Part 24

What Did You Learn?

1. How is heat transferred from one body to another?

2. Could chicks or pigs receive warmth from a heat lamp without the air in the pens becoming warm? Explain.

3. How does a broiler unit in a range cook meat?

4. How does an oven bake food?

5. Tell why iron picture wire was used instead of copper wire for your heating demonstration.

LESSON NO. B-13

Credit Points 2

MYSTERIOUS MAGNETISM

In ancient times, people found certain rocks that clung together in bunches. These rocks were very mysterious. People didn't understand them and many superstitions grew up about lodestones, as these rocks were called. Lodestone (sometimes spelled loadstone) means leading stone. People even told Columbus not to sail out of sight of land because a giant lodestone was just over the horizon waiting to pull all the nails out of his ships.

The Chinese were the first to use magnets. They found that if you hung a lodestone by a string, one end of the stone would always point in the direction of the North Star. They had the first magnetic compasses.

An artificial magnet can be made by stroking or gently rubbing a piece of steel with a lodestone. This piece of steel then can be used to magnetize another piece of steel. This can be continued on and on. Lodestones are not always available but you can get the same results with an electric current. So, magnetism and electricity are very closely related.

What to Do

Learn about magnetism by doing the experiments that follow.

Seeing is believing!

Materials You Will Need

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) Compass 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 pass 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.

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?

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 scrap iron and steel. Then he opens the switch to drop the scrap 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.