Section 36. _grounded Circuits._

Why can a bird sit on a live wire without getting a shock, while a man would get a shock if he reached up and took hold of the same wire?

We have just been laying emphasis on the fact that for electricity to flow out of a dynamo or battery, it must have a complete circuit back to the battery or dynamo. Yet only one wire is needed in order to telegraph between two stations. Likewise, a single wire could be made to carry into a building the current for electric lights. This is because the ground can carry electricity.

If you make all connections from a battery or dynamo just as for any complete circuit, but use the earth for one wire, the electricity will flow perfectly well (Fig. 127). To connect an electric wire with the earth, the wire must go down deep into the ground and be well packed with earth; but since water pipes go down deep and the earth is already packed around them, the most convenient way to ground a circuit is to connect the wire that should go into the ground with the water pipe. The next experiment, the grounding of a circuit, should be done by the class with the help of the teacher.

EXPERIMENT 68. Caution: Keep the switches turned off throughout this experiment.

(a) Put a piece of fuse wire across the fuse gap. Screw the plug with nails in it into the lamp socket. Connect the bare end of a piece of insulated wire to the water faucet and touch the other end to one nail of the plug. If nothing happens, touch it to the other nail instead. The electricity has gone down into the ground through the water pipe, instead of into the other wire. The ground carries the electricity back to the dynamo just as a wire would.

(b) Put a new piece of fuse wire across the gap. Keep switches turned off. Touch the brass disk at the bottom of an electric lamp to the nail which worked, and touch the wire from the faucet to the other brass part of the lamp (Fig. 129). What happens?

Caution: Under no circumstances allow the switch to be turned on while you are doing any part of this experiment. Under no circumstances touch the wire from the faucet to the binding posts of the fuse gap. Do only as directed. Explain what would happen if you disobeyed these rules.

WHY A BIRD IS NOT ELECTROCUTED WHEN IT SITS ON A LIVE WIRE. If a man accidentally touches a live wire that carries a strong current of electricity he is electrocuted; yet birds perch on such a wire in perfect safety. If a man should leap into the air and grasp a live wire, hanging from it without touching the ground, he would be no more hurt by it than a bird is. A person who is electrocuted by touching such a wire must at the same time be standing on the ground or on something connected with it. The ground completes the electric circuit which passes through the body. An electric circuit can always be completed through the ground, and when this is done, it is called grounding a circuit.

APPLICATION 54. Explain why only one wire is needed to telegraph between two stations; why you should not turn an electric light on or off while standing in a tub of water.

APPLICATION 55. In a house in the country, the electric wires passed through a double wall. They were separated from each other and well covered with insulation, but they were not within an iron pipe, as is now required in many cities. The current was alternating. One night when the lights were out a rat in the wall gnawed through the insulation of the wire and also gnawed clear through one of the wires. Did he get a shock? The next morning, the woman of the house wanted to use the electric iron in the kitchen and it would not work. The kitchen had in it a gas stove, a sink with running water, a table, a couple of chairs, and the usual kitchen utensils. There was also a piece of wire about long enough to reach across the kitchen. The electrician could not come out for several hours, and the woman wanted very much to do her ironing. Figure 130 is a diagram of the wires and the kitchen. Show what the woman might have done in order to use her iron until the electrician arrived.

APPLICATION 56. A man wanted to change the location of the wiring in his cement-floored garage. While he was working, would it have been best for him to stand on the bare cement floor, on a wire mat, on an old automobile tire, on a wet rug, or on some skid chains that were there?

INFERENCE EXERCISE

Explain the following:

331. An ungreased wheel squeaks.

332. Lightning rods extend into the earth.

333. A banjo player moves his fingers toward the drum end of the banjo when he plays high notes.

334. When the filament breaks, an electric lamp will no longer glow.

335. An inverted image is formed by the lens of a camera.

336. A blown-out fuse may be replaced temporarily with a hairpin or with a copper cent.

337. Sparks fly when a horse's shoe hits a stone.

338. A room requires less artificial light if the wall paper is light than if it is dark.

339. Phonographs usually have horns, either inside or outside.

340. An electric car needs only one wire to make it go.