2 dry cell batteries (#905) A few feet of No. 18 bell wire 3 steel knitting needles or similar hard steel 2 ft. of light thread Sheet of light cardboard or stiff paper Permanent magnet (bar or horseshoe) Compa.s.s 1 or more large nails or spikes Red and black china-marking pencils or crayons

Iron filings Wire cutters Carpet tacks

(Iron filings usually can be found under the grinding wheel in a shop.

If you can't find any, rub some steel wool pads together to produce bits of metal that will do.)

"See" a Magnetic Field

Cover the permanent magnet with the cardboard or paper. Sprinkle iron filings on the paper. Tap the paper and note the pattern formed. Strings or lines of filings pa.s.s from one pole of the magnet to the other. The area covered by the filings is the center of the magnetic field. To remember this, you might compare the magnetic lines of force that arrange the iron filings to the contour strips in a farmer's field.

This magnetic field is one of the important things in our everyday life with electricity. If it were not for the magnetic field, we would not have electric motors. Telephones, radios, television, and many other things we use every day also depend on this magnetic field.

[Ill.u.s.tration: Figure 1]

Make an Electro-Magnet

You can make magnetism work for you by winding several turns of insulated wire around one or more large nails or spikes (soft iron).

Connect one end of the wire to the battery. Touch the other end of the wire to the other terminal for a few seconds and see how many tacks you can pick up. Repeat the experiment using as many turns as possible. How many more tacks were you able to pick up?

[Ill.u.s.tration: Figure 2]

You have made what we call an electromagnet. When you disconnect the wire, the nails fall off. This is one of the advantages of an electromagnet. We can turn magnetism on and off as we wish. Picture a crane operator throwing the switch and picking up sc.r.a.p iron and steel.

Then he opens the switch to drop the sc.r.a.p metals.

Soft iron can be magnetized easily as you have just seen, but loses its magnetism in a short time. Steel is harder to magnetize but holds its magnetism almost indefinitely.

Make a Permanent Magnet

Wrap the insulated bell wire around the steel knitting needle. The wire should be wrapped the full length of the needle. One end of the wire is connected to the battery. The other end of the wire is then touched for just a few seconds to the other terminal. This should make the needle into a permanent bar magnet. If you did not get results, try two batteries in series, wind more turns of wire on the needle, and leave it connected a little longer. Do the same thing with the second knitting needle. In the same way, you can magnetize a screwdriver, so that you can use it to pick up and hold steel screws. Don't do it unless you want your screwdriver to be magnetized.

[Ill.u.s.tration: Figure 3]

See How They Attract and Repel

Take one of the magnetized needles and hang it with a thread. A thread stirrup (Figure 4) will help keep it level. Be sure it is not near other large pieces of steel. Watch the needle. Does it settle down, pointing in one direction? (Check to see if this is the same direction as your compa.s.s). If it does, you have made a compa.s.s. The tip of the needle pointing north is called the North Pole (North-seeking pole). The other end is called the South Pole. Mark the North Pole with a stroke of the red marking pencil. Mark the South Pole black. Do the same thing with the second needle. You can show this with a sewing needle, and a notched cork, and a bowl of water. Rest the needle in the notched cork, and float it on the water.

[Ill.u.s.tration: Figure 4]

Hold the compa.s.s near the North Pole of the needle. What happens? Does the South Pole of the needle attract the North or South Pole of the compa.s.s? Try this with the second magnetized needle. See if you can prove the rule that like poles repel (drive away) and unlike poles attract.

[Ill.u.s.tration: Figure 5]

Connect one end of a wire loop to the battery and run the wire directly over the compa.s.s. Touch the other end of the wire to the battery. Which way does the compa.s.s point now? If you get some motion out of the compa.s.s needle, this proves there is a magnetic field around the wire when current is flowing. This relation between electricity and magnetism is the thing that makes electric motors and generators work.

[Ill.u.s.tration: Figure 6]

Make Many From One

Lay the third needle (unmagnetized) on a table and stroke it with one of the magnetized needles. (See diagram) Always stroke it in the same direction. Raise the magnetized needle at least two inches on each return stroke. Thus you can magnetize the needle by using the other needle.

[Ill.u.s.tration: Figure 7]

Use the wire cutters to cut the first magnetized needle in short lengths. (Cover the needle with a cloth to keep the pieces from flying.) Can you show by using the compa.s.s that each piece is a complete magnet?

Hold one end, then the other, of each piece to a compa.s.s. Does each piece have both a North Pole and a South Pole?

Magnetism and Animals

The things you have done show that electricity and magnetism are related in many ways. Magnetism is mysterious, and there are still things to discover about it. It is thought that animals and birds are aided in their sense of direction by magnetism. It is commonly known that when a person gets lost in the woods, he tends to go around in circles.

Possibly this is caused by the earth's magnetic field.

What Did You Learn?

1. Where are natural magnets obtained?

2. How can artificial magnets be made?

3. What material is needed for a permanent magnet? For a temporary magnet?

4. How can you find out which is the North Pole of an unmarked magnet?

5. How many poles does a magnet have?

6. Which magnetic poles attract each other?

7. Why couldn't you make a compa.s.s out of a strip of plastic?

8. What causes the compa.s.s to change direction when a wire carrying battery current is held over the needle?

9. List the materials you would need and tell how you would build a homemade compa.s.s.

10. Tell what you enjoyed most about becoming acquainted with mysterious magnetism.

LESSON NO. B-14

Credit Points 2

Give your appliances and lights a square meal