Measurement of forces related to electromagnetic momentum in material media at low frequencies

Abstract

Direct observation of the classical reaction force density −ε 0 μ 0 (∂E × H)/∂t with nonradiating fields (μ 0 H = constant) and inside a low permittivity nonmagnetic material has been carried out for the first time. Its magnitude seems to be ~25% larger than the theoretically predicted value although considerable experimental error (~20%) prevents a final appraisal of the significance of that discrepancy. Successful measurements of the force due to an external magnetic field on polarization currents inside two magnetite specimens at liquid He temperatures are also reported. Together these experiments give substantial credibility to the fine details of the so-called "Abraham force" in the low frequency range. However, possible observable differences arising from the alternative use of ε 0 μ 0 E × H or ε 0 E × B as the classical electromagnetic momentum in magnetic materials still remain within our experimental errors.