Goldfish

I just finished out a four-day physics institute at Dunwoody High School near Atlanta.  I love running these institutes because I meet fun and interesting physics teachers,* and I always learn something new to bring back to my class for next year.

*Is there any other kind?

This year I have been showing off Wayne Mullins' electrostatics demonstration in water, in which I attach 20 V AC across two metal blocks in a shallow container of water.  We use a multimeter to map the voltage at different spots within the water.  The advantages of the water over the more traditional "conductive paper" for this experiment are twofold:  (1) You can feel the voltage viscerally by sticking two fingers of the same hand in the water -- the fingers tingle more with bigger voltage across them -- and (2) Tap water is much cheaper than conductive paper.

Dallas Turner, of Rockford, Illinois, noted that he had seen this very demonstration decades ago.  A college professor he knew had set up the electrodes in a slightly deeper tank than I use, and with dechlorinated water.  

Then the professor dumped in a bunch of little goldfish.

Aha!  The goldfish will align themselves along the lines of equipotential!  Just as you can spread your fingers as far as you want without a tingle as long as you spread them perpendicular to the electric field, so the goldfish can sit comfortably in the water as long as there's no potential difference between head and tail.  

In response to the obvious question from the animal lovers in our group, neither Dallas nor anyone thought that the goldfish were or could be harmed by this demonstration.  For humanitarian reasons,* when I do this demo I will be sure that the fish have a tank and caretaker** awaiting them after they perform for my class and for science.  We'll be sure to videotape the event.

*Defined as "avoiding the wrath of my wife, Burrito Girl"

**my nine-year-old son, who will name each individual goldfish