by Norris Chambers
I have told you how the little electrons with their negative charges follow any suitable path toward a positive charge, and that a positive charge is an absence of electrons. The passage of these electrons through the conductor is called a current and it is measured in amperes. The number of electrons in the source determine the voltage - the more electrons the higher the voltage. Anything that impedes the flow of this current is called resistance, and it is measured in ohms. We will assume that the source of this current is a battery. The battery has a positive and a negative terminal. The electrons move out of the negative terminal and try to get to the positive one. If a conductor, or a wire, is connected between them, they will travel. On batteries the positive terminal is marked with a "+" and the negative with a "-" sign.
We will take a flashlight bulb and connect the metal side of it to a wire and the tip to another wire. When we hold these wires to the two terminals of the battery, the bulb will burn. This is called "direct current," or DC. That means that the current is always flowing in the same direction, from negative to positive and that the resistance of the bulb filament is causing heat, which radiates in the form of light.
But let us try something different...why not reverse the wires at the battery. When we disconnect them, the light goes out and when we reconnect them in the opposite direction, it comes on again. But the current, or electrons, are traveling through it in the opposite direction. If you did this operation five times in a second, the bulb would be flashing off and on at a rate of five times a second. This would be called "alternating current" or AC, and its "frequency " would be 5 cycles per second. (The term cycle per second has been changed by the moderns to "hertz." Hertz was a pioneer in the radio field years ago.) So now its frequency would be 5 hz. Now, if it were possible for you to reverse this connection 60 times in a second, you would have 60hz current, which is the same as that furnished by the power company and used by everyone to do many useful things. You will note that if you were changing them this fast, the light would appear to stay on continuously. This is because your eye cannot follow a change that rapid, and because the filament does not have time to cool and go dark between your connections. So now you know the difference between AC and DC.
Years ago there was a great power struggle between the Edison Company and Westinghouse over which type of current would be the standard for our country. Westinghouse won for several reasons, but primarily because AC can be transformed and transported easily from place to place, where DC would require large and expensive conductors and would be difficult to change from one voltage to another without waste.
Before going further in our electron study, we will look at another puzzling phenomenon - magnetism. You have no doubt played with a small horseshoe magnet, or a bar magnet, and noted how it will attract some metal objects even through paper or wood. The metal attracted to it is usually iron or steel - copper, aluminum and most insulators are not visibly affected. An interesting experiment you can perform is to take some iron filings and sprinkle them on a piece of paper that is held over a magnet. You will notice that the small filings form little lines around the ends of the magnet...these are called magnetic lines of force You can make your own iron filings by filing some off of a piece of soft iron. Why does a magnet behave this way? To understand why, you have to go a little farther back and learn a little bit about the structure of matter. About all that is really necessary to learn is that all matter is composed of atoms, and these atoms have "free electrons" that allow current flow in certain materials. They also have little things we'll call "magnetrons" that have magnetic characteristics. These are normally distributed at random throughout material, but when exposed to an external magnetic field, they tend to align themselves and magnetize the material in which they are dwelling. This is why you can put a nail on a magnet and when you take it off, it too has become a magnet.
Magnetism was first discovered hundreds of years ago. In its natural state, it is in an ore-like material called "lodestone." These pieces are magnetic, and our ancient forefathers discovered that when a piece was suspended on a string, it would align itself in a north and south direction. This constituted the first compass. Later, it was found that a thin piece of iron, when left in the vicinity of a lodestone, became magnetized and acted the same way when suspended. The iron soon became a needle, and was used for navigation. Naturally occurring magnets and magnets charged from them were used for many years.
A horseshoe magnet has strong lines of "flux" between the poles. On a magnet, the magnetic ends are called "poles." The end that points north is called the "north" pole and the one that points south is called the "south" pole. As with electrons, like poles repel each other and unlike poles attract. An electric motor works on the magnetic principle. Large magnets can be very strong when properly charged and made of the proper material. On a single bar that is magnetized, one end becomes a north pole and the other a south pole. Your iron filings will show that the lines of force are strongest at the ends, but tend to blend together near the center.
Magnets and conductors have a peculiar effect on each other. a wire that has electrons flowing in it will form a magnetic field around it, the strength of the field depending on the amount of current. If the conductor is coiled into a coil, a stronger field will be formed, and it will be strongest in the center. If an iron core is placed inside the coil, the core will become magnetized. If it is a soft iron core, the magnetism will disappear when the current is discontinued.
You can observe this principle by taking a bolt and winding fifty or sixty turns of wire around it (more won't hurt) then holding the ends across a battery. You will see that the bolt is magnetized when the current is flowing. You should use "magnet wire." This is enamel coated wire and comes in many diameters. Use about a size 18, 20 or 22 for experimenting. While you are at the electronic store, pick up a couple of permanent magnets to play with. You might also get a 6 volt lantern battery with spring contacts for experimenting.
You have seen how current flowing in a conductor creates a magnetic field around it. Here's another interesting fact. You can move a conductor through a magnetic field, and a voltage will be developed in it. You can make it in the form a coil and the voltage will be higher. The more turns you have, the higher the voltage will be. And if you connect the coil to a resistance, such as a light bulb, it will burn if the coil is moved fast enough and if the magnetic field is strong enough. The faster the coil moves through the magnetic field, the higher the voltage. This is the principle that a generator operates on. The one that charges the battery in your car uses this principle. The battery current induces magnetism in the field coils, which are on the inside of the housing, and the "armature" coils which are rotated through the magnetic field develop the voltage and current that charges your battery.
You have now demonstrated the principle of a motor and acquired a working knowledge of magnetism. You can have a lot of fun with the magnets. The ones you bought are called "permanent" magnets because they remain magnetized. When you create one with a coil, it is called an "electromagnet," and they are magnetized only while current is flowing in the coil. A permanent magnet is made by magnetizing high quality steel rather than soft iron.
We have only touched on the possibilities of magnetism, but I believe the important principles have been presented. Get a book from the library or book store on basic electronics and study it along with these explanations. You will find many refinements that I am not mentioning here.
One other thought I will leave with you - whatever you do, HAVE FUN!
Click on the lesson of your choice:
Lesson Two