Model AchF-3 is the most accurate pendulum clock in the world, capable of an accuracy of 0.002 sec/day (relative units 0.000000002-3), which is comparable to quartz regulators. However the latter need a qualified operator, while Fedchenko's can run years without any need for adjusting or correcting. This superior accuracy is achieved by several revolutionary inventions:1. Isochronous pendulum suspension eliminating circular error. Fedchenko invented this in 1952 and patented a year later, or rather, he registered his idea (registration No. 100085) since in Soviet Russia intellectual property was not patentable. Fedchenko knew that in theory pendulum clocks could maintain a daily accuracy of 0.002 seconds only if their amplitude is maintained with an accuracy of about 2 seconds of 1o. From this it follows that to achieve this accuracy either the amplitude muste kept amazingly constant or the pendulum must be made isochronous. To achieve the first is practically impossible, for even small seismic waves will disturb a pendulum to a quite large degree. Therefore, he concentrated on designing an isochronous pendulum. He was not the first to try but he was the first to succeed. He started to experiment with different shapes and forms of suspension spring, and just as he was about to come to the conclusion that isochronous suspension is impossible, he stumled upon the solution by sheer sloppiness. One day during regular trials with springs, overcompensation occurred that was out of place. After careful examination, Fedchenko noticed that the bracket holding the springs was not tightened as it should have been. This gave him an idea, which he developed in a relatively short time. He was not able to find a solution for the entire range of the amplitudes but he found a range between 90 and 110 minutes in which the period was constant. To keep the pedulum swing within this range was not difficult, it was just a matter of the strength of the impulse.2. More difficult was to minimize the inherent error of all pendulum clocks. The problem was first exposed by British Royal Astronomer Airy in 1826. He proved that the period of a pendulum or a balance is dependent on the duration of the impulse and the moment it is applied. He proved mathematically that the shorter the impulse, and the closer to the center it is applied, the lesser the variations in the period. Fedchenko's impulse devices gradually improved and were finalized in his AchF-3 mode. In it, he designed an electromagnetic mechanism powered by a mercury oxide battery with a capacity of 2.5 amp hours. The unidirectional impulse load is only about 0.002 amp and lasts 0.02 seconds. Never before had such a short impulse been realized. This was done by mounting two magnets to the lower part of the pendulum rod. This lower part is set up in a coil. The magnetic field in the coil interacting with the one of the permanent magnets creates an impulse, via a small invar rod mounted tohe same bracket as the pendulum, running alongside and interacting on a contact bracket set on the pendulum rod.The amplitude can be checked by a microscope mounted at the top of the steel cylinder.The pressure in the cylinder, kept between 8 and 10 mm Hg, is controlled by a baroscope - solid state barometer capable of determining and correcting very small pressure differences.Bibliography:"F.M. Fedchenko and his Pendulum Astronomical Clocks", by Dr. George Feinstein, NAWCC Bulletin, April 1995, pp. 169-84."The Fedchenko Clock", by M. Pleasure, Horological Journal, Sept. 1973, pp. 3-7