Electricity for the 4-H Scientist Part 19

In fact, almost anywhere you find electric power at work you'll find electrical instruments--even in your home. The one you know best measures the amount of electricity used. Another, in the family car, shows whether the generator is charging the battery or if the battery is discharging.

What to Do

1. Make a simple kind of direct-current meter that will show you that there's a magnetic field around a wire carrying an electric current and that will detect a very tiny current.

2. Make a more refined D.C. instrument (galvanoscope) and measure the voltage of different sizes of dry batteries, and show how an electric current can be induced.

Tools and Materials You'll Need:

Pair of pliers, knife, small hammer 30 feet of No. 24 bell or magnet wire Compa.s.s Two coins--a penny and a dime Fine sandpaper Blotting paper Plastic or cellophane tape Wooden blocks (See Figure 4) Glue 2 small nails One #905 dry cell, a penlight battery, and two regular flashlight batteries Table salt Drinking gla.s.s 2 paper clips Two machine bolts

How They Work

Like many electrical things, most electrical instruments depend on the action of magnetism created by an electric current. There is a magnetic _field_ or lines of force around any wire carrying an electric current.

If this field is controlled and made to react on a sensitive device, like an easily moved pointer, we have an electrical instrument.

Detect a Magnetic Field

First, let's prove that there is a magnetic field around any wire carrying an electric current. Take a piece of wire about two feet long and sc.r.a.pe off about an inch of insulation from each end. Connect one end to a battery terminal. Make a loop of wire that crosses the face of your compa.s.s, north to south. Now touch the other end of the wire to the other battery terminal.

(DO NOT attempt to subst.i.tute alternating current, as from a model railroad transformer because its alternating current will cause the compa.s.s needle to swing rapidly from one side to the other.)

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

Put your right hand beneath the wire so that your fingers point the way the needle deflects, and your thumb will point in the direction that the current is flowing.]

What happens? Your compa.s.s needle should move to one side because it is very sensitive to magnetic influences. This proved that the wire created a magnetic field or lines of force when we pa.s.sed electricity through it. (Figure 1)

Detect a Tiny Current

How sensitive is your simple electric meter? Take about five feet of wire and wrap it around your compa.s.s as in Figure 2, keeping the turns bunched together as much as you can. Leave about six inches at both ends of the wire extended for leads. Sc.r.a.pe the insulation off the last inch of both. Rotate the coil and compa.s.s until the needle and coil are parallel, both pointing north and south.

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

Take a copper penny and a dime, and clean off any corrosion or film on the coin faces with a bit of fine sandpaper. Now take a piece of blotting paper about the size of the penny and dip it into strong salt water. Place the damp blotting paper between the penny and the dime.

Place one of your compa.s.s coil leads against the dime, and the other against the penny as shown in Figure 3. Be sure you have good metal-to-metal contact between the wires and the coins.

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

At the instant that you squeeze the leads against the coins, watch what is happening to the compa.s.s needle. It should move for an instant from the north position each time you press the leads against the two coins.

Obviously, the little coin battery you have just made produces a very weak electrical current. Even so, your instrument should be able to detect it.

Make a Simple Galvanoscope

Now let's make a meter that is a little more practical to use. Broadly speaking, a galvanoscope is an instrument that detects the presence of electric currents. It sounds complicated but it is really quite simple.

It is named in honor of an Italian professor named Galvani who made important early experiments with electricity.

A refinement of the galvanoscope is today's galvanometer. Other related instruments are the voltmeter and ammeter. These are very important instruments to the electrical engineer.

Using a gla.s.s or anything three to four inches in diameter, wind about 20 turns of wire in a "bunched" coil as in Figure 4. Wrap the coil at several points with cellophane or plastic tape to keep it from unwinding.

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

Make a wood base for your coil as shown in Figure 4. The compa.s.s support blocks can be thin wood slats. Do not attach them with steel nails or tacks. Use glue instead. Hold the coil in the slot between the blocks with glue or melted wax or use copper staples. Place the compa.s.s on the supports and rotate the base so that the compa.s.s needle and coil are parallel, pointing north and south.

Measure the Voltage of Batteries

Do you know what difference the size of dry cell battery makes in the voltage it supplies? Your meter can tell you.

To test the voltage of batteries we must be able to control our galvanoscope. To do this, connect a gla.s.s of strong salt water in series with the battery as shown in Figure 5. Make sure the wire ends immersed in the salt water are sc.r.a.ped free of enamel.

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

With one of the batteries connected, move the wires in the salt water first closer, then farther apart (keeping them parallel to each other) while watching your compa.s.s needle. When the needle stays 15 to 20 degrees off north, lock the wires in the salt solution in place with paper clips.

Now disconnect the battery you have been using and connect a smaller battery. If both batteries are fresh, the compa.s.s needle should return to almost the same spot. This proves that both batteries regardless of size put out the very same voltage. The larger ones, however, are designed to last longer.

Measure the Difference between Series and Parallel

Using the salt solution as in the previous experiment, connect two flashlight batteries in series as shown in Figure 6. The compa.s.s needle should move about twice as far as it did with one battery connected.

This shows that when you connect batteries this way you double their voltage.

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

Now place your batteries side by side and connect the two top terminals and the two bases as shown in Figure 7. The compa.s.s needle should move only as much as it did for one battery. This is called a parallel connection. You can see that this arrangement does not double the voltage, even though you used two batteries.

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

While you have this hookup, try reversing the position of the leads connected to your batteries. Notice that reversing the direction of current flow in the coil causes the compa.s.s needle to swing in the opposite direction.

Test for Induced Current

Make a simple coil by winding about 50 turns of wire around a machine bolt core. The bolt should be 1/4 to 1/2" in diameter and about two inches long. Connect the coil to your galvanoscope as shown in Figure 8.

Pa.s.s the coil back and forth close to the end of a permanent magnet.

[Ill.u.s.tration: Figure 8]

Notice a slight deflection of the compa.s.s needle with each pa.s.s. You have shown that electricity can be induced in a wire coil by moving it through a magnetic field. Currents generated in this way are called induced currents.

[Ill.u.s.tration: Figure 9]