Statistics are cold facts and are usually uninteresting in a volume of this character, but they tell a story in a concise manner. The development of the modern incandescent lamp has increased the intensity of light available with a great decrease in cost, and this progressive development is shown easily by tables. For example, since the advent of the tungsten lamp the average candle-power and luminous efficiency of all the lamps sold in this country has steadily increased, while the average wattages of the lamps have remained virtually stationary.
Average Candle-Power, Watts, and Efficiency of All the Lamps Sold in This Country
| Year | Candle-power | Watts | Lumens per watt |
|---|---|---|---|
| 1907 | 18.0 | 53 | 3.33 |
| 1908 | 19.0 | 53 | 3.52 |
| 1909 | 21.0 | 52 | 3.96 |
| 1910 | 23.0 | 51 | 4.42 |
| 1911 | 25.0 | 51 | 4.82 |
| 1912 | 26.0 | 49 | 5.20 |
| 1913 | 29.4 | 47 | 6.13 |
| 1914 | 38.2 | 48 | 7.80 |
| 1915 | 42.2 | 47 | 8.74 |
| 1916 | 45.8 | 49 | 9.60 |
| 1917 | 48.7 | 52 | 10.56 |
It will be noted that the luminous intensity of incandescent filament lamps has steadily increased since the carbon lamp was superseded, and that in a period of ten years of organized research behind the tungsten lamp the luminous efficiency (lumens per watt) has trebled. In other words, everything else remaining unchanged, the cost of light in ten years was reduced to one third. But the reduction in cost has been more than this, as will be shown later. During the same span of years the percentage of carbon filament lamps of the total filament lamps sold decreased from 100 per cent. in 1907 to 13 per cent. in 1917. At the same time the percentage of tungsten (Mazda) lamps increased from virtually zero in 1907 to about 87 per cent. in 1917. The tantalum lamp had no opportunity to become established, because the tungsten lamp followed its appearance very closely. In 1910 the sales of the former reached their highest mark, which was only 3.5 per cent. of all the lamps sold in the United States. From a lowly beginning the number of incandescent filament lamps sold for use in this country has grown rapidly, reaching nearly two hundred million in 1919.
XI
THE LIGHT OF THE FUTURE
In viewing the development of artificial light and its manifold effects upon the activities of mankind, it is natural to look into the future. Jules Verne possessed the advantage of being able to write into fiction what his riotous imagination dictated, and so much of what he pictured has come true that his success tempts one to do likewise in prophesying the future of lighting. Surely a forecast based alone upon the past achievements and the present indications will fall short of the actual realizations of the future! If the imagination is permitted to view the future without restrictions, many apparently far-fetched schemes may be devised. It may be possible to turn to nature's supply of daylight and to place some of it in storage for night use. One millionth part of daylight released as desired at night would illuminate sufficiently all of man's nocturnal activities. The fictionist need not heed the scientist's inquiry as to how this daylight would be bottled. Instead of giving time to such inquiries he would pass on to another scheme, whereby earth would be belted with optical devices so that day could never leave. When the sun was shining in China its light would be gathered on a large scale and sent eastward and westward in these great optical "pipe-lines" to the regions of darkness, thus banishing night forever. The writer of fiction need not bother with a consideration of the economic situation which would demand such efforts. This line of conjecture is interesting, for it may suggest possibilities toward which the present trend of artificial lighting does not point; however, the author is constrained to treat the future of light-production on a somewhat more conservative basis.
At the present time the light-source of chief interest in electric lighting is the incandescent filament lamp; but its luminous efficiency is limited, as has been shown in a previous chapter. When light is emitted by virtue of its temperature much invisible radiant energy accompanies the visible energy. The highest luminous efficiency attainable by pure temperature radiation will be reached when the temperature of a normal radiator reaches the vicinity of 10,000°F. to 11,000°F. The melting-points of metals are much lower than this. The tungsten filament in the most efficient lamps employing it is operating near its melting-point at the present time. Carbon is a most attractive element in respect to melting-point, for it melts at a temperature between 6000°F. and 7000°F. Even this is far below the most efficient temperature for the production of light by means of pure temperature radiation. There are possibilities of higher efficiency being obtained by operating arcs or filaments under pressure; however, it appears that highly efficient light of the future will result from a radical departure.
Scientists are becoming more and more intimate with the structure of matter. They are learning secrets every year which apparently are leading to a fundamental knowledge of the subject. When these mysteries are solved, who can say that man will not be able to create elements to suit his needs, or at least to alter the properties of the elements already available? If he could so alter the mechanism of radiation that a hot metal would radiate no invisible energy, he would have made a tremendous stride even in the production of light by virtue of high temperature. This property of selective radiation is possessed by some elements to a slight degree, but if treatment could enhance this property, luminous efficiency would be greatly increased. Certainly the principle of selectivity is a byway of possibilities.