On 21 October 1879, at Menlo Park in New Jersey, a lamp containing a filament of carbonised cotton thread burned for about thirteen and a half hours before it failed.
This is usually reported as the invention of the light bulb. It was not. Incandescent lamps had been made and demonstrated for nearly forty years before that evening, and Edison knew it perfectly well, because he had read the patents.
What happened in 1879 was different and more interesting: somebody worked out which lamp would be economically viable, and the answer came out of Joule’s law.
Who Had Already Done It
The list of people who made electric light glow before Edison is long.
- Humphry Davy produced an arc between carbon rods in 1802, and arc lighting was in commercial use for streets and lighthouses by the 1870s. It was blindingly bright, noisy, and needed constant attention
- Warren de la Rue put a platinum coil in an evacuated tube in 1840. It worked. Platinum was far too expensive to sell
- Frederick de Moleyns held a British patent for an incandescent lamp in 1841
- Joseph Swan in Newcastle had been working on carbonised paper filaments since the 1850s, demonstrated a working lamp publicly in February 1879, months before Edison’s, and had a British patent
- William Sawyer and Albon Man in the United States had their own incandescent lamp and their own patents, which later produced years of litigation
So the physics was not in doubt. Push current through a thin conductor, it gets hot and glows, and if you remove the oxygen it does not burn up immediately. Everybody knew that. What nobody had was a lamp you could sell to a city.
The Real Problem Was Copper
Here is the insight that makes Edison’s work an invention rather than an improvement, and it is an economic argument conducted in physics.
A lamp needs a certain amount of power to produce useful light. Power is current multiplied by voltage, and the heat generated in a conductor goes as the current squared multiplied by the resistance, which is the relationship Joule had established by careful measurement in the 1840s.
Now consider the wires that bring electricity from the generating station to the building. The losses in those wires also go as current squared. So for the same delivered power, a system using high current suffers far greater losses in the distribution network, and the only way to reduce them is to use thicker copper.
Every earlier incandescent lamp, Swan’s included, used a thick low resistance filament, which meant high current at low voltage. Scaling such a system to a city would have required an amount of copper in the streets that no company could have afforded. Edison had his mathematician Francis Upton work the numbers, and the copper bill was the thing that killed it.
The solution was to invert the design: a very thin, high resistance filament, running at higher voltage and lower current. The same light, far less current, far thinner wires, and a distribution network that could be paid for.
That is the actual invention of October 1879, and it is why Edison’s lamp mattered while equally functional lamps did not. He was not trying to make light. He was trying to make a lighting utility, and the lamp is a component specified by the economics of the system it sits in.
What Made It Possible
A thin high resistance filament is fragile and burns out instantly in the presence of oxygen, so the whole approach depended on a vacuum better than anything previously available in a sealed bulb.
The enabling tool was the Sprengel mercury pump, developed in 1865, which used falling mercury to drag air out of a vessel and could reach pressures far lower than mechanical pumps. Edison’s team pushed it further, and combined it with heating the filament during evacuation to drive out absorbed gases. Without that pump there is no high resistance lamp, and it was invented by somebody else for entirely unrelated reasons.
The 1,600 Materials
Then came the search for something to make the filament out of, and this is where the legend lives.
The Menlo Park team tested on the order of sixteen hundred materials. Carbonised cotton thread gave the October result. Carbonised cardboard and paper did better. The breakthrough material was bamboo: a carbonised bamboo filament gave lamp lives of over a thousand hours, and Edison sent men to Japan, China, India and South America to collect and test bamboo species, looking for the best one. A Japanese variety won and supplied his lamps for years.
Edison’s own summary of this process, recorded in an early biography, is better than the version usually quoted: asked about his failures, he said that he had obtained a great many results, and that he knew several thousand things that would not work.
That is a description of an industrial method rather than a personal virtue. Testing sixteen hundred materials is only possible if you have a building full of people, a stock of instruments, a glassblower on staff and a machine shop next door, and that is the thing Edison had actually built three years earlier.
Menlo Park Was the Invention
The laboratory Edison opened in 1876 at Menlo Park is generally regarded as the first industrial research laboratory in the world, and it is his most consequential creation.
It had a machine shop, a chemistry laboratory, a library, a glassblowing facility and a staff of specialists working to a plan. Francis Upton was a trained physicist who had studied under Helmholtz and did the mathematics. Charles Batchelor was an extraordinary experimentalist. John Kruesi built the machinery. Ludwig Boehm blew the glass. Later, Lewis Latimer, an African American engineer and draughtsman, patented an improved process for manufacturing carbon filaments in 1882 and joined the Edison companies, where he also drew the patent illustrations and testified in the lawsuits.
Before Menlo Park, invention meant an individual in a workshop. After it, invention meant a payroll, a research programme and a patent department, and every corporate laboratory since is descended from it. That is a far larger change to the world than any particular bulb.
Swan, and the Settlement
Joseph Swan had the better claim in Britain, having demonstrated and patented first there. He sued. Edison could not win, because in British law he was not first.
The two sides merged in 1883 into the Edison and Swan United Electric Light Company, universally called Ediswan, which is the commercially sensible ending to a priority dispute and therefore an unusual one. Swan got recognition and a business. Edison got the British market.
The System, and the Fight That Followed
The lamp went on sale as part of a package. On 4 September 1882 the Pearl Street Station in lower Manhattan began supplying electricity to 59 customers, with generators, underground conduits, fuses, sockets, switches and meters, all of which had to be invented or adapted, and all of which Edison’s companies sold.
The generators themselves rested on Faraday’s discovery of induction fifty years earlier, and here Edison made the mistake that shaped the rest of his career in electricity. Pearl Street was direct current, and direct current cannot be transformed to high voltage for long distance transmission, so each station could serve only a radius of about a kilometre.
Alternating current could, and the war of the currents that followed is the story of Edison defending, with increasing bad faith, a system he had chosen for good reasons in 1882 that had become the wrong one by 1890.
The irony is exact. He won the lamp by reasoning correctly about resistance, current and the cost of copper over distance. He lost the grid by refusing to follow the same reasoning one step further.
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