Emergency power systems were used as early as World War II on naval ships. In combat, a ship may lose the function of its steam engines, which power the steam driven turbines for the generator. In such a case, one or more Diesel engine(s) are used to drive back-up generators. Early transfer switches relied on manual operation; two switches would be placed horizontally, in line and the "on" position facing each other. a rod is placed in between. In order to operate the switch one source must be turned off, the rod moved to the other side and the other source turned on.