IV - Matter and Force: Ether and Field Theories

Summary

The term ‘magnetic field’ was introduced by Faraday in 1845, and subsequently adopted by Thomson and Maxwell, whose usage clearly echoed Faraday's. Thomson first used the expression ‘field of feree’ in a letter to Faraday in 1849, following their discussion of the nature of magnetism; and Maxwell first referred to a ‘magnetic field’ in a letter to Thomson in 1854, in the context of a discussion of Faraday's ideas. Maxwell gave the term ‘field’ its first clear definition, in consonance with previous usage, in his paper ‘A dynamical theory of the electromagnetic field’ (1865); there he stated, ‘The theory I propose may therefore be called a theory of the Electromagnetic Field, because it has to do with the space in the neighbourhood of the electric or magnetic bodies’. The concept of a field was to be contrasted with an action-at-a-distance theory of electric action; that is, the mediation of forces by the agency of the contiguous elements of the field existing in the space between separated electrified bodies was to be distinguished from the action of forces operating directly between electrified bodies across finite distances of space.

The inclusive breadth of Maxwell's definition of the field makes it apparent that the physical status of the field was not defined uniquely. In a field theory the forces between bodies were mediated by some property of the ambient space or field.