INTERNATIONAL SYSTEM OF UNITS

iii) SECOND:

 Second is the base unit of time. Before 1960, the standard of time was defined in terms of the mean solar day.

1^st Definition:

It is defined as 1/86400 part of an average (mean) solar day of the year 1900 A.D. It is denoted by‘s’.

2^nd Definition:

Second is redefined as that time during which 9192631770 vibrations of cesium-133 atom take place.

iv) KELVIN:

The unit of temperature is Kelvin it can be defined as following:

Definition:

The fraction 1/273.16 of the thermodynamic temperature of the triple point of water. It is denoted by ‘K’.

Note:-

It should be noted that Triple point of a substance means the temperature at which solid, liquid and vapors phases are in equilibrium. The triple point of water is taken as 273.16 K. This definition was adopted in 1967.

v) AMPERE:

The unit of electric current is ampere. It can be defined as following:

Definition:

It is that constant current which if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section and placed a meter apart in vacuum, would produce between these conductors a force equal to 2x10^-7 newton per meter (N/m) of length. It is denoted by ‘A’. This unit of electricity was set up in 1971.

vi) CANDELA:

The unit of luminous intensity is candela. It can be defined ad following:

Definition:

The luminous in the perpendicular direction of a surface of 1/600000 square meter of a black body radiator at the solidification temperature of platinum under standard atmospheric pressure. It is denoted by ‘cd’. This definition was established in 1967.

vii) MOLE:

Mole is the unit of the amount of substance (number o particles)

Definition:

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 Kg of carbon 12. It is denoted by ‘mol’. This unit was adopted in 1971.  

INTERNATIONAL SYSTEM OF UNITS cont....

INTERNATIONAL SYSTEM OF UNITS

In 1900, an international committee agreed on a set of definitions and standard to describe the physical quantities. The system that was established by this committee is called the system international (SI) or international system of units. Since than SI units are used by all the scientific communities in the world. The international system of units is built up from three kinds of units which are the following:

1)      Base units

In SI system there are seven base units of various physical quantities which are given in the following table with their Symbols.

Physical Quantity	SI Unit	Symbol
Length	Meter	m
Mass	Kilogram	kg
Time	Second	s
Electric current	Ampere	A
Thermodynamic temperature	Kelvin	K
Intensity of light	Candela	cd
Amount of substance	Mole	mol

Now we will discuss in some details about the base units in the following lines:

i)  METER:

1^st definition

Before 1960, the meter was defined as “the distance between two lines marked on metal bar of an alloy of platinum and iridium kept under controlled conditions at the International Bureau of Weights and Measures in France”.

2^nd definition

In 1960, an international agreement was reached to define the meter in term of wavelength of orange light emitted by the atoms of krypton-86 in an electrical discharge tube. Thus

“Meter was redefined as 165076373 wavelengths of the orange red light emitted by the atoms of Krypton-86 lamp.”

3^rd definition

In 1983, a new definition was given as “the distance traveled by a light wave in vacuum during a time of 1/299792458 second”. It is denoted by ‘m’

ii) KILOGRAM:

The unit of mass is kilogram following is its definition:

1^st Definition

The mass of platinum (90%) and iridium (10%) alloy cylinder, 3.9 cm in diameter and 3.9 cm in height, kept at the international Bureau of Weights and Measurement in France. It is denoted by ‘Kg’.

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