Coherence Metric system

james clerk maxwell played major role in developing concept of coherent cgs system , in extending metric system include electrical units.

each variant of metric system has degree of coherence—the various derived units directly related base units without need intermediate conversion factors. example, in coherent system units of force, energy , power chosen equations

hold without introduction of unit conversion factors. once set of coherent units have been defined, other relationships in physics use units automatically true. therefore, einstein s mass-energy equation, e = mc, not require extraneous constants when expressed in coherent units.

the cgs system had 2 units of energy, erg related mechanics , calorie related thermal energy; 1 of them (the erg) bear coherent relationship base units. coherence design aim of si resulting in 1 unit of energy being defined – joule.

in si, coherent system, unit of power watt , defined 1 joule per second . in customary system of measurement, non-coherent, unit of power horsepower , defined 550 foot-pounds per second (the pound in context being pound-force). similarly, neither gallon nor imperial gallon 1 cubic foot or 1 cubic yard— gallon 231 cubic inches , imperial gallon 277.42 cubic inches.

the concept of coherence introduced metric system in third quarter of 19th century; in original form metric system non-coherent—in particular litre 0.001 m , (from hectare derives) 100 m. units of mass , length related each other through physical properties of water, gram having been designed being mass of 1 cubic centimetre of water @ freezing point.