An experiment proposed by J. P. Wesley has been carried out to test for the existence of longitudinal ponderomotive forces between current-carrying conductive elements within a single circuit, as predicted by Amp?re but denied by the Lorentz force law. The experiment performed consists in passing a low-frequency alternating current I of a few amperes through a mercury cell of varying cross section and observing the resulting internal pressure (Amp?re tension) differences by measuring height changes of vertical mercury columns situated at positions of different current density. The resulting I2 ponderomotive forces predicted by Amp?re-Wesley are observed (at twice the frequency of electrical excitation) to produce a mechanical resonance corresponding to the natural oscillation frequency of a portion of the total mercury mass. Alternative explanations based on the Lorentz (Biot-Savart) force law are sought but not found. It is concluded that
- Amp?re appears to have been correct in demanding compatibility between the inter-current-element force law and Newton's third law, and
- the action of a noncovariant force may have been observed in nature.
