As used by Einstein, such as in his book: Einstein, Albert (2005). The Meaning of Relativity (1956, 5th ed.). Princeton University Press (2005). pp. 21 ff.
Heaviside, O. (18 November 1882). "The relations between magnetic force and electric current". The Electrician. London, UK.
Alternate source for the same text:
Heaviside, Oliver (1892). "Section 24, The relations between magnetic force and electric current". Electrical Papers (Vol 1.). London: MacMillan and Co. p. 195 (see page 200).