When it was known that Edison, whose genius had already enriched the world with many important applications of electricitywith duplex, quadruplex, and quite recently sextuplex telegraphy; with the electric pen; with some of the best forms of the telephone and its various modifications, as the microphone, the microtasimeter, the megaphone, the aerophone, the phonometer; with the phonographwhen it was reported in 1878 and 1879 that this indefatigable experimenter and versatile inventor had turned his attention to the problem of electric illumination, the public expected that his fertile and practical mind would succeed if it were possible in overcoming the minor but stubborn difficulties which yet stood in the way of electrical illumination. The confidence which was felt in his ability was shown by the fact that during the months in which he was engaged in studying this subject, newspaper rumors of the success or nonsuccess of his laboratory studies made the prices of gas stock rise or fall on the Paris and London Exchanges. He commenced his experiments in September, 1878, and, after fifteen months of research, in the latter part of December, 1879, he published the record of his investigations to the world, and gave a public trial of the elaborated result.
Divining that the practical electric light of moderate illuminating power could not be produced by the voltaic arc, to which recent experiments had been chiefly confined, and with which Jablochkoff, Serrin, Werdermann, and others had obtained remarkable results, but by the incandescence of some resistant material, he confined his attention to the substances of low conducting powers from which the incandescent light can be obtained. These are platinum, iridium, and like metals and alloys, which only fuse at an exceedingly high temperature, and the forms of carbon which possess a high degree of purity and homogeneity. His earlier experiments were expended upon metallic material.
Among the several circuit-closing regulators which he devised was one by which the heated air prest a diaphragm outward, closing and breaking the circuit so rapidly that no variation in the intensity of the light was observable. Another was a device by which the expansion of the luininous conductor itself was made to draw a rod upward, which actuated a circuit-closer through an arrangement of levers. Edison developed in the earlier stages of his investigations a novel kind of lamp, from which he obtained a very brilliant light by the incandescence of a piece of zircon to which the heat-rays of the incandescent platinum spiral were transmitted by reflection. The spiral of platinum and iridium was placed in the focus of an elliptic reflector of copper coated with gold, and the heat-rays were focalized upon a thin piece of zircon, which attained a degree of luminosity greatly exceeding that of the incandescent platinum.
Edison's experiments were necessarily directed mainly to the material to be rendered incandescent, and the form in which it would afford the best results. The brilliancy of the light depends upon the resistance which the incandescent conductor offers to the passage of the electric current. Expecting the best results from platinum, he found that the light was intensified by incorporating fine particles of this conducting agent in a nonconducting, incombustible, and nonfusible material, which was itself rendered luminous by the heat. By imbedding finely divided platinum in a nonconducting substance, he obtained a light from currents too weak to render the spiral luminous. A large spiral of platinum whose coils were coated and separated by magnesia produced a good light; it was with this form of lamp that he employed the regulator in which a metallic cup at the top of the coil pulled a rod upward, actuating a circuit-closing apparatus. Among the other materials upon which he experimented were the oxides of different metals. He obtained a fine light from iridosmine, a natural alloy of osmium and iridium, which he enclosed in a powdered state in a tube of zircon. He tried also a combination of platinum and carbon, the latter becoming highly incandescent as the current passed to it from the platinum rod, encountering a greater resistance.
Still considering platinum the most promising material, he was startled after a couple of months of experimentation by the discovery that the platinum degenerated, and that its incandescence was seriously affected through the action of the atmosphere. Plates and wires of platinum, and also of iridium and other metallic conductors whose point of fusion is at a very high temperature, he found, when heated while exposed to the atmosphere to a temperature near their melting point, by a current of electricity passing through them for a number of hours together, crack and break in innumerable places. These fissures are found under the microscope all over the surface of the metal, running in every direction, and sometimes penetrating to the center of the rod or wire. Holding platinum and alloys of platinum and iridium in the heat of a candle, he observed a loss of weight; and even when they are exposed to heated air there is a diminution of weight. The consumption is sufficient to cause a hydrogen jet to take on a greenish hue. The metal after a while becomes so fractured that it falls to pieces.
He thus perceived that the ordinary platinum or platinum and iridium, as sold in the market, is useless for his purpose, and also that the metal can not be employed for illumination by incandescence, as the cracks cause it gradually to deteriorate and eventually destroy it, while they greatly lessen the degree of incandescence of which its surface is capable. The knowledge of the cause of the disintegration of platinum suggested the remedy. Lodyguine, the Russian physicist, invented a carbon lamp in 1873, in which the cracking and wasting away of the carbon under incandescence, by the action of the oxygen of the atmosphere, was obviated by enclosing the burner in a glass globe from which the air was exhausted. It was necessary to purify the platinum and enclose it in a vacuum to prevent its deterioration when heated to incandescence. Edison devised a method of producing a more perfect vacuum, and at the same time cleansing the platinum burner of all the air and other gases which it contains. A glass globe is connected by an aperture with a mercury air-pump, and the air exhausted. The wires connecting the spiral or other form of burner with the battery pass through holes in the glass which are fused together and hermetically sealed. After the air is exhausted from the glass the current is turned on, heating the platinum to a temperature of about 150 degrees Fahrenheit, at which point it is kept for from ten to fifteen minutes. The gases which issue from the platinum are carried away by the air-pump. The current is then increased until the temperature rises to 300 degrees, at which point it is kept again ten or fifteen minutes. It is thus raised by successive stages until the platinum attains a brilliant incandescence and the glass about the aperture connecting with the mercury-pump melts with the heat and fuses together, hermetically sealing the vacuum.
