"Want me hit you with a bolt of lightening?"

 

"Want me to hit you with a bolt of lightening?"

Think about that question for a second what would your reaction be?  I was asked just this question a number of years ago as I sat inside a Van der Graff generator with a few of my coworkers.  We all looked back and forth at each other in nervous silence for a minute before someone offered up a feeble sounding "sure."

The Museum of Science's Van der Graff Generator (Photo from the Museum of Science Boston)

I should probably explain how we all got to be sitting in the top of the generator in the first place.  One afternoon at lunch the man in charge of the lightening shows at the Museum of Science, I'll call him Len,  mentioned to us he was going to be cleaning out the insides of the generators and if any of us were curious what they looked like on the inside to meet him there tomorrow morning.  The next morning my boss and I along with some other curious coworkers gathered at the agreed on time in the lightening theater.  Len popped open the door at the bottom and instructed us to climb up the ladder to the ball at the top so we could see it innards.  One by one we climbed up the ladder next to a very long conveyer belt which had little brushes over it at the very top.  Once Len had joined us in the big ball at the top he proceeded to explain the generator worked like a big static electricity producer. The faster the conveyer belt ran the more static was produced by the brushes and the extra charge would build up on the outside of  the metal before discharging to create the lightening bolts we saw in his demonstrations.  After a few more minutes of questions from us Len then looks at us with a glint in his eye and asks "want me to hit you with a bolt of lightning?" After agreeing he slips back down the ladder and closes the door at the bottom.  We all let out a nervous giggle as the conveyer belt began to build up its static charge.  "Should this be open?" I shouted down to Len who now sat inside the Faraday cage at the controls.  "You're fine just keep your heads inside" he reassured us.

After a few tense minutes there was an audible crack and we all looked out the hole as the lightening hit the cage Len sat inside.  Cool we all thought as the belt began to spin faster now. After a few minutes of watching rather small lightening bolts hit the cage Len called up "Close the window." We did as instructed an sat completely enclosed in our metal sphere and waited.  Then there was another loud crack and a sound like someone throwing a tennis ball against the side of the metal.  On and on this went for a few minutes the belt spinning faster and faster, the cracks and bangs against the metal getting louder and louder.  When the banging reached deafening proportions we pounded on the sides of the metal sphere, our agreed on signal for Len to stop.  We waited for the belt to stop moving and the door at the bottom to be opened before descending back down the ladder.

"So?" Len asked after we had all safely made it out of the generator.  "Cool!" was our response. "How many volts was that last strike?" someone asked. "Enough to blow up a large redwood." Len casually responded.   

If your trying to figure out how we were all not blasted in to millions of little pieces the answer is simple and complex at the same time; we put our faith in the skin effect. Since I never took physics I can't really explain how this works, but because of the frequency of a bolt of lightening the charge was spread over the surface of the copper metal in which we sat and never penetrated the metal sphere. 

The same idea works if your sitting inside your car in a lightening storm as long as you DON'T touch anything inside the car, just sit there with your feet flat on the floor and hands in your lap. See this in action on BBC's Top Gear.

To see the Museum of Science's Van der Graff Generator in action check out this YouTube Video. https://www.youtube.com/watch?v=1-mFl_YXqiA  But it is MUCH cooler in person!

For More information:

For a great explanation on how the generator works and why Van der Graff created them in the first place see the HowStuffWorks website.
For a really in-depth explination of how the skin effect works. http://fermi.la.asu.edu/w9cf/skin/skin.html
More information on cars and lightening http://www.lightningsafety.com/nlsi_pls/vehicle_strike.html
How to be safe outside in a lightening storm. http://www.ready.gov/thunderstorms-lightning

For a brief glimpse inside the generator I sat in watch Since Bob's video. http://www.sciencebob.com/experiments/videos/video-van_de_graaff.php