The appearance of the gas-filled lamp about 1912 resulted in the addition of a part to some of these lamps. Such lamps were filled with 100% nitrogen to about 0.8 atmosphere. Then, when the lamp operated at full line voltage the heat from the filament resulted in a rise of the gas pressure to about one atmosphere. At this pressure a gas boundary layer of about 1/4-inch thickness exists around the filament. Within this layer the convective flow is laminar and streams upwards due to gravity. Because of the high heat content of this upward flowing gas one was required, in many applications, to operate the lamp only in the base up position. In order to keep the base cement from overheating, a disc was inserted on the glass stem to impede the gas flow. This circular disc was made of mica and was known in the trade as clear muscovite. The thickness of the disc was about 5-10 mils (0.005-0.010 inch).