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An incandescent electric lamp is one in which light is produced by the passage through a solid conductor of a current sufficiently strong to raise it to a temperature of incandescence. In this case the conductor is solid and continuous, while in an arc lamp, the other important type described in Chapters XIV. and XV., light is produced at a gap in the circuit across which the current is carried by the heated vapor present. The ordinary type of incandescent lamp, enormous numbers of which are now used, consists essentially of a high resistance carbon filament hermetically sealed in a nearly perfect vacuum. In fact, these words are substantially the same as the patent claims of Edison,* who developed the incandescent lamp as well as the necessary generator method of distributing current and the various auxiliary devices to a condition of commercial success.
Many forms of incandescent lamp have been devised employing filaments composed of materials other than carbon, and not requiring a vacuum ; but these are special types that will be described later. The present chapter is confined to the ordinary incandescent lamp, as already defined, which has been used to the practical exclusion of any other form since the introduction of incandescent lighting in 1880.
Materials Used for Filaments. — In the earlier lamps made by Edison, the filaments consisted of platinum wire, but that metal soon lost its strength, even at normal working temperature ; and if accidentally raised above this point it was likely to be melted. The high cost of platinum is also a serious objection to its use for this purpose. Consequently Edison soon substituted carbon for platinum in his lamps. After trying many materials, carbonized
* U. S., Paients, 1879.
bamboo was adopted, and generally used in the Edison lamps made for about fifteen years. Other manufacturers employed thread, thin strips of cardboard, or some special compound in place of bamboo. In practically all cases some organic substance carbonized by heat has been used.
For several years the tendency has been to adopt almost universally the so-called “squirted filaments.” They are usually made by dissolving cotton in a solution of zinc chloride producing a viscous, semi-transparent liquid in which the appearance and fibrous character of the cotton are entirely lost. This gelatinous material is forced or squirted through a small hole, and received in a vessel containing alcohol, which causes it to set and harden sufficiently to be handled afterwards. After washing, the material, having the appearance and consistency of cooked vermicelli, is wound upon a large drum and dried, after which it possesses considerable strength, and looks much like a cat-gut string such as is used on a violin. It is then cut into lengths suitable for filaments, and carbonized at a high temperature.
The advantage of using this product in place of some solid substance, such as bamboo, is the fact that it is perfectly homogeneous, and can be made readily and accurately of any desired cross-section or length. On the other hand, there is considerable difficulty in eliminating entirely bubbles of air from the viscous solution of cotton. If they are present, even though very small, they will cause a flaw in the filament at any point where a bubble may happen to exist. In order to get rid of them the solution is filtered and heated under a vacuum. To avoid the presence of impurities and to insure a perfectly homogeneous product the best quality of cotton wool should be used, the specially high grade employed by surgeons as absorbent cotton being adopted by the best manufacturers. Great skill and care are required in making the mixture, the exact density and temperature of the zinc chloride solution as well as the proportion of cotton dissolved in it being matters of particular importance. The formation of lumps in the jelly-like mass is likely to occur, and should be prevented by constant stirring, otherwise the resulting filaments will not be of uniform cross-section.
Measuring and Sorting the Filaments. — After being carbonized, the filaments are carefully measured and sorted according to
length and diameter. The latter is reduced very greatly by the processes of drying and carbonizing, so that it must be determined very exactly by means of a micrometer. A filament made from “ squirted ” cellulose solution is somewhat elliptical in cross-section owing to its having been wound, while soft, upon the drying-drum. For this reason it is necessary to measure both maximum and minimum diameters in order to determine its true cross-section. The filaments suitable for the various types and sizes of lamps are thus selected. In a general way the length is proportional to the voltage, and the surface is proportional to the candle-power for which the lamp is intended.
Flashing or Treating the Filaments. — The object of this process is to render the filaments stronger and more uniform. Formerly, when they were made from bamboo, thread, and similar materials, the filaments obtained were far from uniform throughout their length. The present forms of “squirted” filaments are better in this respect, being more uniform in diameter and more homogeneous; but even these require to be treated after being carbonized. The treatment consists in raising the filament to incandescence by passing through it an electric current in an atmosphere of hydrocarbon vapor or gas.
