Image:Riefler clock drawing.png

English: Drawing of the astronomical regulator clock made by the German firm Clemens Riefler, first sold in 1902, from a catalog of the 1904 St. Louis World's Fair. It could keep time within 10 milliseconds per day, and was one of the most accurate all-mechanical clocks ever made. It was used in astronomical observatories worldwide, and as the source for national time distribution services, including the US Bureau of Standards from 1904 to 1929. It used an Invar temperature compensated pendulum rod, an electric remontoire to rewind the driving weight every 30 seconds, and a unique escapement invented by Sigmund Riefler in 1889 that impulsed the pendulum by flexing its suspension spring. To prevent changes in barometric pressure from affecting the pendulum, it was housed in a glass pressure vessel (C,G) that was kept at a constant pressure with the hand pump (L). The gear train was kept as simple as possible, with separate dials on the face for hours (bottom subdial), minutes (top subdial), and seconds (large hand). It also had a microscope (M) to read the amplitude of the pendulum swing from a small scale (e). About 54 inches (137 cm) high. Alterations to image: Rotated 90°, removed figure number, converted to 32 color PNG.