cross-section and placed 1 metre apart in a vacuum, would produce between these
conductors a force equal to 2π × 10−7 newton, per metre of length.
The kelvin (K), the unit of thermodynamic temperature: The kelvin is the fraction
1/273.16 of the thermodynamic temperature of the triple point of water.
The candela (cd), the unit of luminous intensity: The luminous intensity, in a given
direction, of a source that emits monochromatic radiation of frequency 540 × 1012
hertz and has a radiant intensity in that direction of 1/683 of a watt per steradian.
The mole (mol), the unit of amount of substance: The amount of substance of a
system that contains as many elementary entities as there are atoms in 0.012 kilograms
of carbon-12. When the mole is used, the elementary entities must be specified and
may be atoms, molecules, ions, electrons, other particles, or specified groups of such
particles.
The derived units are formed from products and powers of the base units according
to the algebraic formulae linking the quantities involved. Thus, for example, velocity
is measured as metres per second, the ratio of two of the base units. A few of the
derived units prove to be so useful that they are given special names and symbols;
these are given in Table 1.6.
In order to avoid confusion in the presentation of results the SI conventions should
be adhered to