Re: US grid to float high in freq:
"Awful "bang" every minute when the hands would move. Imagining you were in some sort of penal or mental facility was not difficult."
I would suspect that there was a master clock mechanism somewhere - with a wired connection to the slave clocks.
Every minute it would send a shaped pulse to advance the slave clocks by an escape mechanism - hence the bang.
When a slave clock's hands reached the hour point then it would ignore any further "minute" pulses. The master clock would then send a differently shaped "hour" pulse to get the slaves past that point.
At one minute to the hour the master clock would actually send several "minute" pulses in quick succession. These pulses would compensate for a slave clock having missed a few previous pulses - and get it to the hour waiting point. The "hour" pulse could also be repeated several times to release any slave that had missed the first one.
If a slave clock interpreted noise as a "minute" pulse it would merely get to the hour stop a little earlier - and then wait. Noise that could be interpreted as an "hour" pulse would be ignored except at that critical point. That case might put the slave clock a minute fast during the next hour only. It would resync at the next hour wait point.
Daylight saving can be achieved quite simply. To advance the time the master clock sends a rapid series of "minute" pulses to get the slaves to the next hour. To retard the time it can merely omit the relevant "hour" pulse - keeping the slaves stationary for an hour. Not a problem if no one is likely to be wanting to know the time in the middle of the night.
MSF movements for analogue clocks do that - I watched one at 2am to satisfy my curiosity the first time the clocks went back.
The most novel master clock mechanism is possibly an early 20th century one. It used a very accurate pendulum mechanism - but avoided the problem of friction from a mechanical signal transfer for the slaves. The pendulum shaft had a carefully shaped hole in it. A light shone through the hole onto a photoelectric cell. The electrical output was a sine wave.
Biting the hand that feeds IT
