PURPOSE: Show that electromagnetic theory obeys a relativity principle.

DESCRIPTION: Measure the current (qualitatively) in a coil when the coil and a bar magnet are in relative motion. Einstein (1905) complained that the standard conceptualization (electric forces if the magnet moves, magnetic forces if the coil moves) seems more convoluted than the phenomenon which appears the same whichever component is "at rest." In the upper row of photos below (with a short video of the action linked behind each of them) a coil is moved onto the north pole of a stationary magnet and the north pole of a moving magnet is inserted into a stationary coil. The bottom row of photos shows the reverse setup, with a coil moved onto the south pole of a magnet, and the south pole of a moving magnet inserted into a stationary coil.

SUGGESTIONS:

See Question of the Week #226 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for articles on this topic, including the first section of Einstein's paper on special relativity.

EQUIPMENT: Magnet, earth inductor coil, and overhead projector microammeter.

SETUP TIME: None.