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Cotna
This is of a French maker. It consists of an eddy current motor running from the AC mains and winding a small spring. The spring powers the clock which is a standard mechanical type with a platform balance.
This construction has several advantages:
1) The spring will power the clock for a substantial time if the mains power fails. On test the clock ran about 30 hours before stopping.
2) The Ferraris motor does not require a closely controlled frequency. It will operate over a range, stated on one model as 40-60 Hertz, so the clock still keeps time even if the frequency varies,
3) No elaborate switching is needed for the motor. The Ferraris rotor has little power and can be left stalled when the spring is fully wound.
Movement:
This clock has usually a movement with plated steel plates and wheels. The Ferraris motor has a plated rotor. When the spring is fully wound a felt pad touches the eddy current rotor which restarts after a short time as the spring unwinds.
Cotna also made clocks with a synchronous motor. An example of such a clock is shown in the pictures.