[57] Magnetic field applicator to heat magnetic magnetizable substances or solids in biological tissue, with a magnetic yoke with two pole shoes facing each other on the magnetic yoke and separated by a gap in the exposure volume and with two magnetic coils for the production of a magnetic alternating field, each of which is assigned to a pole shoe, characterized in that the magnetic yoke (2) and the pole shoes (7, 8) consist of ferrite building blocks (16) mounted together and in that the magnetic yoke is made in form of an M with three legs, with two parallel vertical yoke elements (3, 4) at a distance from each other and preferably of identical geometry and with two transversal yoke elements (5, 6) connected between them, each with pole shoes located in their center and facing each other, at least one transversal yoke element with connected pole shoe and associated magnetic coil (10) being displaceable as a component relative to the other transversal yoke element for the adjustment of the width of the gap in the exposure volume.
