An instrument used to measure currents is called an ammeter, and to measure potential difference: voltmeter.
To measure the current in a wire, an ammeter should be connected in series with other elements of the circuit. It is essential that the resistance of the ammeter be very much smaller than other resistances in the circuit. Otherwise, the very presence of the meter will change the current measured.
To find the voltage, i.e. the potential difference between any two points in the circuit, the voltmeter is connected parallel to those 2 points. It is important for the resistance of a voltmeter to be much larger than the resistance of any circuit element across which the voltmeter is connected. Otherwise, the meter alters the potential difference that is to be measured.
Often a single meter – multimeter – is designed so that, by means of a switch, it can be made to serve as either an ammeter or a voltmeter — and usually also as an ohmmeter, designed to measure the resistance of any element connected between its terminals.
• Ammeter measures current in a wire to be connected in series in the circuit.
• Voltmeter measures potential difference between 2 points, to be connected in parallel to those 2 points.
• Multimeter is a device that can include an ammeter, voltmeter and ohmmeter.
current: is the amount of charge moving through a cross-section, over a period of time.
voltage: electrical potential difference, expressed in Volts
resistance: resistance is a measure of the opposition to current flow in an electrical circuit
ammeter: an instrument for measuring electric current in amperes
series: all the components in a series circuit are connected end-to-end
voltmeter: an instrument used for measuring electrical potential difference between two points in an electric circuit
ohmmeter: an electrical instrument that measures electrical resistance