Electro-Magnetic System

3min read

Historical Background

The electro-magnetic system was developed with the assumption that a miniaturised tuning fork which oscillates in a regular manner, would be able to drive the wheels of a wrist watch, enabling time measurement. This theory has been advanced by a Swiss electro-engineer, who gained experience in the physics of tuning forks during his graduation work at the federal engineering school (ETH) in Zurich, Switzerland. This engineer was Max Hetzel, and he has named his revolutionary system ‘Accutron‘.

Max Hetzel presenting the ‘Accutron’ movement. Picture credit (6)

The ‘Accutron‘ (ACCUracy + ElecTRONic) tuning fork system has been thus invented by the Swiss electro-engineer Max Hetzel, working for the American ‘Bulova’ company, in their Swiss facility in Bienne starting from 1950. Initially employed to automatise production machinery for mechanical watches, he received the task to evaluate the new electro-mechanical wrist watch systems announced by ‘LIP’ and ‘Elgin’ in 1952. Bulova was worried that a technical revolution might compromise the survival of the company. Hetzel’s report was cruel: the electro-mechanical system would not be more precise than conventional mechanical watches and argued, that with a tuning fork based system one could build a far superior watch (6). He confirmed his initial opinion during a seminar in Neuchatel in 1964, where he presented most existing, alternative, electric systems to the ‘Accutron‘ system, explaining the reasons of their unsatisfactory performance (1, 2, 5).

The invention of the reliable ‘Accutron’ system its subsequent patent in 1953 and commercialisation in 1960, was one of the main motivations for the Swiss watch industry to engage in the development of battery driven wrist watches, launching among other projects the founding of the CEH (Centre Electronique Horloger) in Neuchâtel (7).

Technological Features

The electro-magnetic system using a miniaturised tuning fork for wrist watches has the advantage of a higher oscillation frequency than a system using a balance wheel, and consequently being more precise, see parent section.

Simplyfied Accutron system. Picture credit (4)

For having a working system, one would need a isochronous oscillating tuning fork at high frequency, which oscillation is controlled by an electronic circuit and alimented by a miniature battery. The electronic circuit produces and controls an electromagnetic field acting on small permanent magnets, stimulating and entertaining the oscillations (300 – 720Hz, depending of manufacturer) of the tuning fork. The developmental genius, apart of the miniaturisation, consists in transforming the isochrone vibration of the tuning fork into rotating action for actuating watch hands. Latter is realised with the help of an index finger (cliquet) advancing an appropriately conceived index wheel (roue d’échappement), which counterclockwise rotation is guaranteed by an additional index finger preventing clockwise counterrotation. A sequence of wheels then adapts the rotation for the appropriate action of the watch hands (3,4).

Ref.:

  1. Coustans M.-P.; Lip, des heures à conter; Editions Glénat, 2022
  2. Trueb L. F., Ramm G., Wenzig P.; Die Elektrifizierung der Armbanduhr; Ebner Verlag, 2011
  3. Hetzel M., Technische Rundschau, Nr. 19, 26.4.1963
  4. Raminagrobis, through Wikipedia
  5. Hetzel M., La Montre Electrique; Conference de Synthese, Societé Suisse de Chronométrie, June 1964
  6. Murphsminutes
  7. Personal communication with a former Executive Vice-President of Ebauches SA, in charge of Research & Engineering.