This parameter A r was introduced into CFE theory by Stern, Gossling and Fowler in 1929 (who called it a "weighted mean area"). For practical emitters, the emission current density used in Fowler–Nordheim-type equations is always the current density at some reference point (though this is usually not stated). Long-established convention denotes this reference current density by the simple symbol J, and the corresponding local field and conversion factor by the simple symbols F and β, without the subscript "r" used above; in what follows, this convention is used.