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. Since elastance is the ratio of voltage over charge, then it follows that the analogue of elastance in another energy domain is the ratio of a generalised force over a generalised displacement. Thus, an elastance can be defined in any energy domain. Elastance is used as the name of the generalised quantity in the formal analysis of systems with multiple energy domains, such as is done with bond graphs. Other analogies Maxwell's analogy is not the only way that analogies can be constructed between mechanical and electrical systems. There are any number of ways to do this. One very common system is the mobility analogy. In this analogy force maps to current instead of voltage. Electrical impedance no longer maps to mechanical impedance, and likewise, electrical elastance no longer maps to mechanical elastance. References Bibliography Blake, F. C., "On electrostatic transformers and coupling coefficients", Journal of the American Institute of Electrical Engineers, vol. 40, no. 1, pp. 23–29, January 1921 Borutzky, Wolfgang, Bond Graph Methodology, Springer, 2009 . Busch-Vishniac, Ilene J., Electromechanical Sensors and Actuators, Springer Science & Business Media, 1999 . Camara, John A., Electrical and Electronics Reference Manual for the Electrical and Computer PE Exam, Professional Publications, 2010 . Cauer, E.; Mathis, W.; Pauli, R., "Life and Work of Wilhelm Cauer (1900 – 1945)", Proceedings of the Fourteenth International Symposium of Mathematical Theory of Networks and Systems (MTNS2000), Perpignan, June, 2000. Enderle, John; Bronzino, Joseph, Introduction to Biomedical Engineering, Academic Press, 2011 . Fuchs, Hans U., The Dynamics of Heat: A Unified Approach to Thermodynamics and Heat Transfer, Springer Science & Business Media, 2010 . Gupta, S. C., Thermodynamics, Pearson Education India, 2005 . Heaviside, Oliver, Electromagnetic Theory: Volume I, Cosimo, 2007 (first published 1893). Hillert, Mats, Phase Equilibria, Phase Diagrams and Phase Transformations, Cambridge University Press, 2007 . Horowitz, Isaac M., Synthesis of Feedback Systems, Elsevier, 2013 . Howe, G. W. O., "The nomenclature of the fundamental concepts of electrical engineering", Journal of the Institution of Electrical Engineers, vol. 70, no. 420, pp. 54–61, December 1931. Jerrard, H. G.,