SIXTEENTH LECTURE THE INCANDESCENT LAMP mHE two main types of electric illuminants are the in- candescent lamp and the arc. In the incandescent lamp the current flows through a solid conductor, usually in a vacuum, and the heat produced in the resistance of the conductor makes it incandescent, thus giving the light. Incandescent lamps in an electric circuit therefore act as non-inductive ohmic resistance and can there- fore be operated equally well on constant potential as on con- stant current. As electric distribution systems are always constant potential, most incandescent lamps are operated on constant potential ; and only for outdoor lighting, that is, for street lighting in cases where the arc lamp is too large and too expensive a unit of light for the requirements, incandescent lamps are used on a constant, direct or alternating current cir- cuit; they are then usually built for the standard arc circuits, and thus for low voltage. For general convenience the efficiency of incandescent lamps is given in watts power consumption per horizontal candle power, when operating on such a voltage, that the candle power of the lamp decreases by 20% in 500 hours run- ning; and the time, in which the candle power decreases by 20% — that is, 500 hours with the present efficiency rating — is called the useful life ; since experience has shown, that after a decrease of candle power of 20%, with the carbon filament lamp, under average conditions, it is more economical to replace the lamp with a new lamp, than to continue its use ; as then the increased cost of light due to the lower efficiency is greater than the cost of the lamp, when distributed over 500 hours. 2IO GENERAL LECTURES In discussing incandescent lamp efficiencies, it is therefore essential to make sure that the efficiency is given at the useful life of 500 hours; since obviously any efficiency can be pro- duced in any lamp, by running it at higher voltage, but the life is greatly shortened thereby. Therefore efficiency compari- sons have a meaning only when based on the same length of useful life, as 500 hours. Obviously, for other types of lamps, the economic life may be greater (as for more expensive lamps) or less than 500 hours. Illuminants are measured and compared by the total flux of light which they give. Usually, however, this is expressed in "mean spherical candle power"; that is, the candle power which would be given by the illuminant if this light were dis- tributed uniformly throughout. Since the object of a lamp is to give light, obviously the only logical way of measuring it is by the total amount of light which it gives, and so by the mean spherical candle power ; this therefore is standard. The conventional rating of the incandescent lamp, in hori- zontal candle power, therefore has to be multiplied by a reduc- tion factor, to give the mean spherical candle power. With the carbon filament lamp, this reduction factor is usually .79; a 16 candle power so has a mean spherical candle power of 16 x .79 = 12.6 c. p., and at an efficiency of 3.1 watts per 3.1 horizontal candle power, it has an efficiency of 779 — 3.92 watts per mean spherical candle power. The carbonized bamboo fibre used in the very early days was very soon replaced by filaments made of structureless cellulose, squirted from a cellulose solution, and then carbon- ized. By "treating" these filaments, that is, heating them in gasolene vapor and therefrom depositing a thin shell of car- THE INCANDESCENT LAMP m bon on them, a considerable increase in efficiency became pos- sible; their efficiency was thus greatly increased, from 5 to 6 watts per candle power in the early days, to 3.5 and 3.1 watts per candle power. Of these two types, the 3.5 watt lamp is used in systems of poor voltage regulation, in which (the more efficient 3.1 watt lamp would have too short a life; with the improvements in the voltage regulation of systems, the less efficient 3.5 watt lamp is thus going out of use. By exposing these "treated" filaments to the highest temperature of the electric furnace, their stability at high temperature is greatly improved ; so that in these "metallized"* filament lamps an efficiency of 2.5 to 2.6 watts per candle power is reached. Whether a still further increase of efficiency of the carbon filament will occur, as is quite possible, or whether the carbon filament will be replaced by the metal fila- ments, remains for the future to decide. In the last years, metal filament lamps giving efficiencies far higher than has so far been possible to reach with the car- bon filament, have been developed. First came the os- mium lamp, of 1.5 watts per candle power. As the total supply of osmium available on (the earth is far less than would be required for one year's production of incandescent lamps, the osmium lamp never could hope for more than a very limited use. The tantalum lamp, which was developed next, and is now quite extensively used, gfives an efficiency of about 2 watts per candle power; that is, it is not quite as efficient as the osmium lamp, since tantalum is somewhat more fusible than osmium. As tantalum is a metal which can be drawn into wire, the tantalum filament is of drawn wire ; while * The name "metallized" is given to the form of carbon produced in the»e filaments by the electric furnace temperature, since it has metallic resistance characteristics; a positive temperature coefficient of resistance, while the other forms of carbon have a negative temperature coefficient. aia GENERAL LECTURES all the other metals which are used for lamp filaments are not ductile, and the