A direct method to measure the localized magnetic shear stress andhence electromagnetic torque or force exerted on a wall by afluctuating magnetic field is presented. A model of the wall is neededwhen the magnetic measurements do not include the normalcomponent. Explicit formulae for thin and thick walls aregiven. Estimates of the force in illustrative Alcator C-Mod tokamakplasmas provide evidence against the theory that resistive magneticdrag determines mode locking.