The base of this bulb has a copper screw shell with a wide skirt above its 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.)   Skirted threaded bases, introduced about 1894, were used for higher wattage bulbs (which employed longer filaments) to insulate the base from the excessive heat generated by the high wattage.  Early high wattage lamps tend to have looped filaments or multiple tungsten filaments or tantalum filaments (mounted in the squirrel cage configuration) which took up much space and hence required a bulb having a wider than average neck. See such examples of filament configurations: [see A116] (filament with anchored loop), [see A117] (two filaments connected in series), and [see A119] (squirrel cage filament configuration).



 
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