## Physics Question #92

Tim Seeburger, a 16 year old male from the Internet asks on November 23, 1999,

How does an analog ammeter works and how could I make one?

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### Ash Parameswaran answered on November 23, 1999

An ammeter measures the electrical current in a circuit. It is measured in amperes (amps). The principle of operation of an analog current meter is based on Lorentz Force. Any current carrying conductor produces a magnetic field around it proportional to the amount of current being carried. When you place a current carrying conductor in an externally applied magnetic field (say for example using a permanent magnet) there is an interaction between the field of lines from the P-Magnet and the magnetic field produced by the current carrying conductor. This results in a relative mechanical force between the magnet and the current carrying conductor. If you fix the magnet (mechanically) then the current carrying conductor deflects due to the force. If you fix the current carrying conductor then the magnet moves (or deflects) due to the force. The former case is called a moving coil ammeter and the latter case is called as moving iron (or moving magnet) ammeter. This mechanism was studied and perfected by the French scientist D'Arsonval and he brought out a meter mechanism which is still being used by all analog meters. It is called D'Arsonval movement. Check out this interactive Lorentz Force illustration by Walter Fendt. Here's a YouTube video that shows a simple way to construct an ammeter with a surplus magnet from an old computer hard drive and a small coil of wire.

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