In a metal housing (Faraday cup) a particle filter is mounted on an insulator. A Faraday cup is a detector that measures the current in a beam of charged (aerosol) particles. Faraday cups are used e.g. in mass spectrometers being an alternative to secondary electron multipliers. The advantage of the Faraday cup is its robustness and the possibility to measure the ion or electron stream absolutely. Furthermore, the sensitiveness is constant by time and not mass-dependent. The simplest form is the following: A Faraday detector consists of a metal cup, that is placed in the path of the particle beam. The aerosol has to pass the filter inside the cup. The filter has to be isolated. It is connected to the electrometer circuit which measures the current.