An unusual electromagnetic surface force

Abstract

IN a recent experiment¹ a disk of high dielectric constant material (barium titanate) was suspended as a torsional pendulum in an intense axial magnetic field. It was found that if a radial electric field was applied to the disk in time quadrature with the magnetic field a substantial torque was produced which, to within experimental accuracy (<5%), was in agreement with a force density Ṗ×μ₀H in the disk. The surprising feature of the torque was that it had a time average, contradicting the ‘Abraham force’ which predicts a force density of ∂(P×μ₀H)/∂t with zero time average. The experiment seemed to suggest that the quantity Ṗ×μ₀H does not produce a force since its time average would be equal but of opposite sign to the time average of Ṗ×μ₀Ḣ.