Anything made in the form of an illuminating device, in which the lighting element is rendered incandescent by electricity, may properly be called an incandescent lamp, whether the medium is incandescent gas as in the Moore lamp, an incandescent vapor as the Cooper Hewitt mercury-vapor lamp, or the incandescent filament of carbon or metal such as is universally used for lighting.

From the year 1879, when Mr. Edison announced the perfection of the incandescent electric lamp, until 1903, when for a short period tantalum lamps were used, very little improvement had been made in the carbon-filament lamp. Immediately following the introduction of the tantalum lamp came the tungsten lamp, which because of its wonderfully increased capability for producing light has extended artificial illumination to a degree almost beyond comprehension. The influence of the tungsten lamp has induced a new era of illumination that has affected the entire civilized world. The development of the high-efficiency incandescent lamp has brought about a revolution in electric lighting. Its use is universal and its application is made in every form of electric illumination.

Regardless of the immense number of tungsten lamps in use, the carbon-filament lamp is still employed in great numbers and will probably continue in use for a long time to come. In places where lamps are required for occasional use and for short intervals of time, the carbon filament still finds efficient use. In one form of manufacture the carbon filament is subjected to a metalizing process that materially increases its efficiency. This form, known commercially as the GEM lamp, fills an important place in electric lighting.

Of the rare-metal filament lamps, those using tungsten and tantalum are in general use, but the tungsten lamps give results so much superior in point of economy in current consumed that the future filament lamps will beyond doubt be of that type unless some other material is found that will give better results.

The filaments of the first tungsten lamps were very fragile and were so easily broken that their use was limited, but in a very short time methods were found for producing filaments capable of withstanding general usage and having an average life of 1000 hours of service. These lamps give an efficiency of 1.1 to 1.25 watts per candlepower of light, as will be later more fully explained. This, as compared with the carbon-filament lamps which average 3.1 to 4.5 watts per candlepower, gives a remarkable advantage to the former. The tungsten lamp has a useful life that for cost of light is practically one-third that of the carbon-filament lamp.

The metal tungsten, from which the lamp filament is made, was discovered in 1871. It is not found in the metallic state but occurs as tungstate of iron and manganese and as calcium tungstate. Up to 1906 it was known only in laboratories and on account of its rarity the price was very high. As greater bodies of ore were found and the process of extraction became better known, the price soon dropped to a point permitting its use for lamp filaments in a commercial scale.

Pure tungsten is hard enough to scratch glass. Its fusing point is higher than any other known metal; under ordinary conditions it is almost impossible to melt it and this property gives its value as an incandescent filament. One of the laws that affect the lighting properties of incandescent lamps is: "the higher the temperature of the glowing filament, the greater will be the amount of light furnished for a given amount of current consumed." The high melting point permits the tungsten filament to be used at a higher temperature than any other known material. Tungsten is not ductile, and in ordinary form cannot be drawn into wire. Because of this fact, the filaments of the first lamps were made by the "paste" process, which consisted of mixing the powdered metal with a binding material, in the form of gums, until the mass acquired a consistency in which it might be squirted through a minute orifice in a diamond dye. The resulting thread was dried, after which it was heated, and finally placed in an atmosphere of gases which attacked the binding material without affecting the metal. When heated by electricity in this condition, the particles of metal fused together to form a filament of tungsten. While the "paste" filaments were never-satisfactory in general use, their efficiency as a light-producing agent inspired a greater diligence in the search for a more durable form.

Although tungsten in ordinary condition is not at all ductile, methods were soon found for making tungsten wire and the wire-filament lamps are now those of general use. One process of producing the drawn wire is that of filling a molten mass of a ductile metal with powdered tungsten after which wire is drawn from the mixture in the usual way. The enclosing metal is then removed by chemical means or volatilized by heat.

Of the difficulties encountered in the use of metal-filament lamps that of the low resistance offered by the wire was overcome by using filaments very small in cross-section and of as great length as could be conveniently handled. The long tungsten filament requires a method of support very different from the carbon lamp. The characteristic form of tungsten lamps is shown in Fig. 217, in which the various parts of the lamp are named.

The filament of an incandescent lamp is heated because of the current which passes through it. The electric pressure furnished by the voltage, forces current through the filament in as great an amount as the resistance will permit. A 16-candle-power carbon lamp attached to a 110-volt circuit requires practically 1/2 ampere of current to render the filament incandescent; the filament resistance must, therefore, allow the passage of 1/2 ampere. With a given size of filament, its length must be such as will produce the desired resistance. A greater length of this filament would give more resistance and a correspondingly less amount of current would give a dim light because of its lower temperature. Likewise, a shorter filament would allow more current to pass and a brighter light would result. When the size and length of filament is once found that will permit the right amount of current to pass, if the voltage is kept constant, the filaments will always burn with the same brightness. This is in accordance with Ohm's law which as stated in a formula is

Fig. 217. - An Edison Mazda lamp and its parts.

E = RC that is E, the electromotive force in volts, is always equal to the product of the resistance R, in ohms, and the current C, in amperes.

In the incandescent lamp, if the electrotromotive force is 110 volts and the current is 1/2 ampere, the resistance will be 220 ohms and as expressed by the law

110 - 220 X 0.5

From this it is seen that any change in the voltage will produce a corresponding change in the current to keep an equality in the equation. If the voltage increases, the current also increases and the lamp burns orighter. Should the voltage decrease the current will decrease and the lamp will burn dim. This dimming effect is noticeable in any lighting system whenever there occurs a change in voltage.

The quantity of electricity used up in such a lamp is expressed in watts, which is the product of the volts and amperes of the circuit. In the lamp described, the product of the voltage (110) by the amount of passing current (1/2 ampere) is 55 watts. With the above conditions the 16 candlepower of light will require 3.43 watts in the production of each candlepower. The best performance of carbon-filament lamps give a candlepower for each 3.1 watts of energy.

The filament of the tungsten lamp must offer a resistance sufficient to prevent only enough current to pass as will raise its temperature to a point giving the greatest permissible amount of light, and yet not destroy the wire. The high fusing point and the low specific heat of tungsten permits the filament to be heated to a higher temperature than the carbon filament and with a less amount of electric energy. These are the properties that give to the tungsten lamp its value over the carbon lamp.

The exact advantage of the tungsten lamp has been investigated with great care and its behavior under general working conditions is definitely known. In light-giving properties where the carbon-filament lamp requires 3.1 watts to produce a candlepower of light, in the tungsten filament only 1.1 watts are necessary to cause the same effect. The tungsten lamp therefore gives almost three times as much light as the carbon lamp for the same energy expended. The manufacturers aim to make lamps that give the greatest efficiency for a definite number of hours of service. It has been agreed that 1000 working hours shall be the life of the lamps and in that period the filament should give its greatest amount of light for the energy consumed.

The Mazda Lamp

The trade name for the lamp giving the greatest efficiency is Mazda. The term is taken as a symbol of efficiency in electric incandescent lighting. At present the Mazda is the tungsten-filament lamp, but should there be found some other more efficient means of lighting, which can take its place to greater advantage, that will become the Mazda lamp.