From 1904-1929, the Riefler Clock served as the first official “Time Standard” of the United States. In other words, it was the specification by which the passage of time was measured. With an accuracy of 1/100th sec/day, or 0.000222%, it still represents the peak of accuracy obtained in pendulum clocks.
Invented by Riefler in 1889, the pendulum of the clock is in a vacuum, helping to eliminate both air friction and the effects of atmospheric pressure on its buoyancy, further eliminating potential sources of time distortion. Further still, the pendulum itself is made of a metal alloy called invar, so named because of its resistance to expansion or contraction (its “invariablity”) in the event of temperature change.
This feat alone was amazing because temperature change can profoundly affect pendulum length, constricting or expanding metal accordingly. Pendulum length then affects the time measurement by effecting the Period, which is the time it takes, in seconds, for a complete cycle or oscillation (a complete back and forth swing, if you will). Represented by T, the Period and Pendulum Length, L, share the following relationship:
Where g is acceleration due to gravity. These obstacles were thus, for the most part, overcome by the Riefler clock, allowing its time keeping to be used for things like precise astronomical measurements, and cementing its place as the highest standard in time measurement for a large part of the 20th Century.
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