The voltage and frequency of AC electricity varies from country to country throughout the world. Most use 220V and 50Hz. About 20% of the countries use 110V and/or 60Hz to power their homes. 220V and 60Hz are the most efficient values, but only a few countries use that combination. The United States uses 110V and 60Hz AC electricity. The choice of frequency for a.c. power represented a compromise among the needs of different types of equipment. During the early years of a.c. systems, numerous different frequencies ranging from 25 to 133 1/3 cycles were used. For generators, lower frequencies tend to be preferable because this requires fewer magnetic poles inside the rotor, though this constraint became less significant as high-speed steam turbines supplemented and replaced slow-moving hydroturbines and reciprocating steam engines. For transmission, lower frequencies are especially desirable because a line