Prior to the 20th century, time was determined astronomically, in terms of seconds derived as a fraction of the time it takes the Earth to make one complete orbit around the Sun. By the 1930s it was clear that this was not constant. The year was slowly lengthening, and it increased and decreased erratically. Hence a method to replace astronomical observations was urgently sought.

The atomic clock, first developed in Britain, was the solution. Scientists at NRC made a Cesium atomic clock (Cs I) which went into operation in 1958. It was stable and accurate enough to became the first in the world to provide the time standard for any country. Cs I was accurate to a few parts in 1010 (about 1 second in 300 years). It used a microwave source in which a specified number of oscillations of the source defined one second. The oval loops around the apparatus carried electric currents to minimize the effects of stray magnetic fields.

Today, Canada’s beam apparatus at INMS, which serves as a primary clock, has an uncertainty of a few parts in 1014 (better than 1 second in a million years!).
Canadian Heritage Information Network
Canadian Heritage Information Network, Canada Museum of Science and Technology, Musée de la civilisation, Stewart Museum, Canadian Medical Hall of Fame, Museum of Health Care at Kingston, University Health Network Artifact Collection, University of Toronto Museum of Scientific Instruments, University of Toronto Museum Studies Program, Suzanne Board, Dr. Randall C. Brooks, Sylvie Toupin, Ana-Laura Baz, Jean-François Gauvin, Betsy Little, Paola Poletto, Dr. James Low, David Kasserra, Kathryn Rumbold, David Pantalony, Dr. Thierry Ruddel, Kim Svendsen

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