A thermally assisted magnetic head which can realize high-density writing onto magnetic recording media is provided. A test medium having an electronic data memory for data is provided, the data being used for testing the sensor. The shapes, sizes, positions and structures of the electrodes are selected to optimize the voltage gradient of the electric field, and to minimize the energy dissipated by the electric field outside the heart. A first layer comprising electrically conductive areas is applied to the side of the element-enveloping compound composite that previously adjoined the carrier. The switch control unit includes a sliding member mounted at the second switch.