History Of The Incandescent Lamp - By John W. Howell And Henry Schroeder (1927)

You are reading: Chapter 2: Edison's Invention of a Practical Incandescent Lamp, a Complete Lighting System and Their Commercial Introduction





Edison first began his study of the incandescent lamp problem in the fall of 1877. He had a well equipped laboratory at Menlo Park, New Jersey, with several able assistants and many workmen, about a hundred people all told. He had already made several important inventions, among which were: the quadruplex telegraph, whereby four messages could be sent simultaneously over one telegraph wire, thereby quadrupling the capacity of the telegraph lines of the country; the carbon telephone transmitter, without which Bell's telephone receiver would have been impracticable; and the phonograph. The lasting value of these inventions proved that Edison was eminently fitted to attack the problem of "subdividing the electric light".

Edison first made many experiments with the object of confirming the failures of others. In July, 1878, his health having been undermined by his unceasing work, he took a trip with an expedition to Wyoming to observe an eclipse of the sun. This he called a "vacation", but he brought with him a delicate instrument he had invented which he called a tasimeter. This was devised to measure the heat transmitted through great distances. In about two months he returned to Menlo Park and again studied the lamp problem, which was but one among many others he was trying to solve.


On the left is the wooden laboratory building, in the left background right foreground is the brick machine shop. The brick building in the right foreground is the office and library.


The men are assembled on the front of the laboratory building. They are, from left to right top row: J. W. Lawson, unknown, unknown, L. K. Boehm, Charles Batchelor, Francis jehl, F. R. Upton, and Dr. A. Haid; second row: J. F. Kelly, David Cunningham, T. A. Edison, Major F. McLaughlin and T. Logan; third row: J. F. Randolph, Charles Flammer, George Dean, George E. Carman, John F. Ott, James Seymour and unknown; bottom row: A. Swanson, Martin N. Force, S. L. Griffin, and Milo Andrus.


This model was made by F. A. Wardlaw, one of Edison's early associates. Each part of the model is made from the original parts of the building itself down to the last detail; the shingles, clapboards, piazza railing and posts, flooring, bricks in the chimney, etc., and even the glass in the windows. Photograph courtesy of the Association of Edison Pioneers.


This photograph was taken February 22, 1880. Several lamps will be seen mounted on the converted gas fixtures hanging from the ceiling. Edison is seated in about the center, his principal assistants gathered about him.

His first experiments having shown the seeming impracticability of carbon for the incandescent burner, he started investigating platinum. He developed a lamp having a platinum spiral for a burner. Inside the spiral was a rod which expanded when the platinum became heated, and if the temperature became too high, the expansion of the rod would cause it to short circuit the burner, thereby allowing the platinum to cool. This took but a fraction of a second, and the rod, contracting almost immediately, opened the short circuit. Thus the lamp only "blinked" when the current was too high. A patent was applied for in October, 1878, and Edison's first lamp patent was granted in April, 1879.


This was the first of a number of lamps he built in his study of making a practical incandescent lamp. It is in the William J. Hammer Collection of Historical Incandescent lamps. Photograph, courtesy of Major Hammer and the Association of Edison Illuminating Companies, in whose custody this collection is kept.

Up to this time Edison had spent quite a lot of money in lamp research, and, in order to raise more money to continue the work, a corporation was organized. On October 17, 1878, the Edison Electric Light Company, with a capital of $300,000, was incorporated by several prominent men for the purpose of backing Edison in his work of trying to develop a complete incandescent electric light system. This company was a forerunner of the present General Electric Company.

Edison's next step in lamp research was to make a more sensitive thermostatic arrangement to short circuit the platinum burner. This was accomplished by means of an expanding diaphragm, a patent being applied for in November, 1878, and granted early the next year. He then made a lamp using platinum foil for the burner. A patent was applied for on this lamp in December, 1878, and granted in August of the following year.

His next development was a lamp having an inverted "U" shaped burner consisting of finely divided iridium mixed with oxide of zirconium. The latter is a non-conductor of electricity when cold, but iridium made the composite burner a conductor and, when heated, the zirconium oxide also became a conductor. A patent on this was applied for in December, 1878, and granted in September, 1879.

