The frequency that wouldn't die hydroelectric generators
Robert D. Barnett · IEEE Spectrum · 1990
North America's Niagara River is the site of operating 25 hertz hydroelectric generators that date to the dawn of the electrical age. The reasons why 25 Hz was chosen for such a large block of power and why that obsolete frequency has lived on for the better part of a century are examined. The reasons for its choice lie in the conditions and attitudes of the period, and it has remained because changing frequencies is very expensive for the customer.>