A measuring instrument is a device for measuring a physical quantity like the voltage, current etc. Voltmeter and Ammeter are the measuring instruments. .A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit. .A voltmeter is connected in parallel with a device to measure its voltage, while an ammeter is connected in series with a device to measure its current.
Ammeters work to measure electrical current by measuring the current through a set of coils with very low resistance and inductive reactance. We have also some Other Measuring Instrument which includes precision meters like energy meter, pH meter, (L/C) Meter etc.