Electric incandescent lamps are the present mainstay of electric illumination and, it might be stated, of progress in lighting.  Wonderful achievements have been accomplished in other modes of lighting and the foregoing statement is not meant to depreciate those achievements.  However, the incandescent filament lamp has many inherent advantages.  The light-source is enclosed in an air-tight bulb which makes for a safe, convenient lamp.  The filament is capable of subdivision, with the result that such lamps vary from the minutest spark of the smallest miniature lamp to the enormous output of the largest gas-filled tungsten lamp.  The outputs of these are respectively a fraction of a lumen and twenty-five thousand lumens; that is, the luminous intensity varies from an equivalent of a small fraction of a standard candle to a single light-source emitting light equivalent to two thousand standard candles.

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

Lumens
Year     Candle-power      Watts         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            51           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be noted that the luminous intensity of incandescent filament lamps has steadily increased since the carbon lamp was superseded, and that in a period of ten years of organized research behind the tungsten lamp the luminous efficiency (lumens per watt) has trebled.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.