember that sunlight brings us something besides light. Does it not bring us heat also (see § 251)? Then, too, sunlight has marvelous chemical powers, as when it tans our skin, or takes a photograph, or helps green plants to build up their food out of water and carbon dioxide (see § 43).