Edison's next application for a lamp patent was made in February, 1879, and covered a long carbon rod pressed upward by a heavy counterweight against a platinum-iridium rod. The light was obtained from the current heating the resistance of the poor contact between the rods. As the heat consumed the end of the carbon rod, it was automatically fed upward by the counterweight. The platinum-iridium rod was also slowly consumed. A patent for this lamp was granted in February, 1880.

Edison's Study of a Complete Incandescent Lighting System

None of the lamps he had made was practical and furthermore he realized that even if he was finally able to make a lamp that would be commercial, it would be impractical if operated on the series system of distributing electricity, as it would be impossible to turn on or shut off one lamp without doing the same thing to all the others on the circuit. In this system the current is constant throughout the circuit, the current flows out of the armature of the dynamo through one brush, through the field coils, through one lamp after another and then back to the armature through the other armature brush. This system was satisfactory for arc lamps, which are inherently a constant current device, and was also suitable for use in street lighting for which arc lamps were most generally used, as in that case there was no need to turn on one lamp at a time.


Prior to 1878, this was the only known method of distributing electric current.


In 1878, Edison invented this system of distributing electricity at a constant pressure and in quantities as required. It is now universally used.

Edison therefore reasoned that another system of distributing electricity to lamps must be used, patterned after the existing gas light system, as small electric lamps would find their greatest usefulness in household, commercial, and industrial lighting. He made an intensive study of gas, obtaining all the literature possible on the subject, and spending several weeks of his time in continuous reading. Gas is distributed through pipes, with mains, feeders and branches supplying it at about constant pressure at the lamps. While the gas escapes into the air after it is burned, electric current must be returned to the dynamo armature after it goes through the lamps.

After much thinking he evolved a constant pressure electrical system, which is called the "multiple" system of distribution. In this system current is generated at a constant pressure and supplied in quantities as desired.


This machine was invented by Edison to fit the multiple system he had also invented. It had an efficiency of 90 per cent which scientists had mathematically "proved" was impossible.

This required the design of a dynamo to supply such a current. This was something that had not been previously done, but undaunted, he attacked the problem.

After much intensive study, he designed a dynamo having an extremely low resistance in the armature. He made a drum wound armature, using large heavy wires in place of small ones in order to reduce the resistance. The field coils were connected directly across the armature in multiple, instead of in series with it. When the machine was run at a certain constant speed, the voltage (pressure) between the two armature brushes was approximately 110 volts and remained about constant, falling but slightly with increasing amounts of current taken from the machine. Up to a certain point, the capacity of the machine, this could be done without undue heating of the armature. He found by tests that the machine, at about full load, converted 90 per cent of the mechanical energy required to drive it into electrical energy, or in other words, it was 90 per cent efficient.

When he announced the invention of this dynamo, some scientists ridiculed it, as it had been proven that the greatest amount of electrical power which could be obtained from a battery was at that point where the internal resistance of the battery was the same as the resistance of its external load. Under these circumstances the battery would have an efficiency of 50 per cent and scientists thought that this should be the condition at which a dynamo could be operated to the best advantage.

Development of a High Resistance Platinum Lamp

Edison now had a dynamo that would give a constant voltage of about 110 volts between the two wire conductors leading from the armature, to which one or more lamps could be connected. By applying Ohm's law, he reasoned that the smaller the amount of electrical power a lamp for this system should take, the higher should be its resistance. For example, suppose an incandescent lamp is to be made to consume 550 watts, which was about the rating of the arc lamps then made, but that this lamp should be designed for use on 110 volts. As the watts are equal to the volts times the amperes, a 550-watt, 110- volt lamp will consume 5 amperes, and by Ohm's law, which is that the amperes equal the volts divided by the ohms, this 550-watt lamp will have a resistance of 22 ohms. Similarly a 110-watt, 110-volt lamp would have a resistance of 110 ohms.

The current in the series circuits on which arc lamps were then commercially operated was about ten amperes, although some systems were later designed for twenty amperes. The lamps that Edison had made previously were designed for use on these 10-ampere circuits and consumed about 110 watts. The voltage across the terminals of the lamp was therefore 11 volts and the resistance of the lamp burner 1.1 ohms. Thus the resistance of the lamps he had previously made had to be increased from 1.1 to 110 ohms before they would be suitable for his 110-volt multiple system.


This had a long, thin platinum wire mounted on pipe clay and coated with zirconium oxide. It had a diaphragm thermostat which cut off the current momentarily if the burner got too hot. This lamp is in the Hammer Historical Collection of Incandescent Lamps. Photograph courtesy of Major Hammer and the Association of Edison Illuminating Companies.

