The outer coil is fixed atop the pillar, and the inner coil is free to rotate. A commutator is used to supply current to the inner coil in both directions as it rotates. Because there are long, straight wires interacting here, it is easy to discuss the operation of the instrument in terms of the attraction between parallel current-carrying wires. 

   The apparatus is described in the 1851 edition of the Manual of Magnetism, but not in the 1842 edition. Davis saw this apparatus as an improvement on the rotating ring. 

   This instrument is in the Harvard Collection of Historical Scientific Instruments. In 1851 its cost was $7.00.