Pierce Experiments on Precision Clocks for Laboratory, Observatory Clocks--One Uses Pendulum Enclosed in Vacuum

During the past year at the Cruft Laboratory Professor G. W. Pierce has been engaged in making some precision clocks for laboratory and observatory use. One of the clocks employs a pendulum enclosed in a vacuum chamber and kept at constant temperature. The pendulum has no escapement, driving spring or other mechanical attachment. Instead it is provided with a small slit through which a beam of light passes at each swing of the pendulum. The beam of light falls on a photoelectric cell and produces an electric current of short duration each second. These current pulses drive an auxiliary second's counter and clock dial for registering the time and also give magnetically impulses to the master pendulum for maintaining its vibration. This clock is timed once a day by radio signals from Washington which registers on a moving tape along with the clock ticks in such a way that the timing can be done to about 1-50 of a second. The clock is still in an experimental state, but maintains a constant rate with an error of about 1-10 of a second per day. Modifications are constantly being introduced in the hope of finding further improvements. He has also a clock making use as its timing element of the longitudinal vibrations of a bar of steel driven by magnetostrietion. This second type of clock produces alternating currents with a constant frequency of 1,000 cycles per second which then pass to a synchronous motor operating the hands. The two clocks are automatically compared by writing the seconds on a moving tape every half-hour.

These clock devices, in addition to constituting a research on horology, are in constant use as timing devices for other researches under way.