On Sun, 19 Aug 2007 04:07:06 -0400, mm
wrote:

On Sat, 18 Aug 2007 10:40:33 -0700, wrote:

I have understood that switching fluorescent lamps - tubes - on and
off was not a good idea and that they should be switched on and left
on. Unlike filament lamps which do not seem to mind.

Why do you say they don't mind? Haven't you noticed that they almost
always burn out at the moment they are turned on?

That's when they heat up, the filament expands (maybe faster than when
it contracts on turning it off) and stresses on the filament are
greatest.

If you leave a filament bulb on, it will generally last much longer
than if you turn it on and off.


During the time it is energised the filament loses metal and gets
thinner, not necessarily uniformly along it's length. Eg. It is
cooled where it is supported and less metal is lost at these points.

Where it is thinnest is also where it is mechanically weak and it's
electrical resistance is greatest. At switch on the whole filament has
a low resistance and takes a big surge of current. The thin weak
points are heated up disproportionately to the rest of the filament
and eventually the filament will fail at one of these points.

I don't think that routine switching on and off shortens the life of
the lamp that much, but rather that a lamp at the end of it's life
which has become frail will tend to fail when it's switched on.

DG