The action of electromagnetism can be ‘explained’ in terms of relativistic corrections to the inverse-square law of electric charges or of any convenient analogy. One of the commonest is that of the electric and magnetic circuits. What constitutes a ‘good’ magnetic circuit and a ‘good’ electric circuit in this context is generally recognised, but it is not so easy to define what one means by a good electrical machine. Yet a combination of the properties of the electric and magnetic circuits must lead to a factor which expresses in some way the ‘goodness’ of the combination. This factor can be evaluated and applied to machine theory with most interesting results. Some examples are given of the ways in which general properties of conventional machines and the performance of some rather curious machines depend on the ‘goodness’ factor.