[Electric engineering.] . d distribute the strains on the glass. The lower carbon B is clamped in the holder C. The top of the bulb is ground smooth and supports a gas cap P with a hole in the center, through which the positive carbon passes. This plate is not fastened to the bulb, but is free to move about a little, and the hole in the center is just large enough to allow the positive carbon to move up and down freely. It is necessary to have the plate itself free to move, in order to get the carbons in line and prevent binding due to any slight irregularity in the carbon. Since the top of th
[Electric engineering.] . d distribute the strains on the glass. The lower carbon B is clamped in the holder C. The top of the bulb is ground smooth and supports a gas cap P with a hole in the center, through which the positive carbon passes. This plate is not fastened to the bulb, but is free to move about a little, and the hole in the center is just large enough to allow the positive carbon to move up and down freely. It is necessary to have the plate itself free to move, in order to get the carbons in line and prevent binding due to any slight irregularity in the carbon. Since the top of the glass and the lower surface of the plate are ground plane, little air can get in between them, and the only place where much air can enter the bulb is at the hole in the center of the top plate, through the small space between the carbon and the plate itself. In the plate shown in the figure, there is an annulargroove a around the carbon. This leaves less surface forthe carbon to rub against and affords a space in which. Fig. 5. 8 ELECTRIC LIGHTING. § 18 eddies are formed by the hot air passing up, thus furthertending to keep out the cold air. The above is intended merely to illustrate the generalarrangement of the enclosed arc. The methods of holdingthe globe and the arrangement of the lower carbon holderdiffer in the various makes of lamp. A^ number of differentstyles of gas cap P are also in use, and the satisfactoryoperation of the lamp depends to a considerable extent on thedesign of this part. 9. As soon as the carbons are drawn apart an arc isformed, as in the ordinary lamp, but the oxygen in the globeis soon burned out and the gases present become rarefied,because the heat of the arc causes them to expand and passout. The globe is not air-tight and there is always a smallamount of oxygen present. This, however, is not enoughto cause anything like the rapid combustion that takes placein the case of the open arc. The arc practically burns ina hot atmosphere of nitrogen
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