EDISON’S INVENTION OF A PRACTICAL INCANDESCENT LAMP
Edison began the study of the problem in the spring of 1878. He had a well-equipped laboratory at Menlo Park, New Jersey, with several able assistants and a number of workmen, about a hundred people all told. He had made a number of well-known inventions, among which were the quadruplex telegraph whereby four messages could be sent simultaneously over one wire, the carbon telephone transmitter without which Bell’s telephone receiver would have been impracticable, and the phonograph. All of these are in use today, so Edison was eminently fitted to attack the problem.
Maxim’s Incandescent Lamp, 1878.
The carbon burner operated in a rarefied hydrocarbon vapor. This lamp is in the collection of the Smithsonian Institution.
Edison’s first experiments were to confirm the failures of other experimenters. Convinced of the seeming impossibility of carbon, he turned his attention to platinum as a light giving element. Realizing the importance of operating platinum close to its melting temperature, he designed a lamp which had a thermostatic arrangement so that the burner would be automatically short circuited the moment its temperature became dangerously close to melting. The burner consisted of a double helix of platinum wire within which was a rod. When the temperature of the platinum became too high, the rod in expanding would short circuit the platinum. The platinum cooled at once, the rod contracted opening the short circuit and allowing current to flow through the burner again. His first incandescent lamp patent covered this lamp. His next patent covered a similar lamp with an improved thermostat consisting of an expanding diaphragm. Both of these lamps were designed for use on series circuits.
Edison’s First Experimental Lamp, 1878.
The burner was a coil of platinum wire which was protected from operating at too high a temperature by a thermostat.
The only system of distributing electricity, known at that time, was the series system. In this system current generated in the dynamo armature flowed through the field coils, out to one lamp after another over a wire, and then back to the dynamo. There were no means by which one lamp could be turned on and off without doing the same with all the others on the circuit. Edison realized that while this was satisfactory for street lighting where arcs were generally used, it never would be commercial for household lighting. He therefore decided that a practical incandescent electric lighting system must be patterned after gas lighting with which it would compete. He therefore made an intensive study of gas distribution and reasoned that a constant pressure electrical system could be made similar to that of gas.
The first problem was therefore to design a dynamo that would give a constant pressure instead of constant current. He therefore reasoned that the internal resistance of the armature must be very low or the voltage would fall as current was taken from the dynamo. Scientists had shown that the most economical use of electricity from a primary battery was where the external resistance of the load was the same as the internal resistance of the battery, or in other words, 50 per cent was the maximum possible efficiency.
Diagram of Constant Current Series System.
This, in 1878, was the only method of distributing electric current.
Diagram of Edison’s Multiple System, 1879.
Edison invented the multiple system of distributing electric current, now universally used.
When Edison proposed a very low resistance armature so that the dynamo would have an efficiency of 90 per cent at full load, he was ridiculed. Nevertheless he went ahead and made one which attained this. The armature consisted of drum-wound insulated copper rods, the armature core having circular sheets of iron with paper between to reduce the eddy currents. There were two vertical fields above and connected in shunt with the armature. It generated electricity at about a hundred volts constant pressure and could supply current up to about 60 amperes at this pressure. It therefore had a capacity, in the present terminology, of about 6 kilowatts (or 8 horsepower).
Edison Dynamo, 1879.
Edison made a dynamo that was 90 per cent efficient which scientists said was impossible. This dynamo is in the collection of the Smithsonian Institution and was one of the machines on the steamship Columbia, the first commercial installation of the Edison lamp.
A multiple system of distribution would make each lamp independent of every other and with a dynamo made for such a system, the next thing was to design a lamp for it. Having a pressure of about a hundred volts to contend with, the lamp, in order to take a small amount of current, must, to comply with Ohm’s law, have a high resistance. He therefore wound many feet of fine platinum wire on a spool of pipe clay and made his first high resistance lamp. He used his diaphragm thermostat to protect the platinum from melting, and, as now seems obvious but was not to all so-called electricians at that time, the thermostat was arranged to open circuit instead of short circuit the burner when it became too hot. This lamp apparently solved the problem, and, in order to protect the platinum from the oxygen of the air, he coated it with oxide of zirconium. Unfortunately zirconia, while an insulator at ordinary temperatures, becomes, as is now known, a conductor of electricity when heated, so that the lamp short circuited itself when it was lighted.
Edison’s High Resistance Platinum Lamp, 1879.
This lamp had a high resistance burner, necessary for the multiple system.
Edison’s High Resistance Platinum in Vacuum Lamp, 1879.
This experimental lamp led to the invention of the successful carbon filament lamp.
During his experiments he had found that platinum became exceedingly hard after it had been heated several times to incandescence by current flowing through it. This apparently raised its melting temperature so he was able to increase the operating temperature and therefore greatly increase the candlepower of his lamps after they had been heated a few times. Examination of the platinum under a microscope showed it to be much less porous after heating, so he reasoned that gases were occluded throughout the platinum and were driven out by the heat. This led him to make a lamp with a platinum wire to operate in vacuum, as he thought that more of the occluded gases would come out under such circumstances.
Edison’s Carbon Lamp of October 21, 1879.
This experimental lamp, having a high resistance carbon filament operating in a high vacuum maintained by an all-glass globe, was the keystone of Edison’s successful incandescent lighting system. All incandescent lamps made today embody the basic features of this lamp. This replica is in the Smithsonian Institution exhibit of Edison lamps. The original was destroyed.
