November 4, 1879: Thomas Edison Invents the Electric Light, Files Patent

By the time Thomas Edison began his effort to develop an incandescent electric light in September 1878, researchers had been working on the problem for forty years. While many of them developed lamps that worked in the laboratory and for short-term demonstrations, none had been able to devise a lamp that would last in long-term commercial use.  Edison was able to succeed where others had failed because he understood that developing a successful commercial lamp also required him to develop an entire electrical system. With the resources of his laboratory, he and his staff were able to design not only a commercially successful lamp but the system that made it possible.

At the time, Edison’s work on telegraphs and telephones largely defined the limits of his knowledge of electrical technology. Unlike some of his contemporaries, he did not even have experience with arc lights and dynamos. Yet he confidently predicted that he could solve the problem and after only a few days of experiment during the second week of September 1878 he announced that he had “struck a bonanza.”  He believed he had solved the problem of creating a long-lasting lamp by designing regulators that would prevent the lamp filament (he was then using platinum and related metals) from melting. Edison reached this solution by thinking of electric lights as analogous to telegraph instruments and lamp regulators as a form of electromechanical switch similar to those he used in telegraphy. Edison’s regulators used the expansion of metals or air heated by the electric current to divert current from the incandescing element in order to prevent it from destruction by overheating. Edison was soon designing lamp regulators in the same fertile manner that he had previously varied the relays and circuits of his telegraph designs.

Edison was also confident that his insights regarding high-resistance lamps and parallel circuits would be key to designing a commercial electric lighting system. Because the regulator temporarily removed the lamp from the circuit, he realized that he had to place the lamps in parallel circuits so that each individual lamp could be turned on and off without affecting any others in the circuit. This was also desirable for customers used to independently operated gas lamps. Even more important was Edison’s grasp of basic electrical laws. He was virtually alone in understanding how to produce an economical distribution system. Other researchers had been stymied by the cost of the copper conductors, which would require a very large cross section to reduce energy lost as excess heat in the system. However, large copper conductors would make the system too expensive. Edison realized that by using high-resistance lamps he could increase the voltage proportionately to the current and thus reduce the size and cost of the conductors.

Edison initially focused his work on the lamp because he saw it as the critical problem and thought that standard arc-lighting dynamos could easily meet the requirements of an incandescent lighting system.  However, after experimenting with one of these dynamos, Edison began to doubt their suitability for his purposes. With the expectation of funds from the newly formed Edison Electric Light Company, he ordered other machines and began to design his own generators as well. By January 1879, Edison’s understanding of generators had advanced sufficiently “after a few weeks hard study on magneto electric principles,” for him to start his machinists building a new design. Edison’s ability to experiment with generators was greatly enhanced by his new financial resources that enabled him to build a large machine shop that could produce not only fine instruments like telegraphs and lamps, but also generators—”in short all the means to set up & test most deliberately every point of the Electric Light.” With the new facilities, machinery, and assistants made possible by his financial backers, Edison could pursue research on a broad front. In fact, by the end of May, he had developed his standard generator design. It would take much longer to develop a commercial lamp.

Just as the dynamo experiments marked a new effort to build up a base of fundamental knowledge, so too did lamp experiments in early 1879 begin to reflect this new spirit of investigation. Instead of continuing to construct numerous prototypes, Edison began observing the behavior of platinum and other metals under the conditions required for incandescence. By studying his filaments under a microscope, he soon discovered that the metal seemed to absorb gases during heating, suggesting that the problem lay less in the composition of the metal than in the environment in which it was heated. The most obvious way to change the environment was to use a vacuum. By improving the existing vacuum-pump technology with the assistance of an experienced German glassblower, Edison was able to better protect his filaments and by the end of the summer he had done away with his complicated electromechanical regulators. The improved vacuum pumps developed by the laboratory staff helped to produce a major breakthrough in the development of a commercial lamp.

Although Edison no longer required a regulator for his platinum filaments and the lamps lasted longer, they were too expensive for commercial use. Not only was platinum a rare and expensive metal, but platinum filaments did not produce the high resistance he needed for his distribution system. With much better vacuum technology capable of preventing the oxidation of carbon filaments, Edison decided to try experimenting with a material that was not only much cheaper and more abundant, but which also produced high-resistance filaments.

The shift to carbon was a product of Edison’s propensity for working on several projects at once. During the spring and summer of 1879, telephone research at times overshadowed the light as Edison sought to improve his instrument for the British market. A crucial element of Edison’s telephone was the carbon button used in his transmitter. These buttons were produced in a little shed at the laboratory complex where day and night kerosene lamps were burned and the resulting carbon, known as lampblack, was collected and formed into buttons. The reason for turning to this familiar material lies in another analogy. Almost from the beginning of the light research, Edison had determined that the most efficient form for his incandescing element would be a thin wire spiral which would allow him to decrease radiating surface so as to reduce the energy lost through radiation of heat rather than light. The spiral form also increased resistance. It was his recognition that the lampblack could be rolled like a wire and then coiled into a spiral like platinum that led Edison to try carbon as a filament material.

Although Edison’s basic carbon-filament lamp patent, filed on November 4, 1879, still retained the spiral form, the laboratory staff had great difficulty in actually winding a carbon spiral. Instead, Edison turned to another form of carbon “wire”–a thread. During the night of October 21–22, the laboratory staff watched as a cotton-thread filament burned for 14 1/2 hours with a resistance of around 100 ohms. This date of this experiment would later be later be associated with the invention of the electric light, but at the time Edison treated it not as a finished invention but rather as the beginning of a new experimental path. The commercial lamp would require another year of research.

Nonetheless, by New Year’s Day 1880, Edison was able to demonstrate his system to the public. Over the course of the next year, he and his staff worked feverishly to bring his system to a state of commercial introduction. In the process, he turned the Menlo Park laboratory into an R&D center, with an emphasis on development. By spring, the staff which previously consisted of some twelve or fifteen experimenters and machinists, was greatly expanded, at times reaching as many as sixty men. Work on the various component was delegated to new members of the staff and over the course of the year, work progressed on each element of the system, including the generator, meter, underground conductors, safety fuses, lamp fixtures and sockets, and the commercial bamboo-carbon filament. By the time all these ancillary components were developed and manufacturing underway in the spring of 1881, Edison had spent over $200,000 on research and development.  Commercial introduction required several thousand additional dollars of research as well as $500,000 to install the first central station system in downtown New York City, which opened on September 4, 1882, four years after Edison first began his research. Though he claimed merely to “have accomplished all I promised,” Edison had done even more by starting a new industry and reorganizing the process of invention.

Historian, Dr. Paul Israel, a former Californian, moved East to NJ over 30 years ago to do research for a book on Thomas Edison & the electric light. Today he is the Director and General Editor of the Thomas A. Edison Papers at Rutgers University, the New Jersey State University. 

The Thomas A. Edison Papers Project, a research center at Rutgers School of Arts and Sciences, is one of the most ambitious editing projects ever undertaken by an American university.

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