Shortly after the excellent stability of frequency control by quartz crystal when applied to valve oscillator circuits was discovered, efforts were directed to producing a clock using this principle for time measurement. Warren A. Marrison of New York was pre-eminent in the application of quartz-crystal clocks to timekeeping, producing his first design c. 1929. Progress in the understanding of the quartz-crystal oscillator and the associated circuits needed to divide the high frequency down to a suitable value for operating small synchronous motors let to the quartz-crystal clock being adopted as a time-measurement standard in place of the pendulum clock in astronomical observatories from c. 1943. A frequency of 100,000 hertz was adopted in all the early clocks, the quartz crystal being kept at a constant temperature by an electrically heated oven. Dr. Louis Essen of the National Physical Laboratory developed the quartz-crystal ring, greatly improving the stability of frequency to about one part in one hundred million, and these were adopted as NPL standards for frequency and time measurement; this represents about 0. 001 second in 24 hours. Modern quartz-crystal oscillators for clocks often operate at 32,768 hertz, which is divided down by binary circuits to 1 hertz for the direct operation of the seconds hand of the clock in the case of analogue display.