Multimeter
A multimeter is a multiple range test instrument that measures voltage, current, and electrical resistance. It was invented in the 1920s to combine the voltmeter and ammeter into one instrument.
There are two basic types of multimeters. One type has a moving coil, indicator pointer, and a graduated scale; the other has a digital display.
The moving coil (and its cousin the moving iron) multimeter makes use of a principal discovered by Hans Christian Oersted (1777-1851) in 1820; an electric current in a wire produces a magnetic field. The magnetic field exerts a force on a coil (or iron magnet), making it turn. This moves the attached pointer, allowing a measurement to be read off the graduated scale. Its greatest limitation is the need to use the very current being measured when it's operating as a voltmeter.
A multimeter with a digital display makes use of integrated electric circuitss and semiconductors and does not have moving parts. A digital multimeter offers greater accuracy and does not draw current, but it is more expensive to build.
In either case, a multimeter has a dial, or push-buttons, on the outside and different resistors and shunts on the inside. Changing the setting allows the user to switch between the multimeter's various functions.
The primary use of the multimeter, whether it be a moving coil or digital display, is as a voltmeter to measure voltage.
Resistance is measured by the multimeter in ohms. The resistance of a conductor is one ohm when a steady current of one ampere flows through the conductor and produces a potential difference of one volt.
When used as an ammeter, the multimeter measures the flow of electrical current in amperes. Most ammeters measure direct current (DC). A very small current can cause a full-scale deflection of the coil, so the ammeter has to be modified (switchable) if it is used to measure high currents.
The more-intelligent digital display multimeters, which are basically digital voltmeters, have caused a decrease in the use of ammeters.
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