"harrogate3"

** Only a BADLY worn or damaged lamp filament fails at switch on.



Er, excuse me. How does the filament of a bulb wear?



** Metal continually evaporates from the surface of the hot tungsten wire
and re-deposits on nearby areas - but not evenly.

Near the end of the filament's life, the wire has a varying cross section
and hence varying resistance along its length.

The thinnest areas get hottest ( I squared R ) and melt, causing lamp
failure.



........ Phil