An Unusual Frequency: 40-Hz Power in the Early Years [History]

In the 1890s, 60-Hz had not yet become the definitive standard frequency to be used for the supply of alternating current (ac) power in the United States. It was in use, but two other frequencies became significant as well. As a result of the huge success of the first 25-Hz power house at Niagara Falls, New York, in 1895, this frequency became a standard for use with large, slow-speed rotating machines.