13. In mechanics, pressures of all kinds are measured by the effects they produce; similarly, in electrotechnics, potential is measured by the effect it produces, and the volt, or practical unit of potential, is that electromotive force which will maintain a current of one ampere in a circuit whose resistance is one ohm. With a known resistance in ohms and a known strength of current in amperes, the electromotive force in volts is determined by Ohm's law, for, by transposing, C = E/R may be written E = CR.