The fact that an electromagnet ceases to become a magnet when the current is turned off can be used to great effect in large and small machines. For example, a powerful magnet can lift very heavy weights of iron and steel in a factory, but that would be no good if the magnet could not be persuaded to release them. With an electromagnet, the current can be stopped and the load released.

Magnets come in two main types: permanent magnets and electromagnets. As its name suggests, a permanent magnet is always magnetized — think of a kitchen magnet that stays stuck to a refrigerator door for years. An electromagnet is different; its magnetism works only when powered by electricity. Although an electromagnet is more complicated than a permanent magnet, it has useful and important advantages.

One of the most important features of an electromagnet is the ability to change its magnetic force. When no electric current flows through the magnet’s wires, it has no magnetic force. Put a little current in the magnet, and it has a small force. A large current gives the magnet a bigger force, able to lift or pull heavier objects. The ability to turn magnetic force on and off has many important uses, ranging from simple household gadgets to giant industrial machines.

The pulling power of a permanent magnet is limited to the type of metal from which it’s made. Currently, the strongest permanent magnets are made of a combination of iron and a metal called neodymium. Although these permanent magnets are strong, the best electromagnets are more than 20 times stronger.

Small electromagnets are used in electronic locks, such as those found on an automobile or the main door of an apartment building. Scrapyard cranes have powerful electromagnets that lift metal car bodies with ease. Magnetic Resonance Imaging machines use very powerful electromagnets to produce highly detailed images of the human body. The strongest electromagnets are those used in scientific research to study the properties of matter.

An electromagnet can save people who live up several flights of stairs from having to walk down to the front door when the bell rings. They can simply find out who is calling by means of an intercom and then press a switch to let the caller in. The switch turns on a current that activates an electromagnet. The magnet attracts the door latch, pulling it back and allowing the visitor to enter. Then a spring allows the latch to slip back into place.

You can find small permanent magnets in toys, handheld gadgets such as electric razors, and clasps for bracelets and watches. Larger permanent magnets are useful in household appliance motors and in stereo speakers. The electric motors in hybrid vehicles use very strong permanent magnets.

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