The figure below shows two dc ammeters connected in series, so that they are both measuring the same current.
An ammeter may be calibrated by comparing it to a more accurate (standard) ammeter measuring the same quantity. The accuracy of the standard instrument should be at least four times better than the instrument to be calibrated.
The current is derived from a dc power supply, and a current-limiting resistor may be included, as shown in the figure above. If the power supply has a current level control, the current-limiting resistor is not needed. The power supply is adjusted to set the measured quantity in convenient steps over the desired range. The two instrument readings are noted, and a calibration chart is prepared. No adjustment is normally made in the case of a Digital Multimeter used as an ammeter, because the DMM current measurement function uses the voltage-measuring facility.
Learn more on: Instrument Calibration Equipment
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