The movements are characterised by: Tuning Fork (photo of Omega's Cal. 1220 tuning fork on the right ) A Transistor acting as a switch No Mechanical contacts

There is a lot of literature on the Internet describing the history and development of the Bulova Accutron, the first tuning fork watch, so I'm not going to repeat it on this site. But as a very brief introduction, the tuning fork watch is like no other battery driven watch, or mechanical for that matter: it is driven by a tuning fork vibrating at 300-720 times a second, depending on the model; as a result, it doesn't tick, but hummms. The index mechanism also means the tuning fork watches are characterised by a constant sweeping seconds hand.

Some good links are:

• The Accutron Watch Page
• The Amazing Bulova Accutron

Nearly all of the hummers have a microscopic indexing system that converts the tuning fork's vibrations into a rotary action to drive the hands. This Index Wheel is at the heart of the movement; it is easily damaged; it has 300+ teeth:

The five photographs below show movements from the various Accutron Series. These movements (or variants of) feature in most of the Bulova Accutron watches and are described more fully in the Bulova Accutron section.

Bulova Accutron 214

Bulova Accutron 2180

Bulova Accutron 2193

Bulova Accutron 2242

Bulova Accutron 2303

Often referred to as the Rolls-Royce of hummers, the ESA 9162 / 9164 (below) featured in many watches including the following on this website:

• Baume & Mercier Tronosonic
• Eterna Sonic
• Longines Ultronic
• Omega F300s
• Tissot Tissonics
• Titus Tuning Fork

Watch manuafacturers using these movements often allocated their own calibre numbers to them. In the two examples below, the Baume & Mercier has a calibre number of 19162 and the Omega of 1260...but in both photographs you can see the ESA numbers stamped into the movement plates just below the bottom of the tuning fork:

Baume & Mercier Tronosonic Cal. 19162 (ESA 9162)

Omega F300 Cal. 1260 (ESA 9164)

But without doubt, the most unique and strangest tuning fork watch ever made was the Omega Cal. 1220 / 1230 or Megasonic movement (photo below). This awesome, and rare, movement featured: an asymmetric tuning fork resonating at 720 Hz instead of the more usual 300 Hz, a very unusual micromotor attached to one of the tuning fork tines and a series of magnetic gears! Read much more about this wonderful movement in the Omega section.

Omega Cal 1230 Megasonic