345. If you were on the moon, you would look up at the earth.

346. Toy balloons burst when they go high up where the air is thin.

347. You have to put on the brakes to stop a car quickly.

348. Telephone wires are strung on gla.s.s supporters.

349. If you pour boiling water into a drinking gla.s.s, frequently the gla.s.s will crack.

350. An asbestos mat tends to keep food from burning.

[Ill.u.s.tration: FIG. 132. Pencils ready for making an arc light.]

SECTION 38. _The electric arc._

How can electricity set a house on fire?

THIS IS ONE OF THE MOST IMPORTANT SECTIONS IN THE BOOK.

Do you know that you can make an arc light with two ordinary pencils?

The next experiment, which should be done by the cla.s.s with the help of the teacher, shows how to do it.

EXPERIMENT 71. Sharpen two pencils. About halfway between the point and the other end of each pencil cut a notch all the way around and down to the "lead," or burn a notch down by means of the glowing resistance wire. What you call the "lead" of the pencil is really graphite, a form of carbon. The leads of your two pencils are almost exactly like the carbons used in arc lights, except, of course, that they are much smaller.

Turn off the electricity both at the snap switch and at the knife switch. Fasten the bare end of a 2-foot piece of fine insulated wire (about No. 24) around the center of the lead in each pencil so that you get a good contact, as shown in Figure 132. Fasten the other bare end of each wire to either side of the open knife switch so that when this switch is open the electricity will have to pa.s.s down one wire to the lead of one pencil, from that to the lead of the other pencil, and from that back through the second wire to the other side of the knife switch and on around the circuit, as shown in Figure 133. Keep the two pencils apart and off the desk, while some one turns on the snap switch and the "flush" switch that lets the electricity through the resistance wire. Now bring the pencil points together for an instant, immediately drawing them apart about half an inch. You should get a brilliant white arc light.

_Caution: Do not look at this brilliant arc for more than a fraction of a second unless you look through a piece of smoked or colored gla.s.s._

Blow out the flame when the wood catches fire. After you have done this two or three times, the inside of the wood below the notches will be burned out so completely that you can pull it off with your fingers, leaving the lead bare all the way up to the wires.

Let the cla.s.s stand well back and watch the teacher do the next part of the experiment.

Connect two heavy insulated copper wires, about No. 12, to the sides of the knife switch just as you connected the fine wires. But this time bring the ends of the copper wires themselves together for an instant, then draw them apart. Hold the ends of the wires over the zinc of the table while you do this, as melted copper will drop from them.

[Ill.u.s.tration: FIG. 133. The pencil points are touched together and immediately drawn apart.]

[Ill.u.s.tration: FIG. 134. A brilliant arc light is the result.]

WHAT HAPPENS WHEN AN ARC IS FORMED. What happens when you form an electric arc is this: As you draw the two ends of the pencils apart, only a speck of the lead in each touches the other. The electricity pa.s.sing for an instant through the last speck at the end of the pencil makes it so hot that it turns to vapor. The vapor will let electricity go through it, and makes a bridge from one pencil point to the other.

But the vapor gets very hot, because it has a rather high resistance.

This heat vaporizes more carbon and makes more vapor for the electricity to pa.s.s through, and so on. The electricity pa.s.sing through the carbon vapor makes it white hot, and that is what causes the brilliant glow. Regular arc lights are made exactly like this experimental one, except that the carbons used are much bigger and are made to stand the heat better than the small carbons in your pencil.

Carbon is one of those substances that turn directly from a solid to a gas without first melting. That is one reason why it is used for arc lights. But copper melts when it becomes very hot, as you saw when you made an arc light with the copper wires. So copper cannot be used for practical arc lights.

FIRES CAUSED BY ARCS. There is one extremely important point about this experiment with arcs: most fires that result from defective wiring are caused by the forming of arcs. You see, if two wires touch each other while the current is pa.s.sing and then move apart a little, an arc is formed. And you have seen how intensely hot such an arc is.

Two wires rubbing against each other, or a wire not screwed tightly to its connection, can arc. A wire broken, but with its ends close enough together to touch and then go apart, can cause an arc. And an arc is very dangerous in a house if there is anything burnable near it.

Wires should never be just twisted together and then bound with tape to form a joint. Twisted wires sometimes break and sometimes come loose; then an arc forms, and the house catches fire. Good wiring always means soldering every joint and s.c.r.e.w.i.n.g the ends of the wires tightly into the switches or sockets to which they lead.

[Ill.u.s.tration: FIG. 135. An arc lamp. The carbons are much larger than the carbons in the pencils, and the arc gives an intense light.]

KEEPING ARCS FROM FORMING. Well-wired houses have the wires brought in through iron pipes, called _conduits_, and the conduits are always grounded; so if an arc should form anywhere along the line, the house would be protected by an iron conduit and if one of the loose ends of wire came in contact with the conduit, the current would rush to the ground through it, blowing out a fuse. The next section tells about the purpose of fuses.

The directions that usually come with electric irons, toasters, and stoves say that the connection should be broken by pulling out the plug rather than by turning off the switch. This is because the switch in the electric-light socket sometimes loses its spring and instead of snapping all the way around and quickly leaving a big gap, it moves only a little way around and an arc is formed in the socket; if you hear a sizzling sound in a socket, you may be pretty sure that an arc has been formed. But when you pull the plug entirely out of the iron or stove, the gap is too big for an arc to form and you are perfectly safe.

Fire commissions usually condemn extension lights, because if the insulation wears out on a lamp cord so that the two wires can come in contact, a dangerous arc may easily form. And the insulation might suddenly be sc.r.a.ped off by something heavy moving across the cord.

This can happen whether the light at the end of the cord is turned on or off. So it is best if you have an extension light always to turn it off at the socket from which the cord leads, not at the lamp itself.

Many people do not do this, and go for years without having a fire.

But so might you live for years with a stick of dynamite in your bureau drawer and never have an explosion. Still, it is not wise to keep dynamite in your bureau.

Arc lights themselves, of course, are no more dangerous than is a fire in a kitchen stove. For an arc light is placed in such a way that nothing can well come near it to catch fire. The danger from the electric arc is like the danger from gasoline spilled and matches dropped where you are not expecting them, so that you are not protected against them.

Fortunately ordinary batteries have not enough voltage to cause dangerous arcs. So you do not have to be as careful in wiring for electric bells and telegraph instruments. It requires the high voltage of a city power line to make a dangerous electric arc.

So many fires are caused by electric arcs forming in buildings, that you had better go back to the beginning of this section and read it all through again carefully. It may save your home and even your life.

After you have reread this section, test your understanding of it by answering the following questions:

1. How can you make an electric arc?

2. Why should wires not be twisted together to make electric connections?

3. Why should wires be brought into houses and through walls in iron conduits?

4. Why should you pull out the plug of an electric iron, percolator, toaster, heater, or stove?

5. Why do fire commissions condemn extension lights?

6. If you use an extension light, where should it be turned off?

7. If you hear a sizzling and sputtering in your electric-light socket, what does it mean? What should you do?

8. Is there any danger in defective sockets with switches that do not snap off completely? What is the danger?

9. In Application 55, page 228, if the rat had gnawed the wire in two while the electric iron was being used, would anything have happened to the rat? Would there have been any danger to the house?

10. Where a wire is screwed into an electric-light socket, what harm, if any, might result from not s.c.r.e.w.i.n.g it in tightly?

11. How can a wire be safely spliced?

12. Why is an electric arc in a circuit dangerous?

INFERENCE EXERCISE

Explain the following:

351. White objects look blue when seen through a blue gla.s.s.