All this reasoning may be difficult for the layman to understand. It was for most electricians in 1879, as they did not thoroughly understand Ohm's law. It was therefore no small accomplishment, although it may not seem so now to those familiar with electrical engineering, for Edison to have developed such a new and complete system of distributing electricity.

The first high resistance lamp that Edison designed had a long thin coiled platinum wire mounted on a piece of pipe clay and coated with oxide of zirconium to protect the platinum from oxidizing. In order to prevent the burner from operating at too high a temperature, it was protected by his diaphragm thermostat, but in this case the circuit was opened to cut off the current from the platinum wire. This was necessary because if the scheme used in former lamps for series circuits of short circuiting the burner were used in the lamp for the new multiple system, the low resistance of the short circuit across the constant pressure would cause such a heavy rush of current to flow that it would melt the conductors almost instantaneously. A patent for this lamp was applied for in February, 1879, and was granted in May, 1880.

Oxide of zirconium, while an insulator when cold, will decrease materially in resistance as it gets hotter. Current, instead of flowing through the long thin platinum wire, would then be shunted through the zirconium oxide coating between the turns of the coiled platinum wire, heating the latter to such high temperature that the lamp would short circuit itself. The lamp was therefore impractical.

During his experiments, Edison had found that platinum became extremely hard after it had been heated several times by the current flowing through it. This made it possible to operate it at much higher temperatures without danger of melting and so give much more light. He believed that the heat drove gases out of the minute pores of the platinum, causing it to become more dense by sintering the particles of platinum closer together. He then thought that if, the platinum were operated in vacuum, more gases would escape from it so that it could perhaps be operated at even higher temperatures.

He therefore wound a long thin platinum wire on a spool of pipe clay, but this time he omitted the zirconium oxide coating. The platinum coil was mounted in a one-piece all-glass globe, all joints being fused by melting the glass together. The ends of the platinum wire passed through the glass, which was fused around the wire to make an air tight joint. The all-glass globe was considered necessary to maintain the high degree of vacuum then obtainable with the recently invented Geissler and Sprengel mercury vacuum pumps. The glass globe was then put inside a glass cover mounted on a holder within which was mounted a diaphragm thermostat which protected the platinum wire from excessive temperature. A patent for this lamp was applied for in April, 1879, which was granted in May, 1880.

This lamp was apparently successful, so a number of them were made to try out. But, since they consumed a lot of power in proportion to the light they gave, were short lived, and very expensive to make, they were not considered commercially practical. The platinum lamp had, it seemed, reached the limit of its possibilities so the problem appeared impossible of solution and, for a time, was abandoned.

Solution of the Incandescent Lamp Problem

Edison had done a lot of experimenting with different forms of carbon for his telephone receiver, which gave him a broad knowledge of the properties of carbon. Several months had passed since he had worked on the incandescent lamp and in the fall of 1879, he began thinking about it again. He knew that carbon had a high resistance compared with platinum. In order to get the requisite resistance, he realized that the carbon would have to be very slender. Thick carbon rods did not last very long when he subjected them to the high temperature of incandescence, so a slender piece should seemingly last but a very short time. He wondered, however, if it would last any longer in the high vacuum he had been able to obtain with his platinum lamp. It seemed foolish to try this but in order to leave no stone unturned he made the bold attempt.

The first problem was that of obtaining carbon of the requisite slenderness, and of determining what its length and diameter should be. After considerable calculation he estimated that the carbon should be not over a sixty-fourth of an inch in diameter, or about the size of ordinary heavy sewing thread. From that he conceived the idea of the possibility of carbonizing a piece of sewing thread by heating it in an air-tight crucible. This in itself was a bold thing to do, for it would not require the presence of much air in order to have the thread burn up. He estimated that the carbon should be about six inches long.

Carbonizing a substance consists of heating it away from the presence of air so that the heat does not oxidize the material, but merely drives off the volatile matter, leaving only the carbon residue behind. This is similar to distilling coal, which is put in closed retorts, heat being applied from the outside. The heat drives out a number of volatile gases from which the coal gas is obtained, which, when enriched with oils, becomes illuminating gas. Coal and many other substances contain hydro-carbon compounds and the heat decomposes them, leaving a carbon residue behind which is known as coke.

