This page recalls Ampere’s Rotator, which was originally discussed in his submission to a French journal in 1821. On this page, you can find a recreation of a similar rotating conductor device created by Joseph Henry and modified by James Marsh.
A transcription of Ampere’s description of the rotating conductor reads:
ABCD (fig. 20) is a zinc vessel, in the center of which there is a cylindrical opening whose edges, also made of zinc, end in a vertical cylindrical surface and rise up inside the vessel. The space between these edges and the inner surface of the vessel ABCD is intended to contain the liquid aid used to excite the electromotive action. The small hollow cylinder placed at the center of the vessel, which ends in a horizontal plane, supports a zinc arc EFG fixed to its outer wall. At the middle point F of this arc rises a vertical copper rod, ending in a cap H designed to hold mercury and to receive the point I of a movable conductor LOM, shown separately (fig. 21).
This conductor is made from a strip of copper with two slender branches, OL and OM, soldered at points L and M to the copper ring LMP, which is fully immersed in the acidic liquid in the vessel ABCD. The cylindrical opening at the center of this vessel allows for the attachment of a bundle of magnetized rods, with like poles all oriented in the same direction. It is around this bundle that the movable conductor LOM and its ring rotate continuously, as soon as the acidic liquid is poured into the vessel ABCD. The direction of rotation can be changed at will by reversing the magnets, which can be explained by considering the horizontal sections of the magnetized bundle as a conductor of a continuous electric current.
It is easy to understand how the six magnetized rods of the apparatus are held together in a regular bundle. Two copper plates, shaped as shown in figure 22, are fixed to each other by two copper rods, hk and mn, of equal length. One or the other of these plates fits onto two hooks attached to the bottom of the vessel ABCD. The rods would slide out of the notches in the plates if they were not held at the bottom by a copper washer pq, which is suspended from the lower plate by two hook-shaped supports pr and qs. When the bundle of magnets is inverted, the washer is also repositioned so that it is always suspended from the lower plate.
One could make the vessel ABCD and its arc EFG out of copper, then connect this arc, as in the previous case, with the movable conductor LOM (fig. 21), which in this case would be made of zinc. The rotation would occur in the opposite direction, with the magnets arranged in the same way; however, the motion would be weaker due to the small surface area of the zinc. The magnets can also be replaced by a wire or a continuous strip, wound several times around the outer surface of the vessel ABCD, with its ends connected to the poles of a voltaic pile. M. Ampère’s apparatus (fig. 20) is mounted on a wooden disk UV, supported by three legs X, Y, and Z. This scientist demonstrated its effects at an Academy session; he had already shown, in the session of November 19, that these phenomena are a necessary consequence of his theory on the action of electric currents and magnets.
By removing the magnets and replacing the movable conductor with another conductor shown in figure 23, due to the action of the Earth, this conductor rotates slowly but continuously, moving from east to west through the south. When the astral pole of a magnet is brought near it from below, the conductor begins to rotate in the opposite direction; but it resumes its original motion as soon as the magnet is removed. This new phenomenon was presented to the Royal Academy of Sciences at the session of December 10, 1821.
Below is his paper, published in the Annales de chimie et de physique, Vol 18. 1821, Page 331. in the original French:
