Tuning Fork

Tuning fork or Diapason widely known as Accutron watches use an acoustic resonator in the form of a vibrating fork as the source of stable resonance/frequency. The concept was invented* in the 1950's by Max Hetzel (a Bulova employee) for commercial use in watches but this breakthrough technology was also used in NASA satellites and spaceflight instruments.  Having achieved proper miniaturization and reliability a precision timing-instrument was sold in wristwatch format in 1960 as the Bulova Accutron 214 (and made Hamilton Electric outdated overnight :). This battery powered wrist device was the first in the World to use a transistor thus it was advertized as the first truly electronic wristwatch. The electric signal transformed by the transistor activates an electromagnetic coil that puts a metal tuning fork into vibration. This motion is then transferred by means of a tiny wire called index finger onto an ordinary mechanical gear train in which the smallest wheel contains 300 teeth unvisible to the naked eye. Shown below in 200x microscope magnification!

These movements have a specific "humming" feature due to the audible 360Hz vibration of the tuning fork thus the entire family of Accutron based watches is widely called "Hummers". They were the most precise watches in the World for more than a decade and were produced until 1977 when reliable and cheap quartz movements reached the market. Early quartz movements (Beta21, Accuquartz, Ultra-Quartz) lacked stepping motors and used Accutron technology as the source of mechanical motion. The Accutron principle is alive also today as all quartz crystals used in modern quartz watches are shaped in the form of tuning fork. Read the whole story from the ultimate Accutron source in the Universe that is maintained by the one and only Rob Berkavicius from Oz/Thailand.

*It is worth to mention that the application of a tuning fork in watchmaking occurred a century before the birth of the Accutron watch. In 1866 Louis Francois Clement Breguet (grandson of the respected Abraham Louis) patented an electric clock with a 100Hz tuning fork escapement - shown here as Shockwave animation (slow upload and player required) or here as frozen image.