Edison cut several pieces of sewing thread and packed them in with a lot of powdered carbon in an earthenware crucible. The threads were packed so that they were "U" shaped in order to reduce the size of the glass globe in which they were to operate. The powdered carbon was partly for the purpose of minimizing the amount of air in the crucible and partly to absorb the oxygen in what little air there was left. The crucible was then covered with an earthenware top, the two being cemented together with fire clay to further exclude any air.

The crucible was then put in a furnace and subjected to a high temperature for several hours. It was then allowed to cool gradually, which took many hours before the inside had become cool enough to prevent the threads from burning up in unpacking. After many patient trials he finally obtained an unbroken carbonized thread, "filament" he called it, which then had to be fastened to a pair of platinum wires. This was finally done, after many failures, by delicate clamps. The platinum wires had been sealed in a piece of glass tubing and the filament was then fastened to the ends of the platinum by the clamps. This mounted filament was then inserted in a glass bulb, the glass tubing being fused to the neck of the bulb to make an air-tight joint. On the opposite end a small glass tube had been fused for the purpose of exhausting the air from the bulb.


On October 21, 1879, Edison made this experimental lamp which embodies the basic features of all lamps made today. It consisted of a carbonized cotton thread operating in a very high vacuum maintained by a one piece all glass globe. This replica was made by Francis jehl, one of Edison's pioneer assistants, by whose courtesy this photograph is reproduced. The original experimental lamp was destroyed.

The lamp was then connected to the mercury vacuum pump until the vacuum reached a high degree. Edison, however, believed from his experience with platinum that gases would also be "occluded" in the carbon filament, so in order to drive them out, he put a small amount of current through the filament to heat it slightly. Immediately the gases began to come out and it took nearly eight hours on the pump before they apparently ceased.

The crucial time had come to try the lamp out. The men in the laboratory were skeptical about it and bets were made that it would last but a few minutes. With a crowd about him, Edison turned the current on gradually by means of external resistance until the filament glowed dimly. It did not burn out. Becoming bold he gradually cut out the resistance until the lamp gave a brilliant light. Still it did not burn out. It continued to burn, and when evening came it was still going strong. This was October 21, 1879, and the lamp burned steadily for nearly two days.

Edison now felt that he was on the right track, and every thing conceivable was carbonized in the endeavor to make a better filament. After many weeks of working almost continuously day and night, he found that carbonized paper (bristol board) would give several hundred hours life. He then felt that he had a practical lamp which could be commercially used, so he decided to announce his invention and demonstrate it to the public.

The announcement was made in an article which took the entire first page of the New York Herald of Sunday, December 21, 1879. Several scientists proclaimed Edison's invention to be a fake. Gas stocks, however, dropped in price and stock in the Edison Electric Light Company soared to thirty-five hundred dollars a share.

The demonstration consisted of about sixty lamps mounted on poles lighting the laboratory grounds and country roads in the neighborhood. Wires were also run to several houses and lamps installed in them. Crowds came out to Menlo Park during the next few days and the Pennsylvania Railroad had to run special trains to accommodate them.


This article appeared in the New York Herald of December 21, 1879, just two months after the "birth" of the lamp. Scientists proclaimed it a fake. Nevertheless the price of gas stocks dropped and stock in the Edison Electric Light Company soared to $3500 a share.

Edison applied for a patent on this lamp on November 4, 1879, and on January 27, 1880, the basic lamp patent No. 223,898 was granted him. All the elements of this lamp are the same as those in the lamps made today; a high resistance filament operating in a high vacuum, maintained by a one piece all-glass globe having all joints sealed by fusion of the glass. While some lamps made today are filled with an inert gas after the lamp has been exhausted, the features are otherwise the same.


Lamps were mounted on poles lighting the neighborhood of the Laboratory at Menlo Park. The various buildings of the Laboratory can be seen in the background.

Edison's Invention

There has been some misconception of exactly what Edison did invent. He was not the first man to make an incandescent lamp, as has been indicated in the previous chapter. The principle of incandescent lighting had been established and demonstrated by several experimenters but no lamp previously made was suitable for use in large numbers over a large area like a city. His analysis of the problem brought him to the conclusion that such lamps must be connected to the circuit in multiple so each one would be independent of the others. He also realized that lamps connected in multiple must be of high resistance, for the higher their resistance the smaller were the conductors necessary to carry electricity to them. So he aimed to make a lamp of the highest practical resistance and he named this high resistance carbon burner a "filament."

