Re: OT: They don't make 'em like they used to

Diesel Damo <Diesel_4WD@[EMAIL PROTECTED]
> wrote:
> John_H wrote:

>> They had a stack of 'em in the physics lab when I was at high school
>> (or was it uni) somewhat less than 108 years ago.  The reason they had
>> them was to demonstrate their negative temperature coefficient of
>> resistance (NTC) -- ie the resistance decreases as they warm up, which
>> is exactly the opposite of a tungsten filament.

>> It is of course the reason why the old carbon filaments didn't often
>> burn out whereas tungsten filaments do....  Perhaps you've never
>> wondered why a tungsten filament light bulb only ever blows when you
>> switch it on (in the absence of vibration or bumping)!  You should
>> also note that this one's hanging on a cord (which protects it from
>> vibration and mechanical shock).

>> Which all goes to show you can actually learn something useful (well
>> almost) in a physics class.  :)

> Or in aus.cars, as that was news to me :-)

LOL.

The tungsten filaments tend to melt very rapidly (sometimes to the point
of va****ising well enough to deposit the vapour on the inside of the glass
of the globe as they fail.  That happens due to the thermal shock of the
sudden heating that occurs when it's turned on, combined with the lower
resistance when cold (positive temperature coefficient) as John mentioned.

The best metal vapour deposition is usually found in automotive globes.

If you fitted soft start circuits (start at low voltage and ramp up slowly
every time the globe is turned on), you could get _much_ more life out of
conventional incandescent globes by avoiding the thermal stress on start
up.

I had a bathroom IR heat lamp blow recently.  That looked quite impressive
as there was some sort of coating that I think was on the filament, which
suddenly dispersed itself across the inside of the face of the globe,
making the face opaque.  When I pulled the globe out, the movement made
the coating collect at the lowest point as a small pile of powder.

-- 
Athol
<http://cust.idl.com.au/athol>
  Linux Registered User # 254000
I'm a Libran Engineer. I don't argue, I discuss.