The Incandescent Lamp: High Resistance, and a Vacuum

Pack a few 30 mm lengths of cotton thread, stretched straight between two small copper wire loops, in graphite powder inside a small steel tin with a pinhole in the lid. Heat the tin outdoors with the propane torch until it glows red, and hold it there for twenty minutes — cellulose only turns into a well-conducting carbon at red heat. Let it cool completely before opening: air reaching hot carbon burns it. The threads come out black, brittle and conducting. Measure one with the multimeter and compare with Edison's hundreds of ohms; a poorly heated thread reads far higher. Handle them with tweezers; they snap at a touch.

Clamp one filament between two stiff copper wires passed through the lid of a thick glass jar and sealed with epoxy or silicone. In AIR, with the lid off, connect it briefly across a 12 V battery through a series resistor: it glows red for a moment and burns through. Mount a fresh filament, close the jar, and pump it down with the vacuum pump through a fitting in the lid. Connect the battery again: the filament glows and lasts far longer, because there is almost no oxygen left to burn it. Your pump will not reach Edison's one-millionth of an atmosphere; the difference is still obvious. Stand the jar behind a clear shield and wear goggles: an evacuated jar can implode.

**US 223,898, 'Electric-Lamp', Thomas A. Edison, patented 27 January 1880.** The specification reports carbonising *"cotton and linen thread, wood splints, papers coiled in various ways"*; carbonised bamboo became the production filament soon after. Joseph Swan in England had been developing carbon lamps in parallel, and the two later merged their British interests. Edison's lamp came with a whole system — parallel distribution, meters, fuses, generators — which is what made it change cities. **Honest limits.** A carbon-filament lamp turns only a few per cent of its power into light; the rest is heat. The filament evaporates and blackens the bulb. Tungsten filaments, gas filling and later LEDs each improved on it many times over.