He found that a carbon filament to be of high resistance must be made very thin and quite long and he also found that such filaments required a very good vacuum to preserve them. He also soon realized that glass chambers made in two separate parts, as previous lamps had been made, would not keep the very good vacuum necessary to preserve the filament. He then made the very bold step of fusing the two glass parts inseparably together and making the glass container closed at all points by fusion of the glass.

That is what Edison invented: a lamp with a high resistance filament of carbon in a vacuum contained in a glass container closed at all points by fusion of the glass and having platinum wires imbedded in the glass to carry current through the glass to the filament. And this was the first incandescent lamp which was suitable for the system of general multiple distribution which solved the problem of the "sub-division of the electric light".

Edison's patent, which the courts upheld as covering the modern incandescent lamp, covered only a particular kind of Incandescent lamp which combined four elements- (1) a high resistance filament of carbon, in (2) a chamber made entirely of glass and closed at all points by fusion of the glass, which contained (3) a high vacuum and through which (4) platinum wires passed to carry current to the filament. It was a patent on a combination of old elements which produced a new thing- A lamp suitable for multiple distribution over large areas.

Commercial Installation of the Incandescent Lamp

The first commercial installation of the lamp was made on the steamship Columbia of the Oregon Railway and Navigation Company. This steamer was being built in Chester, Pa., and was about completed. She took a trip to New York and the Edison Electric Light Company received its first contract to equip the ship with electric light. Four dynamos were installed run from two overhead countershafts driven by a pair of vertical steam engines. Each dynamo had a capacity for sixty lamps, or about six kilowatts (eight horse power), one dynamo being used as an exciter for the other three. In this connection Edison had made another invention, which by the way scientists said was impossible, of connecting two or more dynamos together in multiple, each supplying its proportion of current to a single circuit. The ship was equipped with 115 lamps and the plant was started on May 2, 1880. She sailed around the Horn to San Francisco, where she arrived in July. The Advising Engineer of the Navigation Company reported that the installation was a complete success. The original installation ran for fifteen years, when the ship was overhauled and a more modern plant installed.


This was the first commercial installation of the Edison Lamp and was started May 2 1880. One of these dynamos is on exhibition at the United States National Museum, Washington, D.C.

The next commercial installation was started about the first of the year 1881, in the shop of Hinds, Ketchum & Company, lithographers, 229 Pearl Street, New York. One dynamo was installed having a capacity for sixty lamps.

The commercial success of the incandescent lamp was quickly established. During the two years 1881-82, over 150 other installations were put in, aggregating over 30,000 lamps. These installations included steamships, machine and car shops, mills, stores, offices, theaters, hotel residences, etc.; all of them were entirely successful.


This was the second installation, the first on land which was started about the first of the year, 1881. Photograph, courtesy United States National Museum.

The First Lamp Factory

The first lamps were made in the Menlo Park Laboratory, the glass work being done in a shed there. The shed has been preserved on account of its historical interest and is now at Mazda Brook Farm (near Parsippany, New Jersey), a recreation and meeting place for the employees of the incandescent lamp department of the General Electric Company.

As so many lamps were now being made, it sorely taxed the capacity of the laboratory. In the latter part of 1880 a separate company was formed, called the Edison Lamp Company, to manufacture lamps, and a factory building obtained, located alongside the Pennsylvania Railroad tracks at Menlo Park about half a mile from the laboratory.


In November, 1880, the manufacture of lamps was started in this building, located beside the Pennsylvania Railroad tracks at Menlo Park, about half a mile from the Laboratory. The four men in the foreground from left to right, are Phillip S. Dyer, Accountant; William J. Hammer, Electrician; Francis R. Upton, General Manager; and James Bradley, Master Mechanic.

During the next year, 1881, the demand for lamps had so increased that again it became imperative to get more space. A group of factory buildings were purchased at Harrison, New Jersey, the present headquarters of the Edison Lamp Works. Moving was begun in February, 1882, and manufacture in Harrison began in April of that year, the Menlo Park factory then being shut down. None of the original buildings at Harrison is now standing.

16 C.P., 110 Volts

8 C.P., 55 Volts


The 16 C.P. lamp was called the "A" lamp and the 8 C.P. the "B" lamp, the latter burned two in series on 110 volts. The construction of the lamps as pictured above was standard from 1881 to 1884.

