Thanks everyone for the help. The answer turned out to indeed be Electric Clocks.

 

I don't know about sync signals on the old clocks...I'm not THAT old that I was working with them in the 1950's.

I am not a welder but I'd like to say I saw an inductor being used on an arc welder. An inductor is used to force a constant current through High Pressure Sodium lights (and a few others) and they don't have to be re-ignited 120 times per second, so I think the idea of an AC arc welder being extinguished 120 times per second is somehow faulty.

My complete lack of practical experience invites a real welder to comment on this.

 

Thanks everyone for the help. The answer turned out to indeed be Electric Clocks.

Obviously!

 

Those old clocks in the schools could also be set forward or backward by an hour for daylight savings time. I'm not sure quite how they did it.
...........

Those clocks did not operate off AC line voltage. They had some type of stepper motor which was powered by a master clock controller that sent out periodic pulses to advance the stepper motor (once per minute I think). They also send out a reset pulse once each day (probably at midnight) which reset all the clocks to 12:00. That reset would also take care of daylight time shifts.

If the OP's question were from 50 years ago, I would say electric clock.

Now I would say none of the above.