Here is a very simple way to demonstrate the magnetic force that acts on a current-carrying wire in an external magnetic field. Fashion a piece of solid copper wire (18 gauge works best) into a "D" shape so that most of the central back part of the dee is missing (see Fig. 1) and clip it between the ends of a D-cell battery. The D-shaped wire should be large enough so that it swings clear of the side of the battery. Clipped over the battery, the compressive forces within the wire will keep it attached at either terminal so that it is free to swing. A current now flows through the D-shaped wire. A single neodym-ium "supermagnet" is then placed onto the side of the battery. Since the casing of the battery is ferromagnetic, the magnet sticks to its side and causes a magnetic force to act on the current-carrying wire as the wire is now in the presence of an external magnetic field. The whole arrangement can then be positioned so that the D-shaped wire levitates from the side of the battery against the force of gravity (see Fig. 2). As the directions of both the current flowing in the D-shaped wire and the magnetic force that acts on it are known, the polarity for the face of the magnet showing can be found from the right-hand rule.
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