Re: Science fair help...

["Jim Lux" <jimlux-at-xxxxxxxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

So, your problem is measuring how much power is being transmitted?
This is essentially a standard "radio link" problem.

First off, remember that you've got two ends of the "link" to consider..
The transmitting end (your TC).  and the receiving end (the tube,or
whatever).  Receivers vary in their "efficiency" and in their "aperture". 

Imagine the coil spewing power everywhere, like sunlight....
Imagine your receiver as being a "window" through which some amount of the
light comes in and is detected.  Aperture compares to the "size of the
window"... small apertures collect small amounts of energy.  Efficiency
compares to how much of the power coming through the window is actually
measured or used.

So, whatever you wind up using to do the measurements, you have to separate
the two effects... (i.e. a really bad (inefficient) detector will collect a
significant amount of power if the aperture is big, and vice versa...)

Finally, another thing you have to think about is whether your detector is
"linear"... that is, if you capture twice as much energy, does the meter
read twice as much.  A fluorescent tube is an interesting case... it
doesn't do anything and stays dark until you get to a certain threshold,
then its brightness is roughly proportional to the field strength.


What you probably want to start with is a RF detector, hooked to a small
antenna, and your multimeter.  

Try this: get a small signal diode (like a 1N914), a small capacitor (like
a .01 uF ceramic disc), and a resistor around 10Kohms.  Hook the capacitor,
diode, and resistor all in parallel.  Make an antenna with a couple of
pieces of stiff wire about 6" long, and hook one wire on each end of the
diode,cap, resistor combination. (so the whole assembly is about 12" long,
with the electronics in the middle.)

Then, take a twisted pair of wires a couple feet long and hook your meter
up to measure the voltage across the cap/diode/resistor.  You'll need to
set the multimeter on the most sensitive setting (200 mV DC full scale,
probably). Mount the antenna stuff at one end of a piece of plastic pipe
and the meter at the other (using masking tape or something).  That way,
your hand won't be near the antenna when you hold the meter.

Fire up the TC and stand well back with your antenna/detector/meter
combination. If a streamer hits the antenna, your meter is toast.  As you
move the antenna closer and farther, the voltage should change.

It will also change as you change the orientation of the antenna.


Another way to make a detector is to get a cheap battery powered AM radio.
Tune it between stations, so all you hear is the "static".  Hook up your
multimeter to the earphone output (or across the speaker).  The AC voltage
(use the 200 mV or 2V AC scale) will vary as the RF power intercepted by
the radio changes.  When you fire up the coil and move around, the meter
readings will vary.  The "AGC" in the radio means that it isn't a nice
linear function of the RF power, though.


This will at least get some measurements.  Uncalibrated to be sure... You
can try things like changing the size of the antenna.  You can also measure
the power as a function of distance.


Finally.. you can get a small neon bulb (like a NE-2 or NE-2H) and hook two
wires up to it as an antenna.  It will get brighter and dimmer as you move
it closer and farther from the TC.  Longer antenna wires makes a bigger
"aperture" and should make the bulb brighter (at the same distance).  You
could try to measure the brightness of the bulb with a photocell and your
meter.


Tesla list wrote:
> 
> Original poster: "Michael Deegan" <mdeegan-at-mediaone-dot-net>
> 
> Hidily do,
> 
>     Many of you have probably seen my posts in the last few days. I'm a 8th
> grader in illinois. i decided to do something involving tesla coils for my
> science fair project. the coil is up, running fine but one small problem.
> for my
> purpose i put "How much electricity can be transmitted without wires?" in a
> rather hasty way ( i just wanted to do some coiling). but noww i need to do
> something. tesla coils can light up a flourescent lightbulb right? you
guys are
> probably older than me, and you tribal elders can maybe help me on this
one...
> What should i do to measure the amount??? my multimeter wont exactly work too
> well on that one... any ideas? suggestions???
> 
>             Michael Deegan