# Measurement facts

Physics is a science of measurement. Lord Kelvin (1824-1907), one of the outstanding physicists have emphasised the importance of measurement.

Laws of physics are expressed in terms of physical quantity such as timing, force, temperature, density and numerous others. Physical quantities are often divided into fundamental and derived quantities. Derived quantities those whose definitions are based on other physical quantities e.g speed, area, intensity, etc. Fundamental quantities are not defined in the terms of other physical quantities, e.g. Length, mass and time etc.

Physical quantities may, in general, be divided into two categories

1. Scaler quantities: A scaler quantities is one which has only magnitude. Mass, length, time, volume, speed, energy, work etc are examples of scaler quantities.
2. Vector quantities : A vector quantity has both magnitude and direction. Velocity, momentum, force, acceleration etc are examples of vector quantities.

Measurement is the comparison of an unknown quantity with known fixed quantity.

Measurement involves comparison of the quantity to be measured with a reference standard. The reference standard of the measurement is what we call a unit.

for example unit of density is
Density = Mass/Volume = Mass/(Length) 3 = Kg/m3

SI Base Quantities And Units

Base quantities Units Symbol is Definitions
Length meter m The meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. (1983)
Mass kilogram kg The kilogram is equal to the mass of international prototype of the kilogram (a platinum-irridium alloy cylinder) at international bureau of weights and measures, at Severs, near Paris, France. (1889)
Time second s The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state to the Cesium-133 atom. (1967)
Electric current ampere A The ampere is that constant current, if maintained into straight parallel conductors of in finite length, of negligible circular cross-section, and placed 1 m apart in a vacuum, would produce between these conductors of force equal to 2×10-7 newton per metre of length.(1948)
Thermodynamic temperature Kelvin K The kelvin, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. (1967)
Amount of substance Mole mol The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.(1971)
Luminous intensity candela cd The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x 1012 Hertz and that has a radiant intensity in the direction of 1/683 watt per steradian. (1979)

 Unit of Tempterature:-Celcius The temperature scale adopted by the 1960s conference was based on a fixed temperature point, the triple point of water, at this point solid, liquid and gas are in equilibrium. The temperature of 273.16 K was assigned to this point. The present point of water was designated as 273.15 K, equalling exactly 0� on the Celsius temperature scale. The Celsius scale, which is identical to centigrade scale, is named after the 18th century Swedish astronomer Anders Celsius, who first proposed the use of the scale in which the interval between the freezing and boiling points of water is divided into hundred degrees. By international agreement, the term Celsius has officially replaced centigrade. Unit of Current – Ampere The ampere was defined as the constant current in a conductor when one coulomb charges flowing in one second. Unit of Intensity of light – Candela The International unit of light intensity is Candela, was originally defined as 1/60 of the light radiated from the square centimetre of a black body, a perfect radiator that absorb no light, held at the temperature of freezing platinum.. Unit of Intensity of light – Candela The International unit of light intensity is Candela, was originally defined as 1/60 of the light radiated from the square centimetre of a black body, a perfect radiator that absorb no light, held at the temperature of freezing platinum. Unit of amount of substance – Mole Mole is the base unit of the International system of units, defined as the amount of a substance that contains as many elementary particles (atoms, molecules, ions, electrons or other particles) as the number of carbon atoms in 0.0 12 kg (12 g) of carbon-12