Q. Define Second and write its sub multiples.

Second

            Before 1960, the standard of time was defined in terms of the mean solar day for the year 1900. The rotation of the Earth is now known to vary slightly with time, this motion is not a good one to use for defining a time standard.

In 1967, the second was redefined to take advantage of the high precision attainable in a device

known as an atomic clock,which uses the characteristic frequency of the cesium-133 atom as the “reference clock”.

            The second is now defined as 9 192 631 770 times the period of vibration of radiation from the cesium atom.

 1 hour = 60 min

1 hour = 3600 sec

1min=60sec

1sec=1000ms

1sec=1000000ms