🚀 go-pugleaf

acampane@magnus.acs.ohio-state.edu (Angelo Campanella) writes:
>
>4/ "Halogen: is usually iodine or the like which has heavy molecules that will
>   discourage the sputtering of the filament material away (sputtering
>   depletes the filament and otherwise shortens its life).  Deterioration due
>   to sputtering is evident by darkening of bulb interior.

Actually, the tungsten does evaporate and deposit on the bulb.  But the
chemicals in the gas fill and the high temperature of the bulb cause
the tungsten to be cleaned off the bulb and redeposited on the filament.

Thus, the bulb doesn't get dark as the lamp ages.  The filament doesn't
get gradually thinner, but it does change as tungsten evaporates from
one place and is redeposited elsewhere.  The lamp does eventually fail.

>5/ Quartz is usually used for the envelope because it expands very little when
>   hot.  However, quartz is also very brittle hot or cold, so be careful.

The lamp envelope has to run very hot for the necessary chemical reactions
to take place.  So it has to be small and close to the filament, and has
to withstand high temperatures.  Thus quartz is used.