Edison and the Electric Chair_ A Story of Light and Death - Mark Essig [11]
The source of excitement among the scientists on the western trip was a new version of the electric arc lamp that had just been unveiled at the Paris Universal Exposition by the Russian engineer Paul Jablochkoff. The light's basic principle—running a strong electric current across a gap between two slender carbon rods—had been discovered by Humphry Davy seventy years before, but the technology had changed dramatically over the decades. Whereas Davy had used electricity created by a chemical battery, the Jablochkoff lamps used the latest design of electrical generator.
Because the understanding of electricity as a movement of electrons in a conductor would not emerge until around 1900, in the 1870s not even the greatest electricians could claim to know just what happened inside a copper electric wire. But this lack of theoretical understanding did not prevent scientists from becoming adept at manipulating electrical force. Faraday had discovered that moving a coil of conducting wire through the lines of force of a magnetic field caused current to flow in the conductor, and later experimenters learned that they could increase the strength of the current by using stronger magnets and multiplying the number of coils of conducting wire. The first generators employed permanent steel magnets, which were relatively weak. To skirt this difficulty, inventors in the late 1860s turned to the discovery that first inspired Faraday—the ability of electric current to produce a magnetic field—and built generators that replaced permanent magnets with far more potent electromagnets. At first the current for the electromagnets was supplied by batteries or smaller generators, but in the 1860s and 1870s inventors designed generators that produced the current for their own electromagnets. Because these machines excited their own magnetic fields, they were known as dynamo-electric generators, or dynamos. The new machines could produce a current that was powerful, steady, and inexpensive enough for arc lighting.2
In the Jablochkoff arc lamp system, the creation of light started with the burning of coal, which heated water in a boiler and produced steam. The steam drove the piston in an engine, and the piston moved a driveshaft, which was connected via a leather belt to the dynamo. The belt turned the dynamo's armature, an iron core wrapped with coils of copper wire. As it spun at hundreds of revolutions per minute, the armature repeatedly cut through the lines of force of an electromagnet. The movement of a conductor (the armature) through the magnetic field produced an electric current, which flowed through copper wire to the lamps, each of which contained two pencil-thin rods of carbon a fraction of an inch apart. The current leaped the gap between the carbon rods—producing a powerful light—then flowed on to the next lamp. The process moved from coal to steam to mechanical motion to electricity; it was a simple matter of the transformation of energy, from black coal to white light.
On September 8, 1878, two weeks after returning from his western trip, Edison visited the Connecticut factory of William Wallace, who in the previous few months had developed his own system of arc lighting. A newspaper reporter described the inventor's reaction to Wallace's factory: "Mr. Edison was enraptured. He fairly gloated over it. . . . He ran from the instruments to the lights, and from the lights back to the instruments. He sprawled over a table with the SIMPLICITY OF A CHILD, and made all kinds of calculations." Edison ordered a generator from Wallace, then returned to Menlo Park.3
When he began his lighting experiments, Edison chose