The Arc Lamp: Burning Carbons and Keeping the Gap

**US 190,864, 'Improvement in Carbons for Electric Lights', Paul Jablochkoff, Paris, dated 15 May 1877.** The carbons are placed *"vertically and parallel"* at a short distance apart, the space filled with an insulating substance that burns away with them — the aim, in his words, the *"suppression of any mechanical regulator"*. In service the candles were run on alternating current, so that both carbons burned at the same rate. **US 219,208, 'Electric-Lamp', Charles F. Brush, Cleveland, Ohio, dated 2 September 1879.** A lamp *"having two or more sets of carbons"*, with a lifting device arranged so that *"the carbons of the two sets"* separate one after the other: when the first pair has burned away, the arc passes to the next, doubling the hours a lamp can burn without attention.

The regulator can be built and tested safely at 12 V with no arc at all. Mount a small push-pull solenoid vertically on a plywood stand, its plunger pointing down, and fix a short graphite rod (cut from a graphite block) to the bottom of the plunger. Below it, fix a second graphite rod to the base so the two just touch when the plunger hangs free — that is the lamp 'off'. Wire the 12 V battery, a 10 W power resistor, the solenoid coil and the two rods all in SERIES. Close the circuit: current flows through the touching rods, the solenoid pulls the upper rod up, the contact opens, the current stops, the plunger drops and touches again. It buzzes: that is the regulator's feedback loop hunting, with nothing to hold it steady. Add a light spring or a small weight to the plunger and a drop of oil as a damper, and the buzzing slows and settles — the job Brush's clutch and dashpot did. Keep it at 12 V. Do not try to draw an arc from mains or a welder: the light burns eyes.

Humphry Davy demonstrated the carbon arc early in the nineteenth century; it became practical only with the dynamo. Jablochkoff's candles lit the Avenue de l'Opéra in Paris in 1878, and Brush's series arc systems lit American city streets from 1879. **Hertha Ayrton**'s book *The Electric Arc* (1902) gave the equation the notebook uses. **Honest limits.** Arc lamps are dazzling, hiss, flicker, need their carbons replaced daily and emit dangerous ultraviolet; they gave way to the incandescent lamp (the next rung) indoors and survived only where intense light was needed — searchlights, cinema projectors.