These lamps were expensive to make, and, knowing that he could get the requisite high resistance at much less cost from a long and slender piece of carbon, he thought he might be able to make the carbon last in the high vacuum he had been able to obtain from the newly invented Geissler and Sprengel mercury air pumps. After several trials he finally was able to carbonize a piece of ordinary sewing thread. This he mounted in a one-piece all glass globe, all joints fused by melting the glass together, which he considered was essential in order to maintain the high vacuum. Platinum wires were fused in the glass to connect the carbonized thread inside the bulb with the circuit outside as platinum has the same coefficient of expansion as glass and hence maintains an airtight joint. He reasoned that there would be occluded gases in the carbonized thread which would immediately burn up if the slightest trace of oxygen were present, so he heated the lamp while it was still on the exhaust pump after a high degree of vacuum had been obtained. This was accomplished by passing a small amount of current through the “filament,” as he called it, gently heating it. Immediately the gases started coming out, and it took eight hours more on the pump before they stopped. The lamp was then sealed and ready for trial.
Demonstration of Edison’s Incandescent Lighting System.
Showing view of Menlo Park Laboratory Buildings, 1880.
On October 21, 1879, current was turned into the lamp and it lasted forty-five hours before it failed. A patent was applied for on November 4th of that year and granted January 27, 1880. All incandescent lamps made today embody the basic features of this lamp. Edison immediately began a searching investigation of the best material for a filament and soon found that carbonized paper gave several hundred hours life. This made it commercially possible, so in December, 1879, it was decided that a public demonstration of his incandescent lighting system should be made. Wires were run to several houses in Menlo Park, N. J., and lamps were also mounted on poles, lighting the country roads in the neighborhood. An article appeared in the New York Herald on Sunday, December 21, 1879, describing Edison’s invention and telling of the public demonstration to be given during the Christmas holidays. This occupied the entire first page of the paper, and created such a furor that the Pennsylvania Railroad had to run special trains to Menlo Park to accommodate the crowds. The first commercially successful installation of the Edison incandescent lamps and lighting system was made on the steamship Columbia, which started May 2, 1880, on a voyage around Cape Horn to San Francisco, Calif.
The carbonized paper filament of the first commercial incandescent lamp was quite fragile. Early in 1880 carbonized bamboo was found to be not only sturdy but made an even better filament than paper. The shape of the bulb was also changed from round to pear shape, being blown from one inch tubing. Later the bulbs were blown directly from molten glass.
Dynamo Room, S. S. Columbia.
The first commercial installation of the Edison Lamp, started May 2, 1880. One of these original dynamos is on exhibit at the Smithsonian Institution.
As it was inconvenient to connect the wires to the binding posts of a new lamp every time a burned out lamp had to be replaced, a base and socket for it were developed. The earliest form of base consisted simply of bending the two wires of the lamp back on the neck of the bulb and holding them in place by wrapping string around the neck. The socket consisted of two pieces of sheet copper in a hollow piece of wood. The lamp was inserted in this, the two-wire terminals of the lamp making contact with the two-sheet copper terminals of the socket, the lamp being rigidly held in the socket by a thumb screw which forced the socket terminals tight against the neck of the bulb.
Original Socket for Incandescent Lamps, 1880.
Wire Terminal Base Lamp, 1880.
This crude form of lamp base fitted the original form of lamp socket pictured above. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.
This crude arrangement was changed in the latter part of 1880 to a screw shell and a ring for the base terminals, wood being used for insulation. The socket was correspondingly changed. This was a very bulky affair, so the base was changed to a cone-shaped ring and a screw shell for terminals. Wood was used for insulation, which a short time after was changed to plaster of Paris as this was also used to fasten the base to the bulb. It was soon found that the tension created between the two terminals of the base when the lamp was firmly screwed in the socket often caused the plaster base to pull apart, so the shape of the base was again changed early in 1881, to the form in use today.
An improved method of connecting the ends of the filament to the leading-in wires was adopted early in 1881. Formerly this was accomplished by a delicate clamp having a bolt and nut. The improvement consisted of copper plating the filament to the leading-in wire.
Original Screw Base Lamp, 1880.
This first screw base, consisting of a screw shell and ring for terminals with wood for insulation, was a very bulky affair. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.
Improved Screw Base Lamp, 1881.
The terminals of this base consisted of a cone shaped ring and a screw shell. At first wood was used for insulation, later plaster of paris which was also used to fasten the base to the bulb. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.
In the early part of the year 1881 the lamps were made “eight to the horsepower.” Each lamp, therefore, consumed a little less than 100 watts, and was designed to give 16 candlepower in a horizontal direction. The average candlepower (spherical) in all directions was about 77 per cent of this, hence as the modern term “lumen” is 12.57 spherical candlepower, these lamps had an initial efficiency of about 1.7 lumens per watt. The lamps blackened considerably during their life so that just before they burned out their candlepower was less than half that when new. Thus their mean efficiency throughout life was about 1.1 l-p-w (lumens per watt). These figures are interesting in comparison with the modern 100-watt gas-filled tungsten-filament lamp which has an initial efficiency of 12.9, and a mean efficiency of 11.8, l-p-w. In other words the equivalent (wattage) size of modern lamp gives over seven times when new, and eleven times on the average, as much light for the same energy consumption as Edison’s first commercial lamp. In the latter part of 1881 the efficiency was changed to “ten lamps per horsepower,” equivalent to 2¼ l-p-w initially. Two sizes of lamps were made: 16 cp for use on 110-volt circuits and 8 cp for use either direct on 55 volts or two in series on 110-volt circuits.
Final Form of Screw Base, 1881.
With plaster of paris, the previous form of base was apt to pull apart when the lamp was firmly screwed into the socket. The form of the base was therefore changed to that shown, which overcame these difficulties, and which has been used ever since. The lamp shown was standard for three years and is in the exhibit of Edison lamps in the Smithsonian Institution.