A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. An ammeter is a measuring device used to measure the electric current in a circuit. An analogue voltmeter moves a pointer across a scale in proportion to the circuit’s voltage; a digital voltmeter provides a numerical display. In order for a voltmeter to measure a device’s voltage, it must be connected in parallel to that device. This is necessary because objects in parallel experience the same potential difference
For an ammeter to measure a device’s current, it must be connected in series to that device. This is necessary because objects in series experience the same current. They must not be connected to a voltage source. A galvanometer can function as a voltmeter when it is connected in series with a large resistance R . Then Ris determined by the maximum voltage that will be measured.