filaments have to be made by some squirting process, similar to the carbon filament. The highest efficiency was reached by the tungsten (wolfram) lamp, of i to i ^ watts per candle power; that is, tungsten (or rather wolfram metal, since tungsten is the name of the ore of the metal), has the highest melting point of all known metals, and so can be run at the highest temperature, that is, highest efficiency. All these metals melt far below the temperature where carbon melts or boils, but carbon has the great disadvantage of evaporating considerably below its melting point, while these metals evaporate very little, and so can be run at a temperature fairly close to their melting point; while the carbon filament has to be operated at a temperature very far below the melting point. The g^eat difficulty with all these metal filaments is, that the metals are very much better conductors than carbon; to get the same filament resistance, so as to consume the same current, at the same voltage, the metal filaments must be very much longer and very much thinner than the carbon filament. As the efficiency of the metal filament is far higher, to produce the same candle power at the same voltage, less current and therefore a higher resistance is required, which makes these metal filaments still thinner ; as a result, although the metals are mechanically stronger than carbon, the metal filaments are far more frail, due to their exceeding thinness, and it is very diffi- cult to produce lamps of as low candle power, as is feasible with carbon filaments. For larger units, however, and for larger current low voltage lamps, for series lighting, the metal fila- ments are specially suited. For general use, the i6 candle power lamp has proved the most convenient unit of light. The limitation of voltage, for THE INCANDESCENT LAMP 213 which efficient incandescent lamps of such size can be built, has been the cause of the general use of 1 10 volt distribution. 220 volt 16 candle power carbon filament lamps can be built, but are of necessity less efficient, by about 15%, than no volt lamps: at 220 volts, half the current and so four times the resistance is required for the same power as at no volts; the filament therefore is about twice as long and half as thick, hence more breakable and more rapidly disintegrating ; so that there is no possibility of reaching the same efficiency in a 220 volt 16 candle power lamp, as in no volt lamp made with the same care. For the same reason, the 8 candle power no volt lamp must be less efficient than the 16 candle power no volt lamp. In an incandescent lamp are specified : the candle power, the efficiency, and the voltage. To produce lamps fulfilling simultaneously all three conditions, requires either to allow a large margin in either condition — that is, gives a product inferior in uniformity — or to get a uniform product, a large percentage is thrown out as defective, and the cost of the lamp is thus seriously increased. For this reason, in the manufacture a very close agreement is aimed at in candle power and in efficiency; the lamps are then assorted for voltages, and dif- ferent voltages are then assigned by the organization of illum- inating companies to the different companies, so as to consume the total lamp product. As a result hereof, a far more uniform product is derived than could be derived in any other way, and than is available in any other country. This is the reason, that in distribution systems not one and the same voltage, as no, is employed throughout ; but different cities use different voltages, between 105 and 130. The average incandescent lamp used in this country therefore is decidedly superior in uniformity and in efficiency to those used abroad. The ultimate cause hereof 214 GENERAL LECTURES is, that since the earliest days the illuminating companies have followed the principle of supplying light, and not power ;* and 220 volt distribution, while being more efficient from the gen- erating station to the customer's meter, is decidedly inferior in efficiency from the generating station to the candle power pro- duced at the customer's lamps, as the saving in distribution losses does not make up for the lower efficiency of the 220 volt lamp. For this reason, 220 volt distribution has never found any entrance in this country. In gas lighting, an enormous increase of efficiency resulted from the development of the Welsbach gas mantle. In the same direction, that is, by using what may be called *'heat luminescence" in electric lighting, the Nernst lamp was developed, using the same class of material : refractory metal- lic oxides, as in the Welsbach mantle. The "glower" of the Nernst lamp, however, is a non-conductor at ordinary tempera- ture, and requires some heating device, the "heater", to be made conducting. When conducting, it has a very high nega- tive temperature coefficient; that is, the voltage consumed by the glower decreases with the increase of current, just as in the arc, and it therefore requires a steadying resistance, called the "ballast". The lamp therefore requires some operating mechanism, to cut the heater out of circuit after the glower is started. The glower of the Nernst lamp is not operative in a vacuum, since air seems to be necessary for its heat lumines- cence. Fairly good efficiencies have been reached with these lamps, especially in larger units, as 3 to 6 glower lamps, but not of the same class as with the tungsten lamp. * For a long time, the bills were even made out in "lamp hours" and in the earlier days the machines rated in "lights" and not in kilowatts.