Two sizes of lamps were now being made, 16 candle-power for 110 volts and 8 candle-power for 55 volts, the latter to be burned two in series on 110 volts. The former was called the "A" lamp and the latter the "B" lamp. The A lamps were made "eight to the horse power", the term watts not being in use at that time; the lamps therefore consumed a little over 93 watts. They were rated to give 600 hours life in service, but in the latter part of 1881 the efficiency was increased, the lamps then being made ten to the horse power, rated to give 600 hours life on circuits having good voltage regulation.

Development of Other Parts of Edison's System

In addition to lamps and dynamos, other parts of Edison's incandescent electric lighting system had to be invented, developed and manufactured to make the system complete.

In order to protect the dynamos from accidental overload, such as a short circuit, an automatic device had to be developed to disconnect them from the circuit.


Edison invented the fuse which is universally used. Photograph, courtesy of the New York Edison Company.

Edison invented the well-known lead wire fuse for which he obtained a patent in May, 1880. The same type of fuse was also used to protect the main circuit from troubles on individual branch circuits, so that current would be cut off only from the branch circuit where the trouble occurred.

Lead made into short pieces of wire of various diameters will carry current up to an amount determined by the size of the wire. If the current is increased beyond that point, the lead wire will be heated appreciably and finally melt. if the current suddenly becomes very great, due to a short circuit, the lead wire will melt instantaneously, thereby automatically opening the circuit before any damage is done.

The demand for sockets, switches, fixtures, etc., became so great that a separate organization was formed, known as Bergmann & Company, which obtained a factory at 108 Wooster Street, New York, and started manufacturing early in 1880.


This factory was located on Goerck Street, New York City, the manufacture of dynamos being shifted to it in 1881. In 1886, the Works were moved to Schenectady, N.Y.

The capacity of this factory was soon outgrown and in 1882 the plant was moved to a building on the corner of Avenue B and 17th Street.

As there was insufficient space at the Menlo Park Laboratory machine shop, another separate company was organized, known as the Edison Machine Works. A factory building at Goerck Street, New York City, was obtained and the manufacture of dynamos was started there early in 1881. The capacity of this factory was soon overtaxed and in 1886, the Works were moved to Schenectady, New York.

Edison felt that the wires supplying current from a central station to the various buildings should be underground. This necessitated the design and development of a complete water tight and insulated underground method of distribution, something that had never been previously done; in fact it was considered impossible to prevent current from leaving the wires and being diverted from one part of the system, through the earth, to another part of the system, instead of being supplied to the lamps in the buildings. He finally developed a complete system of underground tubing, joints, junction boxes, branches, etc. These were made by a subsidiary organization, the Electric Tube Company, which obtained a factory at 65 Washington Street, New York.


This registered the amount of current used. Two chemically pure pieces of zinc were put into a glass jar containing a solution of zinc chloride. Current flowing from one zinc to the other through the Solution caused particles of zinc to be transferred from one to the other. The amount of current used was measured by the loss of one and gain of the other. This meter, a double one, is in the historical collection of the Edison Pioneers by whose courtesy this photograph is reproduced.

It was also necessary to design a meter to register the amount of current used by each customer as a basis for bills to be rendered for the service given. An electrolytic meter was finally evolved and in service was found to be extremely accurate.

This meter consisted of a glass jar containing a solution of zinc sulphate and two pieces of chemically pure zinc. Direct current flowing through this cell would cause particles of zinc to be transferred through the solution from one zinc terminal to the other, the amount being in proportion to the current flowing and to the length of time.


This dynamo had a capacity of 1200 lamps and was directly connected to a steam engine. It is one of the original machines of the Pearl Street Station of the Edison Electric Illuminating Company, now the New York Edison Company, by whose courtesy the photograph is reproduced.

Thus one piece of zinc loses and the other gains in weight. This difference measures the total quantity of current in ampere-hours used, which, if multiplied by the voltage, would give the quantity in the modern term of watt-hours. The voltage being approximately constant, the ampere-hours were a direct measure for a basis of rendering bills on the amount of electricity used. Actually only part of the total current used was shunted through the cell so that the zinc electrodes would not have to be inconveniently large in size.


This was the first permanent central station in the world, starting operations on September 4, 1882. Photograph by courtesy of the New York Edison Company.

