Because plaster of paris had a tendency to absorb much moisture, especially when used outdoors, it would eventually crack when the base was placed and tightened into the socket. A substitute had to be found to eliminate these drawbacks.  Thus in 1900, porcelain was introduced as an insulator for the bases.  Porcelain was a white, mildly translucent ceramic material that was harder than plaster of paris.  The copper threaded base has four to six threads (which serve as one contact terminal) over a porcelain insulator and one straight-sided or tapered copper contact (that serves as the second terminal) at the bottom of the base.  The base is cemented and fastened to the neck of the bulb by waterproof cement between bulb and base.  The use of porcelain and waterproof cement in the base eliminated the problem of moisture absorption and the subsequent cracking.

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