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| Mass |
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| The SI unit is kilogram. |
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| 1 kg is defined as the mass of a platinum-iridium cylinder kept at the International Bureau of W and M at Sevres, France, established in 1887. |
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| The atomic structure of mass is yet to be developed. |
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| Atomic mass unit (u) |
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| One amu or u is used as the unit of mass in atomic physics. Mass of a C12 atom is 12 atomic mass units. One u is defined as 1/12th of the mass of one C12 atom. |
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| 1 atomic mass unit = 1 u = 1.66054 x 10-27 kg |
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| Mass of our galaxy is about 2x 1041kg |
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| Mass of one electron is 9.1 x 10-31kg |
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| 1 microgram = 1 mg = 10-9 kg |
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| 1 milligram = 1 mg = 10-6 kg |
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| 1 gram = 1g = 10-3 kg |
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| 1 pound = 1 lb = 0.453592 kg |
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| Time |
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| In 1967, the atomic clock was adopted, choosing caesium-133 atom, which emits electromagnetic radiation of a precise and unvarying frequency, corresponding to the transition between two hyperfine levels of the ground state. |
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| One second (SI unit time) is defined as 9, 192, 631, 770, periods of the radiations from caesium-133 atoms. |
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| The accuracy with which the time interval can be measured, entirely depends upon both the clock and the method of operation (manual or automatic). |
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| Kelvin (K) |
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| This is SI unit of temperature. |
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of the thermodynamic temperature of triple point of water. The triple point
of water is 273.16 K.
0o C = 273.16 K
\ TK = to C + 273.16
t = T - 273.16
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| Ampere (A) |
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| It is the SI unit of electric current. The force between two infinitely long parallel conductors of current I1 and I2, separated by a distance v in vacuum, is given by |
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| Therefore, an ampere is that constant current, which when flowing in two infinitely long straight parallel conductors of negligible circular cross-section, when placed in vacuum, one metre apart, produces between them, a force of 2 x 10-7 N per metre of their length. |
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| Candela (cd) |
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This is the SI unit of luminous intensity
of a source of light in a specific direction. The candela is the luminous
intensity of a black body of surface area 
placed at the temperature of freezing platinum and at a presence of 101,325
N/m2, in the direction perpendicular to the surface. |
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| Candela is redefined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 x 1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian. |
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| [The original definition of candela was based on internationally defined candle burning at a specified rate]. |
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| Mole (mol) |
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| This is the SI unit of amount of substance. One mole is the amount of a system that contains as many elementary entities (atoms, molecules, ions, electrons, etc.,) as there are atoms in 0.012 kg of carbon-12. |
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| Radian |
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| This is the SI unit of (supplementary ) plane angle. One radian is the plane angle between two radii of a circle which cut off on the circumference of an arc equal to the length of the radian. |
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| Steradian (Sr) |
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| This is the SI unit of solid angle. One steradian is the solid angle which, with its vertex at the centre of the sphere, cuts off an area of the surface of the sphere, equal to that of a square having sides of length equal to the radian of the sphere. |
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| If S is the area cut off on the surface of a sphere of radius r, the solid angle at the centre of the sphere is given by |
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| Curie (ci) |
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| It is the SI unit of radioactivity. One curie is the quantity of any radioactive substance which undergoes 3.7 x 1010 disintegrations per second. |
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