Part 19 (1/2)
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.