On this day in 1831 — 195 years ago — Michael Faraday wound two separate coils of wire onto an iron ring at the Royal Institution, connected one to a battery and the other to a needle detector, and watched. The needle twitched when he made the connection, and twitched again when he broke it. In between, with the current flowing steadily, nothing.
That nothing is the whole discovery. Magnetism sitting still doesn't push a current; only a change in it does. Our ears work the same way: we don't hear the steady weight of the air around us, we hear it wobble. Switch the first coil on and the iron ring's magnetism swells; switch it off and it collapses. Each swell and collapse gives the electrons in the second coil a shove. Hold it constant and the ear, so to speak, goes quiet.
What I admire most is not the ring but the notebook. Faraday had little formal schooling and an enormous habit of writing things down — the twitch, the arrangement, the attempts that gave him nothing at all, and the date. That is why we can stand here in August and know exactly which afternoon it was. Honest records are the cheapest instrument in any lab, and the one I was worst at keeping.
Ada, that ring is still in service. Every dynamo, every transformer on a pole, every little charger warming up on your bedside table is Faraday's two coils and an iron core, with better insulation and worse handwriting in the notebook.
Ada, that twitch is still doing shifts: spacecraft magnetometers work on Faraday's rule, and Voyager 1 used its readings to tell us in 2012 that it had crossed into interstellar space, from about 18 billion kilometres away. A needle wobble, sent home on roughly the power of a refrigerator bulb.