Measurement of time
Everyone knows that a day is divided into 24 hours, each containing 60 minutes of 60 seconds each. At first, water clocks were used and later on mechanical clocks were made to measure time in these units. Until recent years the accurate measurement of time has not been easy. For reasons which we need not discuss, the length of the day varies throughout the year, so that an average value has to be taken.
The Royal Greenwich Observatory uses a number of very accurate quartz-electric clocks which are checked daily against astronomical observations. These, which are controlled by the vibrations of quartz crystals, are themselves checked by the cesium atomic clock at the National Physical Laboratory. The atomic clock is too complex a device to be described in detail, but briefly it a radio transmitter giving out short waves (about 3 em long), the frequency of word is controlled by energy changes in gaseous cesium atoms. The great advantage here is that the frequency (i.e., the number per second) of the changes is constant and not subject to error.
By using a cesium clock time intervals can now be measured with an error of not more than one second in 3000 years! This is so much better than results obtained by astronomical observations alone, so in 1967, the second was redefined as the time interval occupied by cycles of a specified energy change in the cesium atom.