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As you learn in Chapter 2 of this minibook, current is the flow of electric charge through a conductor. Current is measured in units called amperes. It should come as no surprise, then, that a meter that measures amperage is called an ammeter.

Don’t ask me why the p is dropped to form the word ammeter, rather than ampmeter. After all, the short form of the word ampere is amp, not am. Go figure.

Very few electronic circuits have currents so strong that they can be measured in actual amperes. So ammeters usually measure current in milliamperes, also called a milliamp and usually abbreviated mA. One mA is one-thousandth of an ampere; in other words, there are 1,000 milliamps in an amp.

The world’s first ammeter was invented by a Dutch physicist named Hans Christian Oersted in 1821, when he accidentally left a compass next to a wire that had an electric current flowing through it. Hans noticed that when the current flowed, the needle moved away from its normal northerly orientation and pointed toward the wire. This is because current moving through a wire creates a magnetic field around the wire, and the magnetic field was strong enough to attract the magnetized end of the compass needle.

After fooling around with it a bit, Hans discovered that the more current he ran through the wire, the farther the needle strayed away from north. It didn’t take him long to figure out that this discovery could be utilized to measure the amount of current flowing through a circuit. Analog ammeters work by this very same principle even today.

Hans Christian Oersted wasn’t the famous writer of children’s stories; that was Hans Christian Andersen. In a strange twist of history, though, Hans Christian Oersted was close friends with Hans Christian Andersen. It’s entirely possible that they were the founding members of some secret society of Evil Mad-Scientist Children’s Book Writers. Perhaps we’ll read about them in Dan Brown’s next novel.