Schematic view of the experimental setup which was used for the measurement of the electric field intensity distributions at the eigenfrequencies of isolated resonances. A cylindrically shaped perturbation body made of magnetic rubber was inserted into the resonator, thereby causing a frequency shift Δ f ( x , y ) , which according to the Slater theorem is proportional to the squared electric field at its position. The electric field intensity distribution E z 2 ( x , y ) was determined by moving the perturbation body along the bottom plate of the resonator with an external guiding magnet, which was fixed to a positioning unit, and measuring Δ f ( x , y ) .