Investigate Lightning Bolts

This experiment allows you to capture the discharge of a tesla coil as it travels through iron filings. This process resembles lightning strikes through the atmosphere.

Materials

6 inch square plexiglas sheet	pie tin with ~5 inch diameter hole in the center
glue stick	iron filings and/or small bb's
1 wire w/ alligator clips	tesla coil
2 large binder clips	computer
Camera, Apple Video Player and Movie Player software or some other means of capturing images	Fractal Analyzer software from the Center for PolymerStudies

Setup and Procedure

1. Using a glue stick, coat a Plexiglas plate with a thin even coat of glue. Attach the pie plate with a hole to the Plexiglas so that it is centered. Sprinkle a thin even coat of iron filings on the Plexiglas in the center of the pie plate. The pie plate will be grounded and is analogous to the earth. The iron filings are the particles in the atmosphere.

2. Ground the pie plate by clipping one end of a wire to the plate and the other to the faucet on the lab table or a metal chair. Use the two large binder clips to make a platform for the apparatus so that it does not sit directly on the table. This will prevent some of your discharge from flowing through the table.

3. You are now ready to start making lightning! Plug in the tesla coil. The tesla coil will create a high electric potential. The electric charges will flow to a lower electric potential (the ground) throughout the iron filings and the pie plate. To start the tesla coil, slowly turn the knob at its base into the tesla coil. You should here a 'whirring' noise. Do not turn the knob all the way in, a small amount will suffice.

4. Place the tip of the tesla coil at the center of the disk of filings. Observe the patterns created by the discharge.

Capturing an image

1. On the computer, open Apple Video Player (it is on the desktop). Position the camera so it sees only the Plexiglas plate and then focus the camera. On the computer choose Windows, Show Control Window. Choose the top button - it looks like a video camera.

2. Turn out all of the lights and shield the apparatus from the light of the computer monitor. Use the tesla coil to produce charges on the plate. When you are ready click Record on the computer. After about 1 minute or less click stop.

3. Save your movie to the desktop of the computer. Open Movie Player. Choose File, Open and your movie.

4. Slide the button at the bottom of your movie until you reach a suitable image to analyze. Choose Edit, Copy.

5. Open Graphic Converter. Choose File, New and OK. Choose Edit, Paste. Your image should appear on the page. Use the same process as last week to invert your picture and trim off excess stuff. Be sure to save the image as a PICT file.

What does it mean?

You may wish to save several images. Analyze them using Fractal Dimension. How are the images different? How are they similar? Does the dimension of the images agree with your observations?

Other Questions to Investigate

• How does the density of the iron filings affect the 'lightning bolts'?
• Try copper or zinc filings. How are they different from iron?
• Try small bb's. How are they different from the filings?
• Is there a maximum distance between bb's for producing a 'lightning bolt'? Look up percolation theory.

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