Kelvin's law
(also Lord Kelvin's law)
noun
Any of several laws enunciated by Lord Kelvin; specifically the law that the most economical cross-sectional area of a conductor used as a transmission line is that for which the cost of the energy dissipated in any period is equal to the charges during the same period on the capital cost of the line.
Origin
Late 19th century; earliest use found in Science.