> When the A/C grid is heavily loaded the generators slow below 60Hz. The[y] make up for it in later by generating higher than 60Hz. That's why the old A/C clocks with synchronous motors never need correction over long periods of time.
Yes, Daryl beat me to the answer. This was true back in the 1960's and into the 1970's, when Manitoba and Winnipeg Hydro went to these lengths to keep people's synchronous motor clocks running extremely accurately for a long time (sometimes well over a year with barely a one second deviation), and usually a power outage would be the limiting factor. However, I've noticed in the last 1-2 decades that this no longer seems to be the case, probably for lack of need. AC synchronous digital clocks I've observed (like on kitchen stoves) don't do that "catch-up/catch-down" any more.
> Most cheap digital clocks haven't had the oscillator regulated so their time creeps off.
Yes, that is (or at least was) one of the benefits of a multi-$100 watch. A $300 Seiko watch I bought around 1981-1982 was guaranteed to deviate no more than 3 seconds per month, and my actual sample was within 2 seconds per month!
Hartmut W Sager - Tel +1-204-339-8331, +1-204-515-1701, +1-204-515-1700, +1-810-471-4600, +1-909-361-6005