[Phys-L] Re: Kelvin Water drop experiment

[Karl Trappe <trappe@PHYSICS.UTEXAS.EDU>]

(1) Priming the pump, as Pete suggested, is the easiest way to guarantee that
an already working Kelvin water drop will actually work quickly enough to be
seen.   By placing a charge on one of the upper cans (or lower), you
preferrentially determine which charge will come off the particular dropper
above it (by induction)  You also guarantee that enough charge is there to
start the process.

(2)  Bigger cans would serve you better. Think of it as being a greater number
of gallons of coulombs you can collect... The cans provide the capacity, and
small cans have less surface on which to store charge.

(3) Good electrical contact is essential as Sharon pointed out.  However, you
don't have to remove paint, except where you connect the wires.

(4)  My guess is that your nice wood support is leaking charge like crazy!  I'd
sure consider mounting the cans on plexiglass rods, and keeping the cans from
being close enough to grounding locations to bleed off the charge.

(5)  I assume that you crossed the wire from the upper left (open can) to the
lower right (closed cans), and a second wire from the upper right (open can) to
the lower left (closed can).

(6) Next is the discharge indicator.  I used a grain of wheat neon lamp.
*However*, you still need a spark gap of about 1 mm between one lead of the
neon lamp and its connecting wire.  This allows enough q to build up in the
q=CV cans at the bottom (+ on one side and - on the other) to be able to jump
the spark gap and have enough current to fire the neon lamp indicator.  Trust
me, its much more visible to see the discharge in neon than in open air.

HOW IT WORKS GIVES INSIGHT:  A preferred charge, say -, appears on the upper
right cup due to your direct contact with a (-) charged rod. This induces an
opposite charge on the droplet forming above that can.  As is passes through
the can, it induces back the - charge on the top can, and falls into the right
bottom can leaving it charged (+).  But this can is connected by the cross wire
to the upper left open can, making it also (+).  This induces a (-) charge on
the droplet forming above it, so that upon falling through the left upper can
and landing in the left lower can, making it the same charge (-) as the upper
right can to which it is connected. Connecting the discharge indicator across
upper or lower cans provides the path for the indicator discharge to take place.

Now you have to get enough charge into the "capacitor cans" to get enough
Voltage = q/C to fire, and enough i=q/t upon discharge to be visible.  The
visibility part is not so obvious, ie, you may be discharging across a small
enough gap, but not be seeing it because there is not enough current to be
visible (like weak Corona discharge), or you maynot be discharging at all.

I hope this helps...Karl


Quoting "Dwight K. Souder" <crvhs_dks@CRESTVIEW-RICHLAND.K12.OH.US>:

> Greetings everyone.  I hope all of you are having a nice school year.
>
> I have a question about a project that one of my students is trying to do.
> She is trying to perform the Kelvin water drop experiment.  She made a nice
> wooden platform/structure to support everything.  At the top is a 1 gallon
> milk container with 2 small plastic tubes coming out of the bottom of the
> milk container.  Underneath the milk container are 2 small metal cans (about
> 1.5" diameter), with both the top and bottoms cutout.  Underneath the 2
> small cans are 2 large metal cans that catch the water.  She is using 2
> gator clip wires to connect the upper right small can with the lower left
> large can (and vice versa).  All of the cans have been sanded down to remove
> any plastic or corrosive coverings and try to make the best contact with the
> gator clips.  We have tried both distilled and tap water to run through the
> system.
>
> We have both tried everything we can to get it working, but we haven't had
> any success...not even a spark.  I did hook up a voltmeter to see if there
> were any changes and it did register a small difference, but nothing too
> great.
>
> Does anyone have any suggestions?  We've been looking online and a couple of
> books for some help, but still haven't had any success.
>
> Thank you,
> Dwight Souder
> Ashland, OH
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