||The invention of the gas-filled tungsten lamp by GE's Irving Langmuir marked a tremendous step forward in terms of lamp efficacy, and is the principle on which all modern incandescent lamps continue to be made. In the higher wattages, efficacy is almost double that of the former Mazda 'B' type tungsten lamps operating in a vacuum atmosphere.
High luminous efficacy could only be attained when thermal conduction and convection losses to the gas filling were minimised, and this required a relatively short filament of large diameter for optimum results. Such conditions initially restricted the gas-filled tungsten lamp to relatively high wattage designs, and the first versions put on the market, in late 1913, were 110V lamps of 1000W and 750W in spherical glass bulbs. The high luminous flux of these lamps resticted their applications somewhat, since up to that time the largest lamp in commercial manufacture was the 500W vacuum tungsten lamp - delivering 3-4 times less light.
In July 1914 two smaller lamps of 500W and 400W were launched - these initially being in spherical bulbs having a long neck, the volume between the bulb and its neck being separated by a mica disc which served to prevent hot gases rising upwards and overheating the cap. By 1915 the design was simplified to the type illustrated here, in which straight-sided bulbs were employed. These were found to blacken rather quickly, and were superseded later that year by the modern pear-shaped bulb with extended neck, such as continues to be used in all modern incandescent lamps. Note the early method of filament clamping in the close-up photo, which shows how small tubes were formed on the ends of the leadwires to fix the still relatively brittle tungsten wires.