One of the first large AC generator plant in America
This unprepossessing manhole cover is one of the few physical traces of one of the early twentieth century's most momentous technological competitions, the "current war" between AC and DC electrical power. Thomas Edison built the first commercial power plant on Pearl St. in Manhattan in 1881, a DC plant. By 1900 it was becoming clear however that the DC system had expensive deficiencies in large, integrated networks, like supplying power to every building in Manhattan. The AC system, championed by George Westinghouse and Nicoii Tesla among others, simplified distribution, while making the end use a bit more complicated. The Kings County Electric Light and Power Company constructed a large AC generating station on Gold St. near the Navy Yards that started operating in May of 1900. The station provided more inexpensive power to customers in Vinegar Hill and Fort Greene than the many other smaller DC generators operating at that time.
The cover design is somewhat generic to Brooklyn covers during that period (circa 1900 to 1919.) Covers with grids formed from raised lines were used by the Brooklyn Department of County Works before consolidation, and also by Brooklyn trolley lines such as the Brooklyn Heights Railroad (see below)
Underground distribution of AC power. Since most electrical motors operated on DC current, there were frequent points where the alternating current was rectified to DC for consumer use.
This example found on Fulton Street east of Flatbush Ave Extension is the only example of a KCEL&P that I am aware of.