In 1933 Floyd A. Firestone proposed a new analogy, the mobility analogy, in which force is analogous to current instead of voltage. Firestone introduced the concept of across and through variables in this paper and presented a structure for extending the analogy into other energy domains.[65] A variation of the force-current analogy was proposed by Horace M. Trent in 1955 and it is this version that is generally meant by the through and across analogy.[66] Trent used a linear graph method of representing networks which has resulted in the force-current analogy historically being associated with linear graphs. The force-voltage analogy is historically used with bond graph representations, introduced in 1960 by Henry M. Paynter, however, it is possible to use either analogy with either representation if desired.[67]