In 1888, the length of the base of a light bulb was increased to accommodate more threads:  from 2 or 3 threads to 4, 5 or 6 threads.  The base of this bulb has a copper screw shell with 4 to 6 threads (which serve as one contact terminal) and one straight-sided or tapered copper contact (which serves as the second terminal) at the bottom of the base, all over a plaster of paris insulator. (NOTE: Plaster of paris is white in color, somewhat rough and porous and easy to scratch.)  The base is cemented to the neck of the bulb by a thick ring of plaster of paris between the bulb and base.  There is no collar above the base as with the first bulbs from the early 1880s.  Instead the bulb is hand-held by its glass bowl to place and turn the base into the female screw of a socket. 

ALSO NOTE: Although two or three-threaded threaded bases were largely discontinued in 1888, they were still used for some applications for years afterwards.

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