The Meteor Smoke Particle Detector (MSPD) is based on the principle of a classical Faraday-cup for detection of charged aerosol particles. The design of the classical Faraday cup is described in Havnes et al. (1996) and comprises a collector electrode (held at payload potential by the negative feedback loop of the electrometer) for the measurement of particles of either positive or negative charge and two grids biased at +/- 3 to 6 V relative to payload potential to shield the collector electrode from electrons and ions. The outermost grid is negatively biased and is connected to an electrometer to measure positive ion density. The MSPD utilizes the same measurement principle, but is designed as a spherical probe. Such a geometry allows for detection of smaller particles, because of its better aerodynamical properties.