Though SI prefixes may also be used to form multiples of the second such as kilosecond (one thousand seconds), such units are rarely used in practice.The more common larger non-SI units of time are not formed by powers of ten; instead, the second is multiplied by 60 to form a minute, which is multiplied by 60 to form an hour, which is multiplied by 24 to form a day.In 1587, Tycho complained that his four clocks disagreed by plus or minus four In 1644, Marin Mersenne calculated that a pendulum with a length of 39.1 inches (0.994 m) would have a period at one standard gravity of precisely two seconds, one second for a swing forward and one second for the return swing, enabling such a pendulum to tick in precise seconds.From 1670 to 1680, Clement made many improvements to his clock and introduced the longcase or grandfather clock to the public.Seconds may be measured using a mechanical, electrical or an atomic clock.SI prefixes are combined with the word second to denote subdivisions of the second, e.g., the millisecond (one thousandth of a second), the microsecond (one millionth of a second), and the nanosecond (one billionth of a second).This clock used an anchor escapement mechanism with a seconds pendulum to display seconds in a small subdial.
This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960, which also established the International System of Units.
Absolute zero implies no movement, and therefore zero external radiation effects (i.e., zero local electric and magnetic fields).
The second thus defined is consistent with the ephemeris second, which was based on astronomical measurements.
Within a few years, most British precision clockmakers were producing longcase clocks and other clockmakers soon followed. In 1832, Gauss proposed using the second as the base unit of time in his millimeter-milligram-second system of units.
The British Association for the Advancement of Science (BAAS) in 1862 stated that "All men of science are agreed to use the second of mean solar time as the unit of time." BAAS formally proposed the CGS system in 1874, although this system was gradually replaced over the next 70 years by MKS units.
They found that the second of ephemeris time (ET) had the duration of 9,192,631,770 ± 20 cycles of the chosen caesium frequency.) During the 1970s it was realized that gravitational time dilation caused the second produced by each atomic clock to differ depending on its altitude.