Precision measurement of time

The fundamental standard of time is made available by means of radio time signals controlled from an observatory. Local time standards may be in the form of chronometers, pendulum clocks or, often more conveniently, quartz crystal clocks. Various forms of chronograph are available enabling events to be recorded in terms of the time scale provided by the local standard. Drum and tape chronographs are simple, robust and of adequate accuracy for many purposes: printing and photographic chronographs possess the advantage of eliminating the need for measurement. Despite the disadvantages involved in employing photographic processes, this form of timing has been found advantageous in, for example, the photographic zenith tube telescope. Interval timers, employing the charge or discharge characteristics of a condenser, are available in convenient portable forms. Specialized cameras have been designed for timing field and track events, and may well find applications in scientific work. An accurate chronograph using a Kerr cell is also described. Decimal counter chronometers are available in various forms, some of which employ cold-cathode tubes. Brief reference is made to oscillographic methods of timing, and two examples are quoted from the work of the Royal Observatory. The review closes with some comments on the present proposals for a new definition of the unit of time.