Georg Ohm
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The German physicist whose 1827 law relates voltage, current and resistance - the first thing anyone learns about circuits, and the last thing they stop using.
The law was poorly received when published and he waited years for recognition, which is a recurring pattern in this timeline rather than an anecdote. What it gave the field was a design tool: with three quantities related by one equation, a circuit can be reasoned about before it is built. Every cable length calculation, every power budget and every fault involving a bad connection is downstream of it.
Also known as: ohm, ohm's law, resistance