| "The revolution of a conductor around a magnet is shown
by the instrument represented [at the left]. Two light frames of copper
wire ... are supported by pivots resting on the poles... of a steel magnet
of the U-form; a small cavity being drilled in each pole to receive
an agate for the bearing of the pivot. The lower extremities of the wires
dip into mercury contained in two circular cisterns sliding on the arms
of the magnet... The cups surmounting the frames should also contain a
On connecting the cups [on the left cistern and the top of the cistern on that side] with the battery, the left-hand frame will revolve in consequence of the action of the north [left-hand] pole of the magnet upon the current flowing in the vertical portions of the frame. By uniting the [top cup and the cup on the right cistern] with the battery, the other frame will rotate. On transmitting the current [from one cistern to the other], it will ascend in one frame, and passing along the brass arm [across the top] will descend in the other, causing them both to revolve in the same direction. Instead of the frame, a single wire may be employed, having the form of a loose helix surrounding the pole, its convolutions being a quarter of an inch or more apart.
(from the 1842 Manual of Magnetism, pp 87-88)
| The device at the left is clearly not by Davis (it is
ca. 1900), but it does have the helical wires mentioned by him. It is at
the collection of scientific apparatus at Transylvania University in Lexington,
Kentucky. The two points connected to the ball on the center column are,
in use, placed in the mercury-filled cups at the tops of the rotating helices.