The movements are characterised by:

• Tuning Fork
• 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

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.

Universal Geneve and Citizen built Accutron movements under license from Bulova and used them in their tuning fork watches; parts are interchangeable.

 

 

ESA

Often referred to as the Rolls-Royce of hummers, the ESA 9162 (date only) and ESA 9164 (day and date) movements featured in many watches in the 1970s including the following on this website:

• Baume & Mercier Tronosonic
• Certina Certronic
• Eterna Sonic
• Longines Ultronic
• Omega f300
• Tissot Tissonics
• Titus Tuning Fork
• Zenith Electronic

Watch manufacturers 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:

In 1970, ESA starts to secretly develop a chronograph mechanism for the ESA 9162. It finally appeared in May 1972 and is known as the ESA 9210; production is limited to only 21,000 units. Several watch manufacturers bring out watches that utilize this movement although the Omega Speedsonic is by far the most common:

• Omega Speedsonic
• Certina C-Tronic Chronolympic
• Longines Ultronic Chrono
• Baume and Mercier Tronosonic Chrono
• Derby

The ESA 9210 is a fascinating and complex movement and for many years, I would only service the motor module, but in December 2013, I finally decided to spend some time in understanding the construction and working of this movement.  Visit my ESA 9210 Tuning Fork Chronograph movement page to read about this.

Omega

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.