[mikethedruid]

This same system was used with AC generation. When a new generator was brought up to speed, its output was connected between it's "hot" output and the output on the already running line. As the generator reached the same speed as the line, the bulb would blink on and off more and more slowly. Finally, one would keep adjusting the speed until the bulb remained out. Then the new generator would be in phase with the existing line, and one could cut its output in to the line.