A lot of the regulators and equipment that provided dusk-to-dawn operation of series streetlighting were activated by a standard photoelectric lighting control connected to a standard, nearby 120V source.

This installation is a little different since it uses a "break" in another series loop for operation. In other words, within this installation, a break (as you can see by the strain insulator on the primary crossarm) connected to the oil switch for regulator operation, instead. Sort of like a "slave" in actuation.

The oil switch is on the left and the big constant current regulator for the series street lights is connected to it, centered on the pole.