In the analogy developed by Maxwell, now known as the impedance analogy, voltage is made analogous to force. The voltage of a source of electric power is still called electromotive force for this reason. Current is analogous to velocity. The time derivative of distance (displacement) is equal to velocity and the time derivative of momentum is equal to force. Quantities in other energy domains that are in this same differential relationship are called respectively generalised displacement, generalised velocity, generalised momentum, and generalised force. In the electrical domain, it can be seen that the generalised displacement is charge, explaining the Maxwellians' use of the term displacement.