The errant compass
Walter B. Gratzer · 2002
Abstract Hans Christian Oersted (1777-1851) was Professor of Physics at the University of Copenhagen when he tried a lecture demonstration that changed the course of physics. Oersted was fascinated by magnetism and he did not hold with the prevailing view that magnetism and electricity were unrelated phenomena— fluids, as Ampere [17] had called them. Rather, he thought, they were forces radiated by all substances, which might well interfere with each other, and, indeed, it was well established that a compass in a ship, struck by lightning, would sometimes reverse its polarity. He conjectured, therefore, that a thin wire carrying an electric current might, when placed beside a compass needle, cause it to deflect. For reasons never made clear Oersted chose to test his hypothesis before an audience at one of his lectures instead of first trying it out for himself. He had, he afterwards confessed, hesitated, for prudence suggested that the experiment might not, after all, work and he would look foolish; but on an impulse he went ahead all the same. Oersted sent the current through his thin platinum wire until it glowed. The compass was lying directly beneath, but before he could move it alongside the wire, the needle deflected. The effect was small and probably invisible to the audience, which manifested apathy, but Oersted was astonished: the current flowing in the direction of the needle s axis had deflected it sideways? How could this be?