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What is an ammeter?

The current flowing in an electric circuit is measured in amperes by an instrument called an ammeter. The word is a short form of ampere-meter. These instruments are used in all kinds of electrical work ranging from the large power stations to very delicate radio and television construction. Some are therefore designed to measure hundreds of amperes and others—for instance, the milli-ammeters—will measure one-thousandth of an ampere.

With one exception all rely for their working on the fact that an electric current, however small, can make a coil of wire behave like a magnet. The outward appearance of these instruments is simply of a small box, showing a scale over which a pointer moves.

The "moving coil" type of ammeter consists of a coil of wire wound on a small frame fixed to a cylindrical block of iron. This is then hung between the ends of a large strong horseshoe magnet in such a way that when a current flows the coil turns, moving the pointer on the scale.

In the "moving iron" ammeter use is made of the fact that two bar magnets will repel each other if placed side by side in a certain way. If two small iron bars are placed next to each other in a coil of wire through which the current is passing, they will become magnetized and repel (push away) each other. One iron bar is fixed and the other moves the needle on the scale.

The third main type is called a "hot wire" ammeter. This is much simpler to make but not so accurate. When a current is passed through a certain type of wire it becomes hot and increases in length, and when the current ceases to flow it cools and therefore returns to its original length. These changes are, by a simple mechanical device, shown by a pointer on a scale.

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