Fig. 122. Which should he choose to connect the broken wires?
Application 50. Robbers had cut the telegraph line between two railroad stations (Fig. 122). The broken ends of the wire fell to the ground, a number of feet apart. A farmer caught sight of the men speeding away in an automobile and he saw the cut wires on the ground. He guessed that they had some evil purpose and decided to repair the damage. He could not bring the two ends of the wire together. He ran to his barn and found the following things there:
A ball of cord, a pickax, a crowbar, some harness, a wooden wagon tongue, a whip, a piece of iron wire around a bale of hay (the wire was not long enough to stretch the whole distance between the two ends of the telegraph wire, even if you think he might have used it to patch the gap), a barrel with four iron hoops, and a rope.
Which of these things could he have made use of in connecting the broken ends of the telegraph wire?
Application 51. A man was about to put in a new socket for an electric lamp in his home. He did not want to turn off the current for the whole house, as it was night and there was no gas to furnish light while he worked.
"I've heard that if you keep your hands wet while you work, the film of water on them will keep you from getting a shock," his wife said.
"Don't you wet your hands, Father," said his 12-year-old boy; "keep them dry, and handle the wires with your pliers, so that you won't have to touch it."
"I advise you to put on your canvas gloves while you work; then you can't get a shock," added another member of the family.
"That's a good idea," said the wife, "but wet the gloves, then you will have the double protection of the water and the cloth."
The man laughed and went to work on the socket. He did not get a shock. Which advice, if any, do you think he followed?
Inference Exercise
Explain the following:
311. A red postage stamp looks greenish gray in the green light of a mercury-vapor lamp.
312. Cracks are left between sections of the roadbed in cement auto highways.
313. Electricity goes up a mountain through a wire.
314. It is impossible to stand sidewise against a wall on one foot, when that foot touches the wall.
315. A charged storage battery will run an electric automobile.
316. An empty house is noisier to walk in and talk in than is a furnished one.
317. Lightning rods are made of metal.
318. It is harder to hold a frying pan by the end of the handle than by part of the handle close to the pan.
319. Diamonds flash many colors.
320. In swimming, if you have hold of a fastened rope and try to pull it toward you, you go toward it.
Section 35. Complete circuits.
Why does a doorbell ring when you push a button?
Why is it that when you touch one electric wire you feel no shock, while if you touch two wires you sometimes get a shock?
When a wire is broken in an electric light, why does it not light?
Suppose you have some water in an open circular trough like the one shown in Figure 123. Then suppose you have a paddle and keep pushing the water to your right from one point. The water you push pushes the water next to it, that pushes the water next to it, and so on all around the trough until the water just behind your paddle pushes in toward the paddle; the water goes around and around the trough in a complete circuit. There never is too much water in one place; you never run out of water. But then suppose a partition is put across the trough somewhere along the circuit. When the water reaches that, it cannot pass; it has no place to flow to, and the current of water stops.
The electric circuit. The flow of electricity in an electric circuit may be compared to the flow of the water in the tank we have been imagining. The long loop of wire extending out from the dynamo to your house and back again corresponds to the tank. The electricity corresponds to the water. Your dynamo pushes the electricity around and around the circuit, as the paddle pushes the water. But let some one break the circuit by putting a partition between two parts of it, and the electricity immediately stops flowing. One of the most effective partitions we can put into an electric circuit is a gap of air. It is very difficult for any electricity to flow through the air; so if we simply cut the wire in two, electricity can no longer flow from one part to the other, and the current is broken.
