[Electric engineering.] . of the air, andthe final exhaustion is then accomplished either by meansof a mercurial air pump orby the chemical method. The principle of opera-tion of the Sprengel mer-curial air pump is shownby Fig 6; c d is a glasstube with a T joint at x,from which a branch tubeleads to the lamp. Thelower end of the tube dipsbelow the surface of themercury in the vessel is allowed to runfrom A by opening thepinch-cock c, and in doingso draws the air out ofthe bulb by carrying downa stream of air bubblesuntil the air is completelyexhausted. When the airhas become exhaust
[Electric engineering.] . of the air, andthe final exhaustion is then accomplished either by meansof a mercurial air pump orby the chemical method. The principle of opera-tion of the Sprengel mer-curial air pump is shownby Fig 6; c d is a glasstube with a T joint at x,from which a branch tubeleads to the lamp. Thelower end of the tube dipsbelow the surface of themercury in the vessel is allowed to runfrom A by opening thepinch-cock c, and in doingso draws the air out ofthe bulb by carrying downa stream of air bubblesuntil the air is completelyexhausted. When the airhas become exhausted, themercury falls from the topto the bottom of the tubewith a sharp click. Thisstyle of pump is capableof producing a high de-gree of exhaustion, but,unfortunately, it is ratherslow in its action. Thepump has been modifiedin various ways to adapt it to commercial work, but itsaction is briefly as outlined above. Another method of exhaustion, known as the chemicalmethod, has recently come into use, and has rendered the. fig. 6. 10 ELECTRIC LIGHTING. § 16 process of exhaustion much more rapid. In this process,the air is first exhausted to quite a high degree by mechan-ical pumps. A gas is then introduced, which combines withthe remaining gases and renders them incapable of actingon the filament. The process is in the main kept secret;it produces a vacuum that gives as good results as thatproduced by a mercurial pump, and the process is much morerapid. The chemical that is often used is phosphorus, asmall quantity of which is placed in the stem of the bulband heated when the mechanical pumps have produced theproper degree of exhaustion. 15. Bases.—After the lamp has been exhausted, it iscomplete with the exception of the base N, Fig. 1, with whichit must be provided in order that it may be readily attachedto the socket. These bases are usually made of brass andporcelain, the lamp being held in them by a setting ofplaster of Paris or cement. In Fig. 5, the lower part of the l
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