Saturday, April 16, 2016

Worst (electric) shock of my life

I was recently reminded of a bad shock I took once, in the name of science of course. One of my high school students had really enjoyed our film can Leyden jar. He enjoyed dabbling in science on the side (who doesn't?) and made me a massive Leyden jar out of a pickle jar and a terra cotta pot. It is over a foot tall and while we presumed it was functional I was a new enough teacher that I did not want to try it out. It was covered in orange duct tape with "DANGER" written all over it. In fact, I hid it whenever there were substitutes in my room so no one would even be tempted.

Years later that same student came back to visit me, while studying physics in college, and we reminisced about his project. With another physics teacher in the room we decided to test it. We charged it up with my Van de Graaff generator and I touched it. It hurt. A lot. In fact I felt the shock all the way from my hand to my shoulder on the same arm. It was larger than I had anticipated and it hurt for awhile afterwards.

Now I regularly allow the current from my Van de Graaff generator to flow through a long aluminum rod through me to the ground while demonstrating. I try to avoid the shock from my dissectable Leyden jar demo but I'll take it if necessary. The shock from this massive Leyden jar was worse. By a lot. Now at least I can tell my students how bad the shock was and use it as a starting point to our discussion about shocks and safety.

It was my understanding that if the shock had been large enough to continue across my heart it could have been a lot worse. I would like to measure the size of the "shock" off of it, and the smaller Leyden jars I have students make but never have. I would think the large one would surpass my analog ammeter and RAFT multimeters. 

I recently posed this question to the Exploratorium's Teacher Institute Listserv to see if anyone had any ideas about how to achieve this. They agreed the equipment available to me would almost certainly be fried by my massive Leyden jar. The level of equipment necessary was apparently expensive and usually only available at universities and research facilities. Public high school: poor, fewer cool toys. Got it. 
The other take-away was that measuring the "shock" was difficult because there were many things to measure.  I show my students this table when we begin our electrostatics unit and continue to reference it through our current electricity unit. While the level of shock is often determined by current, as with this table, you can also measure the energy, power, capacitance and even the distance it jumps. The shock is incredibly short lived, however, making measuring it difficult.

I wanted to share my story as a cautionary tale and hoped to add, "If you wanted to test how bad the shock from a demonstration might be you should measure the shock before by [detailed safe procedures]." 

Since there is no easy way to do so I'll just say it's not a good idea because it really hurt.


Paul Doherty said...

An important rule : Never mix a Van de Graaff generator and a Leyden jar.....NEVER.

Dean Baird said...

Benjamin Franklin took a pretty good shock. And then wrote about it.

"I then felt what I know not how well to describe; as universal Blow thro'out my whole Body from head to foot which seem'd within as well as without; after which the first thing I took notice of was a violent quick Shaking of my body which gradually remitting, my sense as gradually return'd, and then I tho't the Bottles must be discharged but Could not conceive how, till att last I Perceived the Chain in my hand, and Recollected what I had been About to do: that part of my hand and fingers which held the Chain was left white as tho' the Blood had been Driven Out, and Remained so 8 or 10 Minutes After, feeling like Dead flesh, and I had a Numbness in my Arms and the back of my Neck, which Continued till the Next Morning but wore off. Nothing Remains now of this Shock but a Soreness in my breast Bone, which feels As if it had been Brused. I Did not fall, but Suppose I should have been Knocked Down if I had Received the Stroke in my head: the whole was Over in less than a minute."