In order that investigator in different parts of the country and different parts of the world may compare the results of their experiments on a consistent basis. It is necessary to establish certain standard units of length, mass, time, temperature, pressure, etc.
A Dimension defines a physical variable that is used to describe some aspect of a physical system. The fundamental value associated with any dimension is given by a Unit.
A Unit defines a measure of a dimension.
Fundamental Dimension: Length, Mass, Time, Temperature, Electrical Current, Luminous Intensity.
Derived Dimension: Acceleration, Area, Density, Velocity and Force.
A Primary Standard defines the value of a unit it provides the means to describe the unit with a unique number that can be understood throughout the world. The primary standard then assigns a unique value to a unit by definition.
The standard of length:
The meter is defined as the length of a platinum-Iridium bar maintained to certain specified condition at the International Bureau of Weights and Measured.
1 foot = 0.30480060 meters. 1 inch = 0.02540005 meters.
The standard of Mass:
The kilogram is defined as the mass of a particular platinum-Iridium bar which is maintained under very specified conditions at the International Bureau of Weights and Measured.
1 lbm = 0.45359237 Kg.
The standard of time:
The second has been defined as 1/86400 of a mean solar day. The solar day is measured as the time interval between two successive transits of the sun across a meridian of the earth.
The standard of Temperature:
The basic unit of temperature, the Kelvin, K, is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water, the temperature at which the solid, liquid, and vapor phases of water exist in equilibrium.
°C = K – 273.15 °F = °R – 459.67 °F = 1.8 °C + 32
All electrical units originate from the definition of the Ampere. One ampere is the current that produce a magnetic force of 2*10-7 N/m on a pair of thin parallel wires carrying that current and separated by 1 m. The remaining electrical units, such as volts and ohms, can all be derived from the value of the ampere and the basic units of mass, length and time.
References: Wikipedia Amílcar A. Rincón Charris