School systems clocks have lots of different correction protocols - I call them "predictive motion" one of the standards sends a short pulse once an hour and a longer one twice a day - sometimes this signal was sent with large frequency generators tied to the local power grid and notch filters added to clock. One old story was I believe Stromberg installed a system at a NYC airport in the 50's and everything worked fine during the day until they buffed the floors at night. There are 24VDC Reverse Polarity movements available out of Europe - that can be easily run with a H-Bridge. Telechron - a firm which my company grew out of (due to a patent on a self starting clock motor) - developed master clocks to keep line frequency accurate (based on Tesla's ideas) - they also made large clock systems with a switch that allowed a 120HZ generator to manually advance the clocks - I believe some of these systems are still working almost 100 years after installation. Thomas D. Erb o: 508-359-9684 p: 508-359-4396 x 1700 f: 508-359-4482 a: 97 West Street, Medfield, MA 02052 USA e: tde@electrictime.com w: www.electrictime.com<http://www.electrictime.com> Tower & Street Clocks Since 1928