The movement of charged particles in electromagnetic and magnetostatic field gradients is examined. It is shown that when the orientation and amplitude of such gradients are suitably arranged, the mean force in relation to time acting on these particles remains constant in direction. Discussed are this mean force and, more generally, the validity of the theory of mean value in relation to time.The results are applied to the case of a low-density plasma, and a brief description is given of four possible applications of a device based upon such forces: as injector, plasma propellant, vacuum pump and direct converter of thermal energy into u.h.f. electromagnetic energy. Orders of magnitude are stated. In two annexes, the authors determine the limits of validity of the theory based on mean value, and a comparison is made with an exact theory. The curves obtained are described and classified by analogue computer.