  This article describes the dynamics of a charge particle in a electromagnetic
field in presence of a scalar fifth force. Focusing to the fifth force that
would be induced by a chameleon field, the profile of which can be designed
properly in the laboratory, it draws its physical effects on the cyclotron
motion of a particle in a static and uniform magnetic field. The fifth force
induces a drift of the trajectory that is estimated analytically and compared
to numerical computations for profiles motivated by the ones of a chameleon
field within two nested cylinders. The magnitude of the effect and the
detectability of this drift are discussed to argue that this may offer a new
experimental design to test small fifth force in the laboratory. More
important, at the macroscopic level it induces a current that can in principle
also be measured, and would even allow one to access the transverse profile of
the scalar field within the cavity. In both cases, aligning the magnetic field
with the local gravity field suppresses the effects of Newtonian gravity that
would be several order larger than the ones of the fifth force otherwise and
the Newtonian gravity of the cavity on the particle is also argued to be
negligible. Given this insight, this experimental set-up, with its two effects
-- on a single particle and at the macroscopic level -- may require attention
to demonstrate its actual feasability in the laboratory.
