Invalidation of the Kelvin Force in Ferrofluids

Abstract

Direct and unambiguous experimental evidence for the magnetic force density being of the form $M\mathbf{\nabla }B$ in a certain geometry—rather than being the Kelvin force $M\mathbf{\nabla }H$—is provided for the first time. ( $M$ is the magnetization, $H$ is the field, and $B$ is the flux density.)