This base is similar to that of earlier bulbs except that the collar used for holding and twisting the bulb into a socket is now eliminated.  It was introduced in 1884 and includes a copper screw shell with two or three threads (which serve as one contact terminal) over a plaster of paris insulator.  (NOTE: Plaster of paris is white in color, somewhat rough and porous and easy to scratch.)  The bottom of this base has a straight-sided or tapered copper contact which serves as the second terminal, also over the plaster of paris insulator and separated from and below the threads.  The base is cemented to neck of bulb by a thick ring of plaster of paris between the bulb and base.  Since the plaster of paris collar above the base has now been eliminated, the light bulb is hand-held by the glass bowl to place and turn the base into the female screw of socket.

close window