The reason for withdrawing the air from the bulb is that if the filament were heated in the air the oxygen of the air would combine with the carbon, causing combustion and consequent destruction. Even enclosed as it is in a vacuum the filament is slowly destroyed by the intense heat at which it is operated. Aside from the advantage named is the fact that there is no heat-conducting medium between the filament and the globe, practically all the heat that is emitted being that which radiates from the filament (a small amount is lost by conduction through the leading-in wires). If there were any gas or vapor within the bulb it would con duct additional heat to the glass walls, and also dissipate heat by convection, so that with a given current in a given filament the temperature of the filament would be reduced, and, therefore, less light would be obtained. The destruction of the filament referred to is not due to com bustion because as just explained there is no oxygen left in the bulb. There is a disintegra tion of the filament by some process that has never been positively identified or explained, the results being that impalpable particles of carbon are deposited on the inside surface of the bulb, causing a gradual darkening of the glass that is readily discernible.
The leading-in wires are made ofplatinum because that metal has the same coefficient of expansion by heat as that of glass. Were the coefficient different, small cracks would form in the bulb and the vacuum would be spoiled. Carbon is an exception to the general rule that almost all conductors increase in resistance when the temperature is raised. Its resistance decreases rapidly with an increase in tempera ture up to the red point. Thereafter, up to the white, the resistance decreases more slowly. The ordinary carbon filament, when at its work ing point, has about one-half the resistance as when cold. The standard filament gives out a mean illumination of 16-candle power at right angles with the axis of the lamp from base to top.
The power required for the usual 16-candle power standard lamp varies from 50 to 64 watts, depending upon the temperature at which the filament is operated. The higher the tem perature of the filament the higher the efficiency in watts per candle power, and also the shorter will be the life. The life of the filament in this way limits the efficiency of the lamp. The candle power of the incandescent lamp may be greatly increased by simply increasing its fila ment temperature by the simple expedient of increasing the current, but thereby its life is shortened. The increase in candle power is not directly in proportion to the increase in current but in a considerably higher ratio.
It is important to maintain the potential on the terminals of an incandescent lamp at the normal working point. Any slight excess ma terially shortens its life. The resistance of an incandescent lamp filament is much lower when hot than cold, approximately in the ratio of two to one; that is, the resistance is twice as high cold as when hot when at normal burning temperature. The illumination is rated in candle power and power consumed in watts per candle measured when the lamp is giving its rated candle power, but this condition is not obtained with any incandescent lamp through a large portion of its active life. When a new
lamp is placed in circuit it will usually give the full candle power or a trifle more at the start, and the candle power will rise to a value from 5 per cent to 11 per cent higher than the rating. It soon, however, begins to fall off with a constant diminution up to the breaking point.
The rise and fall of candle power are due to changes in the structure of the filament. The resistance at first decreases, allowing more cur rent to flow and consequently higher tempera ture and more light. This is indicated by the rise of the curve from 16.2 candle power to 17.3 candle power during the first 50 hours. Then the diminution of resistance ceases and is fol lowed by the gradual wasting away of the filament, which causes a gradual increase in resistance, by reducing its cross section. The current thus gradually falling off, taken together with the decreased surface of the filament and the deposit of carbon upon the globe, causes the fall in candle power indicated. The decrease in candle power is not directly proportional to the decrease in current. thus the enersry ner candle power increases rapidly after the first few hundred hours.
The Tungsten The tungsten fila ment has revolutionized the incandescent light ing industry and is reducing the use of arc lamps. The carbon filament is rapidly going out, and the old-fashioned arc lamps are being replaced either by groups of tungsten lamps or Nernst or vapor lamps. The tungsten filament has a life of 1,000 hours, and some have been made of twice that durability, whereas 125 to 200 hours is long for other filaments and arc lamps. The tungsten also has the advantage of diffusing the light more, there being more turns in the filament, and it is closer to day light in its coloration. The tungsten is a truly automatic light, requiring no attention beyond an occasional dusting. Tungsten is not so rare a metal as once supposed, and its great resist ance to heat — it melts at 3200° C.— renders it a most fit metal for filaments. At first it was found exceedingly difficult to form it into the minute diameter wire form needed. It is ex ceedingly brittle and fragile, and early attempts to draw it through fine dies, such as are used for making the most delicate strands of copper and steel Wire, were failures. Kuzel solved the problem by making a solution of tungsten, evaporating the surplus moisture and squirting the residuum through a die, getting a thread which would bear handling. Snch a filament is "flashed" in a mixture of steam and hydro gen, and as the steam decomposes the oxygen unites with the carbon of the filament. Later a method of drawing the metal was worked out successfully and is preferred to the squirted film. The character of tungsten is such that a very long and frequently lapped filament is required to give the necessary light. This is an advantage, reducing the glare.