The interaction between electric and magnetic fields on the one hand, and hot gases (plasma) on the other is of great practical importance. Known as “magnetogasdynamics” it has developed into a wide field of research. In contradistinction, the corresponding problem of the interaction between electromagnetic fields and solid bodies has long remained dormant. Of course, certain special effects as, for instance, piezoelectricity or photoelasticity, have been well studied and put to technological use. But it is only for a relatively recent period of time that a general theory has been developed.