Measuring device for the electrically decoupled functional checking of working systems

The arrangement has a current carrying conductor (2) directly mechanically coupled to a mechanical-electrical transducer (8) via an essentially incompressible medium. The transducer detects vibrations caused by the conductor and feeds the signal to an evaluation unit via an amplifier. The mechanical-electrical transducer is in the form of a strain gauge system or a piezoceramic element. The piezoceramic element can be a knock detector and the current carrying conductor can be the ignition coil of an internal combustion engine.