In article ,
"Christian McArdle" writes:
Theft reduction I think - the bulbs on the old underground train were some
weird voltage for that reason as well - something like 80 volts

I seem to recall that 80V is near to the optimum voltage for an incandescent
bulb. It is something about the thickness of the filament being just right
for the most efficient operation.

55V is normally held to be the optimum for 100W lamp.
However, it doesn't drop away quickly as you move away
from 55V. There have been 84V lamps made which are
designed to run on half-wave rectified 120V US mains,
with the aim of getting higher efficiency than 120V
lamps. Unfortunately, designing lamps to run off 240V
is about as bad as it gets in this respect (on the
basis that no one uses even higher voltage filament
lamps).

--
Andrew Gabriel