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Smiths Sectronic Mk I Movement
(Impulsed; moving coil; electronically switched)

Sectronic Mk I
This movement from the 1960s is a forerunner of the simpler moving magnet movements made by Smiths Industries in England. There is a fixed magnet (to the left of the balance wheel), a coil mounted at right-angles on the balance wheel, and it is transistor switched.

The principle of the movement is this. The coil on the balance wheel has two concentric windings. As the coil passes the fixed magnet, one winding generates a small current which is used to turn a transistor on, while the other winding is in the collector circuit of the transistor and has a larger current pass through it when the transistor turns on. This larger current generates a magnetic field in the coil which interacts with the magnet and gives impulse to the balance. There is no mechanical switching of the current and therefore no switch contacts to fail. The hands are driven by a pin pallet escapement working in reverse.

Both coils are on the balance wheel, which means three connections to be taken to the coils (there is one common connection). This is done by having three hair springs and much complicated insulation!


Voltage: 1.5v
Battery type: one LR14/MN1400/C size cell
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Copyright  Text & Pictures - Martin Ridout.   Last updated Jan 2005.