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. Firestone's analogy became popular amongst mechanical filter designers because it has the property of preserving network topologies when transforming between the mechanical and electrical domains.[10][11]