The wire is usually insulated (often with a tough, transparent and nearly invisible enamel coating for commercial solenoids), but doesn’t have to be. (Note that the field lines in the centre are truncated to save space, but would form very large loops as mentioned above.) Using the elegant differential notation, Maxwell’s second equation is written like this: This is a consequence of Maxwell’s second equation of Electromagnetism (one of a system of four equations developed by James Clark Maxwell in 1873 that summarise our current understanding of electromagnetism). ‘Seem to’ because magnetic field lines always form closed loops.
The field lines seem to begin at the north pole and end at the south pole. Permanent magnets now became as cheap as chips.Ī permanent bar magnet is wrapped in an invisible evanescent magnetic field that, given sufficient poetic license, can remind one of the soft gossamery wings of a butterfly… Orient a bar magnet vertically so that students can see the ‘butterfly field’ analogy…
Over many centuries, by patient trial-and-error, humans learned how to magnetise a piece of iron to make a permanent magnet. In ancient times, lodestones were so rare and precious that they were worth more than their weight in gold. (The origin of the name is probably not what you think - it’s named after the region, Magnesia, where it was first found). Lodestones are pieces of the ore magnetite that can attract iron.
Very few minerals are naturally magnetised.
An alternate universe version of the Troggs’ famous 1966 hit recordĪnd they are such an all-embracingly useful bit of kit that one might imagine an alternate universe where The Troggs might have sang: Pipe-thing! You make my heart sing!Īnd pipe-things do indeed make everything groovy: solenoids are at the heart of the magnetic pickups that capture the magnificent guitar riffs of The Troggs at their finest.
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