  Credit Points 4 YOU CAN MEASURE ELECTRICITYInstruments that can detect or measure the flow of electricity have helped to make possible the wonders of electricity as we know them today. Scientists in laboratories must have measuring devices for experiments leading to new uses of electricity. Power suppliers must have instruments that tell what the generating equipment is doing and to measure the amount of electricity being sold to users. Factories need instruments that keep tab on electrical equipment to make sure electricity is being used efficiently. 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 Do1. 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: How They WorkLike 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 FieldFirst, 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 scrape 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 compass, north to south. Now touch the other end of the wire to the other battery terminal. (DO NOT attempt to substitute alternating current, as from a model railroad transformer because its alternating current will cause the compass needle to swing rapidly from one side to the other.) 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 compass 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 passed electricity through it. (Figure 1) Detect a Tiny CurrentHow sensitive is your simple electric meter? Take about five feet of wire and wrap it around your compass 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. Scrape the insulation off the last inch of both. Rotate the coil and compass until the needle and coil are parallel, both pointing north and south. 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 compass 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. Figure 3 At the instant that you squeeze the leads against the coins, watch what is happening to the compass 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 GalvanoscopeNow 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 glass 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. Figure 4 Make a wood base for your coil as shown in Figure 4. The compass 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 compass on the supports and rotate the base so that the compass needle and coil are parallel, pointing north and south. Measure the Voltage of BatteriesDo 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 glass 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 scraped free of enamel. 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 compass 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 compass 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 compass 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. 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 compass 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. 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 compass needle to swing in the opposite direction. Test for Induced CurrentMake 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. Pass the coil back and forth close to the end of a permanent magnet. Figure 8 Notice a slight deflection of the compass needle with each pass. 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. Figure 9 Now make another coil and core just like the first one and arrange them and a connection as shown in Figure 9. If you make and break the current to the second coil, you will build up and collapse a magnetic field around the first coil and again induce a current in it. You will see the compass needle swing back and forth again. These last two experiments give you a crude idea of how an electric generator works, producing electric current by induction as a coil-wound rotor revolves within a magnetic field. What Did You Learn?What does every current-carrying wire have around it? How does this help us to measure electricity? How sensitive are electrical instruments? What is the difference in voltage between (a) a large and a small dry cell? (b) batteries connected in series and in parallel? (c) your original connection and the reverse of it? What similarity does the test for induced current show between movement through a magnetic field and the making and breaking of a direct current? Demonstrations You Can GiveShow others how your galvanoscope can detect: whether a battery is producing current, which way the current is flowing, and whether a current is strong or weak. Demonstrate how a current can be generated using magnetism. For More InformationAsk your power supplier representative to show you some of the instruments used by his organization, and to give you a brief explanation of how they work. Ask him or an electrician to give you a demonstration of a split-core ammeter. This and all associated files of various formats will be found in: Updated editions will replace the previous one--the old editions will be renamed. |
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