A voltmeter is an instrument that measures the difference in electrical potential between two points in an electric circuit. An analog voltmeter moves a pointer across a scale in proportion to the circuit's voltage; a digital voltmeter provides a numerical display. A voltmeter works differently. Instead of reading electrical flow, it reads voltage. This means the gauge will measure voltage across a ground and a positive wire. New cars use voltmeters, which are preferred by most street rodders. Most alternators typically put out between 13.8 and 14.5 volts while charging.