represented as an electrical resistance in the electrical equivalent circuit. It is also natural to think of force as analogous to voltage (generator voltages are often called electromotive force) and velocity as analogous to current. It is this basic analogy that leads to the analogy between electrical and mechanical impedance. The principal disadvantage of the impedance analogy is that it does not preserve the topology of the mechanical system. Elements that are in series in the mechanical system are in parallel in the electrical equivalent circuit and vice versa. The impedance matrix representation of a transducer transforms force in the mechanical domain into current in the electrical domain. Likewise, velocity in the mechanical domain is transformed into voltage in the electrical domain. A two-port device that transforms a voltage into an analogous quantity can be represented as a simple transformer. A device that transforms a voltage into an analogue of the dual property of voltage (that is, current, whose analogue is velocity) is represented as a gyrator. Since force is analogous to voltage, not current, this may seem like a disadvantage on the face of it. However, many practical transducers, especially at audio frequencies, work by electromagnetic induction and are governed by just such a relationship. For instance, the force on a current-carrying conductor is given by, where History The impedance analogy is sometimes called the Maxwell analogy after James Clerk Maxwell (1831–1879) who used mechanical analogies to explain his ideas of electromagnetic fields. However, the term impedance was not coined until 1886 (by Oliver Heaviside), the idea of complex impedance was introduced by Arthur E. Kennelly in 1893, and the concept of impedance was not extended into the mechanical domain until 1920 by Kennelly and Arthur Gordon Webster. Henri Poincaré in 1907 was the first to describe a transducer as a pair of linear algebraic equations relating electrical variables (voltage and current) to mechanical variables (force and velocity). Wegel, in 1921, was the first to express these equations in terms of mechanical impedance as well as electrical impedance. References Bibliography Beranek, Leo Leroy; Mellow, Tim J., Acoustics: Sound Fields and Transducers, Academic Press, 2012 . Busch-Vishniac, Ilene J., Electromechanical Sensors and Actuators, Springer Science & Business Media, 1999 . Carr, Joseph J., RF Components and Circuits, Newnes, 2002 . Darlington, S. "A history of network synthesis and filter theory for circuits composed of resistors, inductors, and capacitors", IEEE Transactions