In 1880, Edison decided to build a large dynamo capable of being directly connected to a steam engine instead of being belt driven. Up to this time the dynamos he had made had a capacity of sixty lamps, which in the present terminology would be rated at six kilowatts (about eight horse power).

A central station of even reasonable capacity would have to have a vast number of these six kilowatt dynamos, requiring a very large space and great investment. In order to deliver 110 volts they had to be run at high speed, about a thousand revolutions per minute, far beyond that possible with a steam engine. It was, therefore, no small matter to design a large dynamo to be directly connected to a steam engine whose maximum speed at that time was about one hundred revolutions per minute.

Edison was finally able to get an engine maker to make a steam engine of about 120 horse power to run at 350 revolutions per minute and then he made a dynamo of 1200 lamp capacity to be directly connected to this machine. At this time lamps were being made ten to the horse power, each consuming about 75 watts. This 1200-light dynamo therefore had a capacity of 90 kilowatts (about 120 horsepower) and was nicknamed the "Jumbo" dynamo after the well-known elephant, then the largest in captivity.

Edison had always believed that the most economical method of supplying current for incandescent lamps was by the generation of current in a large central plant instead of by individual plants. In the latter part of 1880, plans were started for a central lighting station in New York City and the first central station, the Edison Electric Illuminating Company of New York (now the New York Edison Company) was incorporated in December, of that year.

The construction of the power plant, the more than fourteen miles of underground mains, covering an area of about one-sixth of a square mile between Spruce Street, Ferry Street and Peck Slip on the north, the East River on the east, Wall Street on the south, and Nassau Street on the west, and the wiring of consumers' premises, took nearly two years of work. Finally, on September 4, 1882, the Edison Electric Illuminating Company of New York started operations with a load of about 300 amperes supplying about 59 customers having a total of 1284 sockets. It had six Jumbo dynamos with a rated capacity of 7200 lamps, or about 540 kilowatts (720 horse power). The station was located at 257 Pearl Street, New York, and its design was quite equal to that of a modern plant. Real estate was so expensive that in order to save space the boilers were located on the ground floor and the dynamos and engines on the second floor. On the top floor was a test rack with sockets for a thousand lamps which was used to test out the station before it was put into regular operation. The great weight of the dynamos and engines on the second floor was supported by special steel beams.

The Three-wire System

Further study of the central station showed that the amount of copper required in the mains to distribute the current would have to be very great if the distance and amount of current used was large. The investment for such a great amount of copper would be very heavy, almost prohibitive. After much thought, Edison evolved the "three-wire" system of distribution which resulted in a saving of 60 per cent of the amount of copper required by his former two-wire system.

In the three-wire system, two 110-volt dynamos are connected in series to give 220 volts. The circuit consists of three wires, two connected to the outside wires of the dynamos so that the voltage between them is 220 volts. The third wire, called the neutral wire, is connected to the connection between the two dynamos and runs wherever the outside wires run. The voltage between the neutral wire and either outside wire is 110 volts, and all lamps are connected between the neutral wire and one or the other of the outside wires, the load being about evenly divided. It is good practice to make motors for 220 volts and connect them to the outside wires, as this preserves the balance between the two sides. A 110-volt motor on one side disturbs the balance a great deal.

The current flowing through the outside wires of a three-wire distributing system, provided the lamps are evenly divided, is half that which flows through the wires of a two-wire system having the same aggregate number of lamps. As the amount of power lost in these wires is equal to the square of the current flowing in them times their resistance (the C²R loss), the resistance of the outside wires can be quadrupled for the same loss (the current being halved) by making them one-quarter the size of those used in a two-wire system. Therefore, if the load were equally balanced at all times on each side of a three-wire system, the neutral distributing wire could be dispensed with, making a theoretical saving of 75 per cent in copper. In practice there are, however, at one time or another, more lamps burning on one side of the system than the other, so that a neutral distributing wire becomes necessary. Even so, it is possible to obtain a 60 per cent saving in copper. Edison obtained a patent on the three-wire system early in 1883.

This system is now universally used where direct current is distributed, and is largely used on alternating-current local distributing systems. Its invention has caused the saving of untold millions of dollars of investment and it is probable that without it the central station industry would have been retarded for many years; at least until the alternating-current high voltage distributing system had been established.


This system reduced the amount of copper necessary in his former two-wire distributing system by 60 per cent.