File:Riefler clock NIST.jpg

English: Photo of astronomical regulator clock made by German firm Clemens Riefler, which was the primary US time standard from 1904 to 1929, in the NIST Museum in Gaithersburg, MD. About 54 inches (134 cm) long. The pendulum, which swings once per second, is made of Invar and is temperature compensated by the small length of aluminum seen under the weight. In order to prevent changes in atmospheric pressure from affecting the swing of the pendulum, the clock is housed in a glass pressure vessel, which is kept at a constant pressure. The batteries seen on a shelf behind the clock power an electric remontoire in the movement, which winds the clock every 30 seconds. The half-moon shaped hole in the face on the lower left edge of the upper subdial allows the operation of the remontoire to be seen. The gear train is made as simply as possible, with separate dials for hours (bottom subdial), minutes (top subdial), and seconds (large dial). It was accurate to 15 milliseconds per day, one of the most accurate all-mechanical clocks ever made. Alterations to image: cropped out caption.