Dear  Sigrid Arnott & List members,
 
The most significant datable traits of electrical lightbulbs are: the
location of vacuum vent, the overall shape of glass body, the type of base,
the material and shape of the filament., and sometimes, the mark.
 
For mid-twentieth types, the best source are trade catalogs from electrical
companies such as General Electric, Westinghouse, Amalgamated, Philips, to
name a few. Frequently reedited and always dated, these catalogs give an
approximation as to when new types were introduced on the market and when
they were withdrawn. They are also relatively easy to find in public
libraries and archives. I hope this helps.
 
Gerard Gusset
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