Measuring apparatus and measuring method for detecting ferromagnetic particles which are movably arranged in a liquid volume, characterized by a magnetic means for generating a magnetic field in the liquid volume, a measuring means for the total electrical resistance of the liquid volume with the ferromagnetic particles therein and a means for checking the function of the measuring apparatus with at least one low-value resistor of between 50 and 500Ω for carrying out a resistance measurement. The invention also describes a magnetic means with an electromagnet which, when in need of cleaning, can be demagnetized by means of correct energization (measured using the normal resistance measurement) and can be magnetized again when started up again.