 In 1933 Firestone proposed an alternative to the mechanical-electrical analogy of James Clerk Maxwell in which force is made the analogy of voltage (the impedance analogy).Firestone's analogy (now called the mobility analogy) makes force the analogy of current.In this work he introduced the concept of "through" and "across" variables and demonstrated that there were analogies for these variables in other energy domains, making it possible to treat a complex system as a unified whole in analysis.