The entire threaded base (with 4 to 6 threads) is fastened to the neck of the bulb mechanically without the use of waterproof cement. Base has one copper contact terminal on the end of the base. The base has black or blue glass insulator fastened to the neck of the bulb mechanically without the use of waterproof cement.  The previous base design employed cement for fastening the neck of the bulb to the base.  However, this cement would crack under severe operating conditions (such as outdoors under extreme temperature changes – think Arizona and Alaska), quite often before the end of the bulb’s useful life span.  In 1933, General Electric then introduced the mechanical base.  Bulbs with mechanical bases were manufactured with the neck of the bulb molded with four indentations onto which an inner brass collar was snapped.  Over this collar a regular threaded base was fitted.  The result was a bulb and base that could be used in the frigid cold winters of the northeast, the hot dry summers of the southwest, or even in the extreme temperatures of freezers and ovens.  This is our current base type still in use today.  Sometime before the beginning of the Second World War, bases of brass or aluminum were introduced.  Although brass-based bulbs were more costly than the aluminum-based bubs, brass was preferred over aluminum since aluminum tended to seize up in the socket when the bulb was to be removed.


 
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