The concept of electrical conductivity, or, as initially coined by Stephen Gray (1666–1736), ‘electrical communication’, has always been assigned an important role in the history of electrical research. Some thirty-five years after Gray's ‘electrical communication’ acquired wide attention, Priestley employed the concept of conductivity to define physical reality, thus giving a privileged position to the science he himself endeavoured to cultivate. As he argued in the introduction to The History and Present State of Electricity (1767), ‘the electrical fluid is no local, or occasional agent in the theatre of the world. Late discoveries show that its presence and effects are every where … It is not, like magnetism, confined to one kind of bodies, but every thing we know is a conductor or nonconductor of electricity’. Contemporary historians, for example, Heilbron, Home and Hackmann, link the concept of conductivity to a radical transformation of electrical research which pertained to its mode of organization and the definition of its subject-matter, and which culminated in its emergence as a distinctive branch of eighteenth-century ‘experimental philosophy’.