The Chronolith

Light-powered clock.

This was the first radiometric clock ever made: two lamps on either side of the clock light up alternately, thus pushing the pendulum each time. The radiometric force acts as a motor for the pendulum, but also as a brake, regulating its amplitude within a very precise range. The inside of the tube is under vacuum, with a pressure of about 0.01 bar. The balance is started by a magnet placed near the glass. The oscillation frequency of the pendulum is one second and the fine adjustment of the time base is done from the outside by turning the 4 planetary weights around the sphere of the pendulum with the help of the same magnet. This avoids having to fill it with air, dismantle the clock and recreate the vacuum for each adjustment. The clock therefore consists of a glass tube, a pendulum, a few sheets of mica, two relays and a hand drive mechanism: that’s all. It is assembled without screws or bolts and is held together by the sole force of an atmospheric pressure of 7.7 tons. To dismantle it, all you have to do is let air into the tube.
At the time, its accuracy was of the order of 2 seconds per month. It was so accurate that on 3 February 2002 it recorded the Afyon earthquake in Turkey in Switzerland, as well as its aftershock two hours later. This pendulum, which was more of a scientific experiment than a clock, was exhibited for a year at the Museum of Clockmaking in La Chaux-De-Fonds and then disarmed in 2010: it has not worked since.

Those who are more interested in the technical part will find it easier to go here