Charged particles collected by the filter generate an electric current which is measured by the electrometer.
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.
The simplest form is the following: A Faraday detector consists of a metal cup, that is placed in the path of the particle beam.