ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be .602176634× – when expressed in the unit C , which is equal to A s, where the second is defined in terms of ∆ ν cs . Thermo kelvin K The kelvin, symbol K, is the SI unit of thermodynamic temperature. dynamic It is defined by taking the fixed numerical value of the Boltzmann constant Temperature k to be .380649× – when expressed in the unit J K – , which is equal to kg m s – k – , where the kilogram, metre and second are defined in terms of h , c and ∆ ν cs .
Amount of mole mol The mole, symbol mol, is the SI unit of amount of substance. One mole substance contains exactly .02214076× elementary entities. This number is the fixed numerical value of the Avogadro constant, N A , when expressed in the unit mol – and is called the Avogadro number. The amount of substance, symbol n , of a system is a measure of the number of specified elementary entities.
An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. Luminous candela cd The candela, symbol cd, is the SI unit of luminous intensity in given direction. intensity It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency × Hz, K cd , to be when expressed in the unit lm W – , which is equal to cd sr W – , or cd sr kg – m – s , where the kilogram, metre and second are defined in terms of h , c and ∆ ν cs . * The values mentioned here need not be remembered or asked in a test.
They are given here only to indicate the extent of accuracy to which they are measured. With progress in technology, the measuring techniques get