 
Magnets are used in many kinds of devices. Electromagnets are useful because they only become magnetised when a switch is closed. On this page we'll show you how to make a cassette tape eraser using a simple electromagnet. You can buy a fancy device that erases music cassettes, so that you can rerecord them with more music, but the one you'll build here does the same job for a fraction of the price. The supplies you'll need are shown on the right: a U-bolt with bracket and two nuts, some electrical tape, several metres of plastic-covered wire, and a battery. We're using a 6-volt lantern battery because it lasts a long time, but a small 9-volt battery would also work. We won't be making a switch to turn the 'tape eraser' on and off; we'll just hook it to the battery whenever we want to use it. Leaving 10-20 cm of wire sticking out, start by wrapping the wire around the far side of the bolt, as shown in the first picture below. Wrap under the bolt, and continue wrapping until you've reached the top of that side, where the bolt begins to curve. Now loop the wire over the near side of the bolt, and continue wrapping in that direction, as shown. 
 When you're done, carefully wrap both sides of the wire-covered bolt with electrical tape, to hold the loops in place. Then attach the end plate, and the nuts. Don't tighten them too much, or you might damage the wire. 
 
If the ends of the wires are too short to reach the battery, as ours were, twist on another piece to each end, and wrap up the joins with tape. Bare the ends and attach them to the battery.Be careful: if you're using a 6-volt lantern battery, the wire and bolt will get hot quite quickly. You might try wrapping the curved top of the bolt with many layers of tape, to make it easier to hold when it's hot. In any event, don't leave the battery connected too long. You should test each end of the bolt with a compass; one end should be a north pole, and the other a south. Then test how strong it is by picking up small metal objects, like paperclips. If you find your magnet is very weak, you should take it apart and rewrap it, winding the wire up, down, and up the top side of the bolt, and then down, up, and down the near side, to make three layers of coiled wire on each side. The more coils there are, the stronger the magnet will be. With a strong electromagnet, you can run the bolt along the side of a cassette, moving it around in small circles, for just a few seconds, and both sides of the tape will be completely erased. Now you can record more music on a completely blank tape, which will considerably improve the quality of the music when you listen to it. |