Sci/Hist: What Oersted found

Hans Christian Oersted (1777 – 1851) (Danes write his name ‘Ørsted’), gave his name to the unit of magnetic field strength, the oersted. He also made up the word ‘electromagnetic’, so you shouldn’t be surprised to learn that he discovered the magnetic effect of an electric current.

He connected a battery with a wire that carried electricity over a magnetic compass, and saw that the compass needle moved. If he reversed the current, the needle moved the other way, but if he ran the wire under the compass, this also reversed the needle’s movement.

A home version of Oersted’s experiment. Notice how the compass needle changes when the wire is either over or under the compass.

You can see how it worked in the photo above, and as this is simple enough for the reader to try, let me note that the entire apparatus is one compass, one AA cell, a length of insulated wire and some sticky tape. The aluminium ruler is optional, which would have pleased Oersted (I’m deliberately not saying why!). I bared one end of the wire, taped it to one end of the dry cell (this was sloppy but good enough) and I bared the other end of the wire.

I taped the dry cell to the ruler, taped the compass to the ruler to stabilise it, and that was it. As you can see, the current from a single dry cell was enough to bring about a noticeable swing. Incidentally, if you reverse the wire (and as a result, the current), the swing reverses. Play with it!

A quarter of a century later Oersted’s discovery was essential to the working of the telegraph lines that had started to link the world. In his 1846 Introductory Lecture to the Course on Natural Philosophy, William Thomson (who later became Lord Kelvin), commented on what Oersted saw:

Oersted would never have made his great discovery of the action of galvanic currents on magnets had he stopped in his researches to consider in what manner they could possibly be turned to practical account; and so we would not now be able to boast of the wonders done by the electric telegraphs. Indeed, no great law in Natural Philosophy has ever been discovered for its practical implications, but the instances are innumerable of investigations apparently quite useless in this narrow sense of the word which have led to the most valuable results.

So curiosity is good!

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