The use of high-frequency electric fields for heating dielectric
materials had been proposed in the 1930s, for example US patent
2,147,689 (application by Bell Telephone Laboratories, dated 1937)
states "This invention relates to heating systems for dielectric
materials and the object of the invention is to heat such materials
uniformly and substantially simultaneously throughout their mass. ...
It has been proposed therefore to heat such materials simultaneously
throughout their mass by means of the dielectric loss produced in them
when they are subjected to a high voltage, high frequency field."