Some experiments have indicated the possible existence of particles with a negative inertial mass. It is shown under which conditions Weber's electrodynamics gives rise to this effect. Some specific experiments related to this aspect of Weber's law are described. Two particles equally electrified with charges of the same sign would then move toward one another if they had negative effective inertial masses. A new concept for nuclear fusion is presented based on the possibility of creating a negative effective inertial mass for ions. It is then considered some properties of the inertial dipole, that is, a system composed by a pair of particles in which one particle has a positive effective inertial mass while the other particle has a negative effective inertial mass. The possible utilization of the inertial dipole as a propulsion system is briefly discussed.41 7782