"This instrument, represented by [the cut at the right, below] is designed to show the change produced in the direction of rotation by altering the course of the current. The helix, which surrounds the lower end of the bar, is represented in the section, in order to show a wide glass tube within, which can be filled with mercury. The iron bar rests in an agate cup at the bottom of the tube. Its upper end connects with a glass cup in an ivory arm proceeding from the upright brass pillar, This cup may be united by a bent wire with a similar one in the pillar, as represented by the cut. When the screw cups on the base board are connected with a battery, the current passes from one of the cups up the pillar and along the [missing] bent wire at the top; thence it passes down the inclined bar. A little mercury in the glass tube conveys the current to the brass at the bottom of the tube. It then passes through the helix to the other screw cup. The bar now rotates on its axis; a circular card attached to it renders the motion perceptible as some distance." (from the 1851 edition of the Manual of Magnetism, pg 176) This apparatus is at Harvard, and the original cost was $5.00 to $6.00.