Realization of the units of alternating electric quantities
Electric alternating quantities – voltage and current – are measured through reduction to the more precise measurable DC quantities (AC-DC-Transfer). Presently, the most accurate method is the comparison of direct voltage and alternating voltage with thermal converters.
Simply envision them as plain electric bulbs that are heated to the same temperature at AC and DC current. To measure this temperature a thermocouple is attached to a filament. The same temperature of the heating filament at AC and DC current means that the effective value of the alternating current corresponds to an equivalent direct current with the same thermal output.