The wires purified by this process are found to have a gloss and brightness greater than that of silver. Their light-giving power is increased in a remarkable ratio. The same burner which will give when new a light of only three candles, emits in the vacuum a light of twenty-five. Testing spirals which had been prepared and sealed in a glass vacuum in this manner by subjecting them to sudden currents of electricity which raised them to incandescence a great number of times, no cracks or breaks were discoverable, nor the slightest loss of weight. Wires of chemically pure iron and nickel were found to give a light in the vacuum equal to that of platinum exposed to the atmosphere; and carbon sticks, freed from air and inclosed in a vacuum in the same manner, may be heated until they become soft and plastic, and then regain their former consistency when cool again. Edison next tried the combination of platinum and iridium alloy with magnesia in a vacuum. He found that the oxide will unite with the metal, hardening it and rendering it more refractory to such a degree that a spiral so fine that it would melt without the coating of magnesia could be raised to a dazzling incandescence and remain quite elastic. Such a spiral, with a surface of only three-sixteenths of an inch, will give a light of forty candles. He next turned his attention to securing the greatest possible amount of resistance in the conductor. Instead of using lamps of only one or two ohms of resistance, he reached the conclusion that the light could be more economically produced from conductors having two hundred ohms of resistance or more.
The perfected form of the platinum lamp consists of a long coil of wire coated with magnesia, supported in a glass vacuum tube by a rod of platinum, the tube resting upon a metallic frame containing a regulating apparatus in a chamber within. The conducting wires pass through the bottom of the globe and into this chamber, where the circuit can be instantaneously broken and closed again by the regulator. Around the vacuum tube is a glass globe resting upon the frame, with openings into an aneroid chamber below, whose bottom is a diaphragm which distends sufficiently when the air within the globe is heated to a certain degree to press a pin in its center downward against a straight spring, which rests with an upward pressure upon a metallic block, through which the current is transmitted through the spring to the wire which leads it to the incandescent spiral. When the contact between the spring and the block is broken, the flow of electricity is interrupted, to be restored again by the immediate cooling and contraction of the air in the globe and aneroid chamber, which is so instantaneous that no variation in the intensity of the light is perceptible. While bringing the platinum lamp to this high state of perfection, Edison set on foot inquiries regarding a larger supply of platinum; and the miners of the gold regions, incited by his advertisements, discovered such frequent indications of its presence that this exceedingly valuable metal may be expected to be produced in much larger quantities than the present supplies. The vacuum which Edison's method produced was much nearer perfect than had been before attained. One of the reasons for the want of success of lamps in which the light was produced by the incandescence of carbon in a vacuum was the impossibility of sufficiently exhausting the air in the glass chamber. By the present process it could be reduced to but little over one-millionth of an atmosphere.
The inventor thought that he had elaborated a lamp which embodied the best principles, and which was sure to prove a commercial success. He had introduced improvements in the electric machine by which the equivalent of about 90 per cent. of the power expended was returned in electricity. When he was nearly ready to give the lamp in this form to the world, he began, led partly by accident, to experiment with carbon, with results which induced him to alter his whole system and adopt a carbon instead of a metallic burner. A prominent cause for the failure of carbon burners had been the impossibility of obtaining a form of carbon sufficiently pure in substance and homogeneous and even in texture. Edison was encouraged to try new forms from obtaining a remarkably brilliant light in the vacuum by the incandescence of a piece of calcined cotton thread. He placed in the glass a thread of ordinary sewing-cotton, which had been placed in a groove between two blocks of iron and charred by long exposure in a furnace, exhausted the air, and sealed the tube. He then turned on the electrical current, and increased it until the most intense incandescence was obtained before the slight filament broke. Examining then the fragments under the microscope, he found that the fragile substance had hardened under the excessive heat, and that its surface had become smooth and glossy.
This led him into a long series of experiments with carbon. After carbonizing and testing a great variety of fibrous substances, he found that paper yielded the most satisfactory results. The burner on which he finally settled was made from Bristol cardboard in the form of a tiny horseshoe. Strips about two inches long and an eighth of an inch wide, curved in the shape of an elongated horeshoe, are struck from a sheet of cardboard, and a number of them laid one upon another, with pieces of tissue-paper between, in an iron mold; this is tightly closed and placed in an oven, which is gradually raised to a temperature of 600 degrees; the mold is next placed in a furnace and allowed to come to a white heat, and then removed and left to cool. The carbonized paper horseshoe is then taken out with the utmost care, mounted in a diminutive glass globe, and connected with the wires. The air is then pumped out and the glass hermetically closed. The form of the lamp is a small bulb-shaped glass vacuum, globular above, with an elongated end resting upon a standard, through which the wires leading to and from the generator pass, connecting with thin platinum wires, which penetrate the thick end of the glass; to these the carbon burner is attached by clasps made from the same metal. No regulating apparatus is attached to the lamp, as the current can be regulated at the central station where the electricity is generated.
The inventor has developed a method by which the currents can be cut off from any of the lamps and the lights extinguished, without affecting the supply of electricity to those which are left burning. He proposes to supply the electricity in cities for lighting the houses and public places from stations in which a number of electric machines adequate for supplying an area of about a third of a square mile are driven by one or two powerful steam-engines. Each generator is capable of supplying about fifty burners.
1 From an article in Appleton's "Annual Cyclopedia" for 1879. By permission of D. Appleton & Company.