<P> The piezoelectric properties of crystalline quartz were discovered by Jacques and Pierre Curie in 1880 . The first quartz crystal oscillator was built by Walter G. Cady in 1921, and in 1927 the first quartz clock was built by Warren Marrison and J.W. Horton at Bell Telephone Laboratories in Canada . The following decades saw the development of quartz clocks as precision time measurement devices in laboratory settings--the bulky and delicate counting electronics, built with vacuum tubes, limited their practical use elsewhere . In 1932, a quartz clock able to measure small weekly variations in the rotation rate of the Earth was developed . The National Bureau of Standards (now NIST) based the time standard of the United States on quartz clocks from late 1929 until the 1960s, when it changed to atomic clocks . In 1969, Seiko produced the world's first quartz wristwatch, the Astron . Their inherent accuracy and low cost of production has resulted in the subsequent proliferation of quartz clocks and watches . </P> <P> Atomic clocks are the most accurate timekeeping devices in practical use today . Accurate to within a few seconds over many thousands of years, they are used to calibrate other clocks and timekeeping instruments . </P> <P> The idea of using atomic transitions to measure time was first suggested by Lord Kelvin in 1879, although it was only in the 1930s with the development of Magnetic resonance that there was a practical method for doing this . A prototype ammonia maser device was built in 1949 at the U.S. National Bureau of Standards (NBS, now NIST). Although it was less accurate than existing quartz clocks, it served to demonstrate the concept . </P> <P> The first accurate atomic clock, a caesium standard based on a certain transition of the caesium - 133 atom, was built by Louis Essen in 1955 at the National Physical Laboratory in the UK . Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time (ET). </P>

Who knew what time it was when the clock was made