ColerÂÂs original electro-magnetic apparatus was claimed to provide an unexplained source of energy. An apparatus able to supply unlimited power output would have clear potential uses in deep-space missions. The equipment consists of a unique arrangement of electromagnetic excitation coils and ferromagnetic cores, each having a length much larger than the core diameter. Recent investigations concerning this apparatus have explained certain phenomena by hypothesizing that there exist longitudinal acoustic resonant modes within the magnetic cores used. The large length/diameter ratio allows consideration of modes with wave-vector parallel to the principal axis independently of possible modes in other directions that are not excited by the excitation coil arrangement. However, differences in the amplitudes of the observed modes were left unexplained. In the present paper, it is shown that the hypothesis of exciting longitudinal acoustic modes will explain the observed differences in resonance amplitudes when the lengths of the excitation and pick-up coils on the magnetic core are considered. Some modes are excited more than others according to the physical overlap of each normal mode with the excitation and pick-up coils. As a result, the coils can be designed to maximize the resonant amplitudes, thereby optimizing the operation of the apparatus.