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Smiths Sectric Tuning Fork Mantel Clock
(Impulsed; moving magnet; electronically switched)

Smiths Tuning Fork Clock

This mantel clock has a tuning fork movement similar in principle to the Bulova Accutron wrist watch. Whereas the Accutron drives the hands by a mechanical linkage from the tuning fork, this clock does so by means of a magnetic escapement (developed by C.F.Clifford in the 1940s). The tuning fork vibrates at 300Hz, which makes the second hand move smoothly, and look as though the clock is powered by a motor.

The clock is powered by a single 1.5 volt battery.
Smiths tuning fork clock
Smiths tuning fork movement

The electronics (one transistor, one resistor and two capacitors)  are on a single board along with the coil assembly.

The 300 Hz tuning fork can be seen in the lower half of this photograph. It is fixed to the back plate at the centre of the curve at the left hand end. The right hand end of the lower tine carries a cylindrical magnet that vibrates within the coil. The coil has two windings; one generates current from the movement of the cylindrical magnet which turns on a transistor; a second coil which in the collector circuit of the transistor then gives impulse to the tuning fork, maintaining its vibrations. 

The right hand end of the upper vibrating tine carries a small horse-shoe magnet which interacts with the mu-metal escape wheel and drives the wheel round in continuous motion. The escape wheel is not self-starting, but is spun into motion by the hand-setting action. Once rotating, the escape wheel's speed is locked to the vibrations of the horse-shoe magnet. There is no mechanical connection between this magnet and the escape wheel. 
Smtihs tunung fork escapement
Voltage: 1.5v
Battery type: one LR14/MN1400/C size cell
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Copyright  Text & Pictures - Martin Ridout.   Last updated Jan 2005.