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For resistance measurements, usually a small constant current is passed through the device under test and the digital multimeter reads the resultant voltage drop; this eliminates the scale compression found in analog meters, but requires a source of precise current.
In the past, a common time measuring instrument was the sundial.Today, the usual measuring instruments for time are clocks and watches.For highly accurate measurement of time an atomic clock is used.
Combined with an internal voltage source, the current measuring mode can be adapted to measure very high resistances, of the order of 10 17 Ω. Finally, by calculation from the known capacitance of the electrometer's input terminal, the instrument can measure very small electric charges, down to a small fraction of a picocoulomb.
Ammeters must be connected in series with the circuit to be measured. For relatively small currents (up to a few amperes), an ammeter may pass the whole of the circuit current. For larger direct currents, a shunt resistor carries most of the circuit current and a small, accurately-known fraction of the current passes through the meter movement.
An early D'Arsonval galvanometer showing magnet and rotating coil. A galvanometer is an electromechanical measuring instrument for electric current.Early galvanometers were uncalibrated, but improved versions, called ammeters, were calibrated and could measure the flow of current more precisely.
In electrical engineering, current sensing is any one of several techniques used to measure electric current. The measurement of current ranges from picoamps to tens of thousands of amperes. The selection of a current sensing method depends on requirements such as magnitude, accuracy, bandwidth, robustness, cost, isolation or size. The current ...
A string galvanometer is a sensitive fast-responding measuring instrument that uses a single fine filament of wire suspended in a strong magnetic field to measure small currents. In use, a strong light source is used to illuminate the fine filament, and the optical system magnifies the movement of the filament allowing it to be observed or ...
Meters which respond to true RMS rather than mean current are described as "true RMS". Typical hand-held Hall effect units can read currents as low as 200 mA, and units that can read down to 1 mA are available. The Columbia tong test ammeter (illustrated) is an example of the iron vane type, used for measuring large AC currents up to 1000 amperes.