In an air flow entrained with particles, charge transfer (frictional electrification) occurs between the particles and the probe when particles collide with the probe. The particles in the air flow also carries a certain net charge, the induced charge (i.e. electrostatic induction) will therefore be generated on the probe when the particles pass near the probe.

When a particle collides with the probe, the amount of charge transfer depends on the physical and chemical properties (such as size, chemical composition, electric inductivity, etc.) and velocity of the particle. The comprehensive result of a group of particles colliding with the probe is the microelectronic current signal generated on the probe, and the intensity of the signal proportionate to the number of particles colliding with the probe at a certain time interval. The particles are not usually evenly distributed around the probe, and their flow velocity also fluctuates around the average, therefore the intensity of the collision electronic current signal also fluctuates around a certain mean value.

When a particle passes over the probe, the amount of induced charge depends on the shape of the probe, the radial distance between the probe and the particle, and the net charge carried by the particle and the axial velocity of the particle. When the particle passes through the cross section of the duct where the probe is located, if the charge it carries is regarded as a pulse signal, the electronic current signal generated by the induced charge on the probe is the pulse response of the induction system. Therefore, the probe and the space around it will form a signal filter namely spatial filtering effect. When the randomly distributed particles pass the probe, the induced current signal is the filtered result of the original random signal, which contains important information about the particle flow and the signal filter itself.

When the electronic current signals from the above two sources are monitored, processed and analyzed, the flow amount and concentration of particles are therefore obtained.