How does it work?

The following article by Mike Richards published in Popular Electronics, December 1960 explains how a tuning fork watch works.
A 1960 Accutron Model 214 featuring a battery hatch and a setting handle at the back.	Transistorized Watch "Tickless timepiece" controlled by electromagnetic tuning fork boasts minute-a-month accuracy A new sound of time - a "microsonic" tone to replace the centuries-old ticking sound is given off by a new Bulova Watch Company timepiece called the "Accutron". Guaranteed accurate to plus or minus one minute per month, this transistorized device is about ten times as accurate as a conventional fine-quality wristwatch. From the outside, the Accutron looks like a conventional watch, except that there is no winding or setting stem. Instead, on the back of the case, there is a recessed handle for setting the hands and a removable cap for mercury cell replacement.
On the inside, there is the power cell, a set of drive coils, a transistor switching circuit, and an electromagnetic tuning fork - it's the latter that gives off the barely-audible 360 cycle tone. Drive coils. A pair of drive coils mounted near the tuning-fork tips keep the fork vibrating. A sensing coil picks up pulses from the fork and triggers the transistor to deliver current to the drive coils. One of the drive coils has 8000 turns of very fine wire, the other has 6000 turns, with the remaining 2000 turns making up the sensing coil. Attached to one of the tuning fork tines is a tiny index spring. A jewel on the tip of the spring engages ratchet teeth on an index wheel which is moved forward one tooth for each cycle of the tuning fork. To prevent the index wheel from moving backwards and returning to its original position, a pawl finger rests on the wheel's teeth. The wheel which turns the gear train connected to the Accutron's hands, is 0.095" in diameter (about the size of a pin head); its 300 precisely-machined teeth are separated by one-thousandth of an inch about one-third the diameter of a human hair! In operation, the voltage induced in the phase sensing coil is added to the powercell voltage to charge a capacitor (see schematic). A resistor slowly discharges the capacitor. The recharging pulses from the phase sensing coil cause the base circuit to conduct, allowing a driving pulse to flow in the drive coils. Amplitude Control. An important feature of the circuit is that it will return the tuning fork's amplitude to normal after any disturbance. The proper amplitude is maintained by controlling the size of the drive pulses.

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The collector circuit conducts at the instant the induced voltage in the drive coils is at a maximum and opposite in polarity to the power-cell voltage. If the tuning fork's amplitude is high and the induced voltage equals the power-cell voltage, no current will flow and the amplitude will drop. If the fork's amplitude is low - more current will flow in the drive coils and bring the amplitude up to normal.

The specially designed mercury cell will power the timepiece for at least one year before replacement is necessary - the Accutron requires only about eight-millionths of a watt for operation.

Mike Richards