The high temperature of the filament decomposes the hydrocarbon, and causes carbon to be deposited upon it. This deposit occurs over the entire surface of the filament, but is greater at any point where the electrical resistance may be abnormally high, because the temperature there will also be higher. Hence the tendency is to produce a filament of uniform resistance throughout its length. In the same way the strength is made more uniform, because any part thinner or weaker than the rest is likely to have a higher electrical resistance, so that it will be reinforced by receiving a heavier deposit of carbon. On the other hand the deposited carbon is graphitic in character, and has a lower specific resistance of 10 to 15 per cent that of the original filament, which is undesirable especially for high voltage lamps.
The filaments are treated after they have been carbonized, but before they have been mounted, the process being performed in a jar containing hydrocarbon vapor. The stopper of the jar carries metallic holders into which the ends of a filament are inserted, the latter being then introduced into the jar. By means of the metallic holders which serve also as electrical connections, a current is caused to flow through the filament in order to bring it to incandescence. The resulting deposit of carbon reduces the resistance until a certain value is reached, when the current is interrupted, and the filament is taken out, to be followed by another and so on. The proper resistance is predetermined by experience or calculation for each type of lamp. It may be measured during the process of treatment by disconnecting the filament from the current supply, and connecting it to some resistance measuring device, such as an ohmmeter, a double-throw switch being used to make the change. In this case the measurement is made while the filament is cold, and it is generally assumed that at working temperature the resistance is reduced one half. It is also an easy matter to determine the resistance when the filament is incandescent, and the carbon is being deposited upon it. By measuring the voltage across the terminals of the filament and the current flowing in it, we know from Ohm's law that ohms = volts = amperes. In another method the filament is made one arm of a Wheatstone bridge, and the other three resistances are so adjusted that no current flows in the galvanometer circuit when the filament reaches the proper resistance. A relay put in place of the galvanometer will release its armature at that moment, and may be arranged to stop automatically the current through the filament. The increase in diameter resulting from the deposit of carbon is about 10 per cent, but varies in different sizes and makes of filament.
Mounting the Filaments. — In order to mount the filaments, that is, connect them to the “ leading-in” wires (C C in Fig. 352), that are to supply them with current, various methods have been devised and used. One plan consists in electroplating a sleeve of copper around the end of the filament and of the wire, thereby mechanically binding and electrically connecting them together. In lamps formerly made from carbonized cardboard, the ends of the filaments were enlarged so that they could be attached to the ends of the wire by very small bolts. Another method consists in forming a socket at the end of the wire into which the end of the filament is inserted, and held in place by squeezing the socket around it. These means of connection have been in most cases abandoned for the simpler and cheaper joint, made by pasting together the ends of the filament and wire, using a
mixture of powdered carbon and molasses, or other similar sticky material. Still another form of joint is made by heating the junction of the filament and the wires by an electric current in an atmosphere of hydrocarbon gas or under a hydrocarbon liquid. In this way a deposit of solid carbon is formed around the filament and wire, which binds them together. The deposit takes place more rapidly in the liquid, but the latter is objectionable because it adheres to the filament and wires. This so-called deposited carbon joint is a very good one, but is more troublesome and expensive to make than the pasted joint.
Platinum “ Leading-In "Wires. — To insure a perfectly airtight seal where the “ leading-in” wires pass through the glass, they are made of platinum, because its coefficient of expansion by heat agrees with that of the glass which is used. If the two coefficients of expansion differed materially, it is obvious that there would be a tendency either to crack the glass or to let in air when temperature changes occurred.
Platinum being a very expensive metal, even the small amount required in an incandescent lamp is a considerable item in the cost, so that many attempts have been made to substitute some cheaper metal or alloy. While alloys having about the same coefficient of expansion as that of glass can be made, they are open to the objection of not being able to stand the high temperature of melting glass or the action of the blowpipe flame without melting or burning while being sealed in, so that there is likely to be a leak owing to the imperfect fusing of the glass to the wire.
In order to economize as much as possible in the cost of platinum for each lamp, the greater portion of the length of the leading-in wires is composed of copper, platinum being used only where the wire passes through the glass. For example, in Fig. 352, the longer parts, D C, of the leading-in wires are of copper, and the shorter parts, D E, are of platinum, the joints between the two being made by electrical welding.
Glass Portions of Incandescent Lamps. — The several steps in their manufacture are indicated in Figs. 347–353, a standard Edison 16 c. p. lamp being represented on a half scale. To form the “inside part,” a glass tube A is used, being first softened by heat and Aared out at one end B. The other end is then softened, the leading-in wires C E introduced, and the plastic