A device for converting an input quantity X (e.g. a magnetic field) into an output quantity Z (e.g. an electric field) via another quantity Y (e.g. a mechanical force) by means of a composite material consisting of a heterogenous mixture of at least two component phases which statistically periodically alternate one with another throughout the material and are in intimate contact with one another, one phase of which converts X into Y while the other phase converts Y into Z.