The entire copper threaded base has 4 to 6 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.  Look for a white, mildly translucent ceramic material at bottom of the base between the base eyelet and screw.  Porcelain should be a bit shinier than plaster of paris and harder to scratch.  The base is cemented and fastened to the neck of the bulb by a waterproof cement between bulb and base. Porcelain insulator was introduced in 1900 because plaster of paris had a tendency to absorb much moisture, especially when used outdoors, so that it would eventually crack when the base was placed and tightened into the socket.  The use of porcelain and waterproof cement in the base eliminated the problem of moisture absorption and the subsequent cracking.



 
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