Voltmeters. Digital voltmeters used for measuring electricity supplies, in this case alternating currents (A.C.). The voltmeter screen at centre shows the electrical voltage supply in volts (V) and its frequency in cycles per second (hertz, Hz). In the top centre of its screen are figures showing the maximum amplitude of the waves in the A.C. current and its average (mean) value. A generalised A.C. waveform is shown at the top left of the screen. The voltmeter at right shows the average value of an A.C. supply.