Standard Units

 

Unit of Length

 

Any fundamental unit must be unalterable and one with which other sub-standards can be compared and verified. In 1801 the French republic defined the unit of length to be one ten millionth of an arc of the earth measured from the pole to the equator and termed this length the meter. In fact they did not get the value quite correct (0.2mm short) and is now redefined as the length at 0^oC of a certain length of platinum rod kept in the Archives of Paris.  Later it was suggested that using earth dimensions might not be so permanent as one might think and it was suggested that a more absolute unit might be based on the wavelength of light chosen a particular frequency. The yard is defined in a similar manner as the distance between two marks on a certain bronze bar at 62^oF kept by the Board of Trade in London.

 

Unit of Mass

The unit of mass in the CGS system is defined from the unit of length and is the mass of water occupying a volume of one cubic centimeter at 4^oC and is known as the gramme (or gram). The gramme is now more practically defined as one-thousandth part of a mass of platinum kept in the Archives of Paris.  Likewise the pound is defined as the mass of a lump of platinum also in possession of the Board of Trade.

 

Unit of Time

 

The unit of time in the CGS, MKS, and FPS was originally the mean solar second. The earth, turning on its axis revolves round the sun but the period of revolution as observed from a star would be different from that observed from the sun. The earth makes approximately 366.25 revolutions on its axis in one year. Viewed from the sun, however, it would appear to be only 365.25 revolutions. The true revolution period of the earth (i.e. the interval that elapses between two consecutive passages of the same star across the meridian) is called a sidereal day. The interval that elapses between two consecutive passages of the sun across the meridian (i.e. the time between true noon one day and the next) is called a solar day. The length of the solar day is not constant but depends on the time of the year. Its average length is called a mean solar day and from it the second is derived.   

                           Thus 1 second is of a mean solar day.

 

The period of the revolution of the earth is probably increasing due to the drag of the tides the change however, is insignificant in terms of the life span of civilization.

 

The units of time have also been determined from the period of the swing of a pendulum and even the heart beat. Today however, the unit of time is obtained from the caesium clock (based on radioactive decay). The caesium clock is so accurate that it will neither gain nor loose one second in 80 million years.