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rrr@hpdmd48.boi.hp.com (Rudi Rynders) writes:

> Can someone versed in physics please explain this phenomenon?
> Given an incandescent lamp with a dimmer switch. The lamp is on.
> One can then slowly turn the dimmer knob and reduce the light
> intensity until the filament is just barely glowing.
> If , however the lamp is off, and one slowly turns the dimmer
> switch up, no matter how slow, the light always comes on rather
> suddenly and much brighter than in the reverse process .
> Why is that so ?
> Thanks in advance!

> Rudi (Not a physicist-just an ME) Rynders
> (rrr@hpdmd48.boi.hp.com)

The resistance of the filament is very much dependent on its
temperature.  When the filament is cool the resistance is low
so that at a given setting of the dimmer most of the energy is
disapated in the dimmer and not the filament.  There is insufficient
energy being dissipated in the filament to raise its temperature
significantly.

In order to get it to glow one has to raise the current more and as
soon as it begins to glow, (temperature up) the resistance rises and
it absorbs a larger fraction of the energy so it would heat up faster.
This would happen quite suddenly and it would come on fairly brightly.

However if the filament is hot to start with (viz. approach the
same dimmer setting from the hot side) its resistance is high and
now for the same setting of the dimmer a greater fraction of the
energy is dissipated in the filament - enough to keep it hot.  

	Jim