farad (F−1). The term daraf is sometimes used for this unit, but it is not approved by SI and its use is discouraged. The term is formed by writing farad backwards, in much the same way as the unit mho (unit of conductance, also not approved by SI) is formed by writing ohm backwards. The term daraf was coined by Arthur E. Kennelly. He used it from at least 1920. History The terms elastance and elastivity were coined by Oliver Heaviside in 1886. Heaviside coined a great many of the terms used in circuit analysis today, such as impedance, inductance, admittance, and conductance. Heaviside's terminology followed the model of resistance and resistivity with the -ance ending used for extensive properties and the -ivity ending used for intensive properties. The extensive properties are used in circuit analysis (they are the "values" of components) and the intensive properties are used in field analysis. Heaviside's nomenclature was designed to highlight the connection between corresponding quantities in field and circuit. Elastivity is the intensive property of a material corresponding to the bulk property of a component, elastance. It is the reciprocal of permittivity. As Heaviside put it, Here, permittance is Heaviside's term for capacitance. He did not like any term that suggested that a capacitor was a container for holding charge. He rejected the terms capacity (capacitance) and capacious (capacitive) and their inverses incapacity and incapacious. The terms current in his time for a capacitor were condenser (suggesting that the "electric fluid" could be condensed out) and leyden after the Leyden jar, an early form of capacitor, also suggesting some sort of storage. Heaviside preferred the analogy of a mechanical spring under compression, hence his preference for terms that suggested a property of a spring. This preference was a result of Heaviside following James Clerk Maxwell's view of electric current, or at least, Heaviside's interpretation of it. In this view, electric current is a flow caused by the electromotive force and is the analogue of velocity caused by a mechanical force. At the capacitor, this current causes a "displacement" whose rate of change is equal to the current. The displacement is viewed as an electric strain, like a mechanical strain in a compressed spring. The existence of a flow of physical charge is denied, as is the buildup of charge on the capacitor plates. This is replaced with the concept of divergence of the