Magnetic Suction
This experiment shows how your doorbell works.
A coil of wire with current flowing through itforms an electromagnet that acts very much like a bar magnet. The coil will magnetize an iron nail and attract it in a remarkably vigorous way.

• 40 feet (12 m) of insulated bell wire.
• A plastic or cardboard tube 4 to 6 inches (10 to 15 cm) long and about 1/4 inch (6 mm) in diameter.
• A large battery, 6 volts or more. (An ordinary 1.5-volt D battery will work, but may go dead very quickly and will require more coils to get the same effect.)
• The largest iron nail that will fit in the tube loosely.

(15 minutes or less)

Tightly wrap as many coils of wire as possible around the tube, leaving the two ends free so that you can strip the insulation off them and connect them to a battery.

(15 minutes or more)

Insert the nail part of the way into the coil and briefly connect the ends of the wires to the battery. (Leaving the wires connected too long will result in death for your battery and perhaps a burn for you from the hot wires.) The nail should be sucked into the coil. Reverse the leads to the battery and repeat the experiment, after predicting what will happen.

Any moving electric charge creates a magnetic field around it. A loop of wire with a current creates a magnetic field through the loop. You can increase the strength of this field by piling up a lot of loops. The more loops, the stronger the magnet. Like a bar magnet, this coil of wire now has a north pole and a south pole.

Because of the motion of electrons around its nucleus, each iron atom can be thought of as a tiny loop of moving charge. Each atom therefore acts like a small magnet. Ordinarily, all these "loops" point in different directions, so the iron has no overall magnetism. But suppose you bring a nail near the south pole of your electromagnet. The north poles of the iron atoms will be attracted to the south pole of the electromagnet and will all line up pointing in the same direction. The nail is now magnetized, with its north poles facing the south pole of the electromagnet. The opposite poles attract each other, and the nail is sucked into the electromagnet.

When the direction of current is reversed, the poles of the electromagnet reverse. Knowing this, you might think that when you bring the nail near the same end of the electromagnet as you did previously, the nail would now be repelled by the electromagnet, rather than attracted and sucked into it again. But when you try it, the nail does the same thing it did before. That's because the nail's iron atoms all reorient so that they line up with their opposite poles pointing toward whatever pole the electromagnet presents. Thus the nail will always be attracted to the electromagnet and will never be repelled.

You can find which end of the coil is the magnetic north pole by wrapping the fingers of your right hand around the coil in the direction the current is flowing; your thumb will point to the north end of the coil. You can also use a magnetic compass.

The principle of magnetic suction is used to make a variety of devices, from doorbells (in which an iron rod is sucked into a coil to strike a chime) to pinball machines (in which current goes through a coil, sucking in a rod that is attached to the flipper) to the starter switch on your car.

To extend the original activity, hold the coil vertically and repeat the experiment. Try smaller nails and straightened paper clips in the coil Remove the nail from the coil and test its magnetic properties by seeing if you can pick up some paper clips with it. If the electromagnet is not strong enough, the nail will not stay magnetized after the battery is disconnected, so to see this effect use as large a current source as possible. If the electromagnet is strong enough, the nail may stay magnetized for a while, until the random jiggling of the iron atoms eventually moves them out of alignment again. To demagnetize the nail rapidly, drop it onto a solid surface, such as a cement floor, a couple of times. This knocks the iron atoms out of alignment. Try to pick up paper clips with the demagnetized nail.