[Electric engineering.] . 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
[Electric engineering.] . 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 lamp is made of suchshape that the base will be held securely when the plaster ofParis is put in place. The rib b prevents the base from pull-ing off. The base must, of course, provide two terminals for. the leads from the filament, these terminals being arrangedso that when the lamp is placed in the socket, contact willbe made with two corresponding terminals. There arethree different bases commonly used in America; these are § 16 ELECTRIC LIGHTING. 11 the Edison ; the Thomson-Houston^ or T. H, as it is morecommonly called; and the Westingkouse, or bases are shown in Fig. 7. 16. Fig. 7 (a) shows the Edison base, of which thereare more in use than all- the others put together. One endof the filament is attached tothe outer shell /, which isprovided with a coarse screwthread. The other terminalis connected to the project-ing center piece /, the twobrass pieces being separatedby means of a porcelainpiece c. When the lamp isscrewed into the socket, thescrew shell makes one con-nection and the center piecethe other. Fig. 8 shows alamp screwed into an ordi-nary Edison key socket. Fig. 7 (&) shows the T. , so called because itwas brought out
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