"Phil Allison" wrote in message
...

"Graham."


An incandescent light bulb is a good example, If it lasts 1000 hours
when run continuously, its life will be considerably shorter if run
(say) 4 hours a day and the time it is on added up.


** Where ever did you get that nonsense from ??

Some web forum ?

I believe turning the bulbs on and off can induce thermal shock which causes
premature failure.

http://en.wikipedia.org/wiki/Thermal_shock

See also: http://en.wikipedia.org/wiki/Thermionic_valves

"The common wisdom was that valves—which, like light bulbs, contained a hot
glowing filament—could never be used satisfactorily in large numbers, for
they were unreliable, and in a large installation too many would fail in too
short a time".[13] Tommy Flowers, who later designed Colossus, "discovered
that, so long as valves were switched on and left on, they could operate
reliably for very long periods, especially if their 'heaters' were run on a
reduced current".