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Re: Light Bulb Experiment (ala Brent Turner)



Tesla List wrote:
> 
> > > Subject: Light Bulb Experiment (ala Brent Turner)
> > Subject: Re: Light Bulb Experiment (ala Brent Turner)
> 
> >From bert.hickman-at-aquila-dot-comThu Sep 19 22:25:27 1996
> Date: Wed, 18 Sep 1996 23:55:40 -0700

> Tesla List wrote:

<<SNIP>>

> 
> Thanks Brent!
> 
> I thought tungsten had a very positive temperature coefficient, so that
> the resistance went up when the filament got hot. If you retune for the
> larger overall loading capacitance, do you still tune for maximum
> voltage output at the new setting or for something else??
> 
> -- Bert --


In all honesty, we happened to get lucky in that 2 extra turns on the
primary was enough to bring the system close into tune. Not perfectly
spot-on, but close enough that the experiment works. I am sure that
we are running off-tune a bit, but bear in mind that the purpose is
for demonstration, and the idea of having to fiddle-fart with the
thing each time we set up for a show is a real pain in the butt.

Obviously, we attempted to tune for maximum spark length, as indicates
that we are at 1/4-wave resonance with the system. What I was referring
to was Richard's comments regarding running (and tuning) a coil so that
with a humongeous toroid on it, you get no spark at all, hence you wind
up with one hellatious radiated energy level.

I *remember* that incandescent lamps have negative-resistance
characteristics, but I could be wrong. I do recall that the negative
resistance change is one reason why a lamp is used in a Wein-bridge
oscillator though.

You could attempt to measure this by placing a small thermally stable
resistance in series with an ordinary lamp and plug the whole shebang
into a variac. Plot the voltage drop across the resistor as a function
of input voltage and actual lamp intensity. I think you will find that
there is actually a 'knee' in the curve where the filament does it's
resistance-drop trick.

- Brent