The Electrostatic Accutron

The Accutron watch didn’t have a terribly long life, as things go in horology, but it had a pretty interesting one. The first year of production, 1960, was preceded by a nearly decades-long period of research and development, and when the Accutron debuted, it was the first really commercially and technically successful electronic watch. At the heart of the Accutron is a tuning-fork oscillator, made to vibrate thanks to current provided by a small battery. One tine of the tuning fork has a tiny pawl attached to it, and as the tuning fork vibrates, the pawl pushes the teeth of an index wheel (both the pawl and the 320 teeth of the index wheel are so small that they’re almost impossible to see without the aid of a microscope). The movement of the index wheel drives the hands, and of course, the whole system is regulated by the frequency of the vibration of the tuning fork.

Original tuning-fork Accutron Spaceview, 1974

Two of the signature features of the Accutron, other than its unprecedented accuracy (Accutrons were generally guaranteed to run to within a minute a month or less) were the smooth sweep of the seconds hand, and the high-pitched hum emitted by the tuning fork. Production ceased in 1977 but in their heyday, Accutrons were indisputably the most accurate wristwatches on Earth and were used in some of the most demanding environments imaginable. They were used in experimental aviation, as wristwatches, for both the A-12 spy plane pilots, and for the X-15 rocket plane program, and Accutron movements were used as cockpit instruments for the Apollo lunar missions. Even prior to that, Accutron movements were in space, often as timing instruments on communications satellites.

At Baselworld this year, Citizen and Bulova debuted a new concept movement which has some of the features of the original Accutron, but which also uses a new-to-wristwatches technology to both generate power for the watch, and to move the hands.

The Accutron concept movement will remind Accutron enthusiasts immediately of the Accutron Spaceview, an original-production Accutron which had an open dial, allowing the tuning fork mechanism and transistor to be seen. The concept movement likewise exposes the mechanism, however, there’s no tuning fork. Instead, two small turbine-like structures are visible in the lower half of the dial, which rotate rapidly when the watch is in motion, while the upper part of the dial is dominated by a near-identical, but larger, turbine – this rotates continuously, as it drives the seconds hand in a smooth forward motion that recalls the original Accutron watches.

The Accutron, And The Fastest Plane Ever Made

The tuning fork Accutron movement was used in a number of aerospace applications, and the Accutron Astronaut model was the official watch of the top secret CIA Project OXCART, which flew the fastest planes ever made. Find out more about the A-12/SR-71 aircraft, and the Accutron Astronaut, right here.

The two lower turbines are actually electrostatic generators, while the upper, larger, driving turbine is an electrostatic motor. An electrostatic motor is different from a conventional electric motor in several key aspects. A standard electric motor consists of a central rotor, wound with wire through which an electrical current travels; this rotor sits inside the stator, which is a ring-shaped permanent magnet. As current passes through the rotor coils, a magnetic field is produced and the attraction and repulsion of this field with respect to the permanent magnet surrounding it, causes the rotor to rotate, turning the driving shaft of the motor.

An electrostatic motor, by contrast, uses the direct attraction and repulsion of positive and negative electrical charge. In its simplest version, an electrostatic motor’s rotor sits between two electrodes, one of which carries a positive electrical charge, and one, a negative. As the edge of the rotor passes an electrode, it acquires a charge identical to the electrode, which pushes that part of the rotor away (like charges repel) causing the rotor to turn. 

The first simple electrostatic motors were made all the way back in the 18th century, by Ben Franklin and a Scottish monk named Andrew Gordon – Franklin’s “electric wheel” as he called it, rotated at about 15 rpm and he mused in his notes that ” … if a large fowl were spitted on the upright shaft, it would be carried round before a fire with a motion fit for roasting.” Electrostatic motors eventually gave way to more powerful electromagnetic motors, which are the type almost universally used today, in everything from power tools to toys to propulsion systems for planes, trains, automobiles and ships at sea.

The Wimshurst machine, a type of electrostatic generator developed by inventor James  Wimshurst, around 1880; image, Wikipedia.

While the electromagnetic motor is more powerful, the electrostatic motor has an advantage over it. Electrostatic motors don’t require a coil-wound rotor, and they can therefore be made much more compact than electromagnetic motors. Today, one niche application for electrostatic motors is in MEMS applications (Microelectromechanical systems) and in nanotechnology; the smallest known electrostatic motor consists of just 18 atoms. Electrostatic motors also, in general, use less energy than electromagnetic motors – an advantage that becomes more pronounced as size decreases.

The electrostatic drive system reproduces the gliding motion of the original Accutron seconds hand.

The concept Accutron electrostatic movement’s electrostatic generators are powered by arm movements, just as in a conventional automatic watch. The current produced is stored in a capacitor, and delivered to the larger electrostatic motor. The blades of the motor’s rotor are alternately attracted and repelled by the electrical charge of the stator – in this case, a static arrangement of blades which are superimposed on the rotor – causing the rotor to turn. This system drives the second hand in a smooth, sweeping motion reminiscent of the orginal Accutron; the hour and minute hands are driven by a conventional stepper motor. Both drive systems are controlled by a quartz timing package.

The watch is very interesting to see in action, and the visuals aren’t like any other watch. The electrostatic motor and generators move with a flickering, almost kaleidescope-like effect. The only other system out there with any similarities to the Accutron’s electrostatic drive system is Seiko’s Spring Drive, but while there is some conceptual crossover, there are major differences – Spring Drive’s glide wheel is basically an electromagnetic generator with a quartz controlled braking system, while the Accutron uses an electrostatic motor; moreover Spring Drive is, as the name implies, driven by a mainspring, while the Accutron is driven by electrical energy produced by electrostatic generators, and stored in a capacitor.

The technology certainly produces unique aesthetics and while the movement was presented as a concept movement, rather than a new line of watches, the technology looks mature and ready for market, so I wouldn’t be surprised at all if at some point in the near future we saw it in commercially available watches. Citizen released its caliber 0100 as a concept movement a year before the commercially available limited editions came out, so there’s a company precedent for the strategy. The resemblance, visually and kinetically, to the original Spaceview tuning fork Accutron watch is a powerful nostalgia play (though this is a concept movement, I’d honestly be just fine if these were the aesthetics of a finished watch) but with a drive system unique to the new electrostatic Accutron concept movement, and it’ll be interesting to see what next steps are planned. If you want news on the Accutron concept movement direct from Citizen, there’s a teaser site up, right here. The press release also says in part, “Accutron is committed to upholding an extraordinary legacy of excellence in design, style and technology as it reenters the watch industry,” so it looks like Accutron is on the comeback trail.