Human Battery

In my electrochemistry lectures I point out that a person's fingers can be a salt bridge in an electrochemical cell.  In an energy exhibit at the St. Louis Science Center yesterday I saw a great demonstration of a person being an integral part of a battery.  There were two pairs of metal plates (each pair had a copper and a zinc plate connected by a wire) connected to a sensitive voltmeter.  When my hands were  placed on plates of the same metals (Cu and Cu or Zn and Zn), no electrochemical potential was established and the voltmeter read zero.  When my hands were placed on a Cu and a Zn plate that were electrically connected away from the voltmeter, the voltmeter read zero.  When my hands were placed on a Cu and a Zn plate that were electrically connected through the voltmeter, the voltmeter needle deflected as oxidation took place at the Zn plate and reduction took place at the Cu plate.  The direction of the needle deflection depended on which specific plates were touched. The magnitude of the deflection seemed to vary with the quality of the contact made with the plate (illustrating the significance of resistance in the cell).  I was reminded of a "potato clock" only this time it was a couch potato involved in the battery!  Another nice exhibit located nearby had a rod that one could gently displace to demonstrate the magnitude of a joule of energy.