In this GSR-Monkey (Skin Conductance Measurement System) Martin Sona uses the principle of galvanic skin response to plot some data onto a grid. Two people can be connected and monitored at the same time. I can imagine using this system as a two way lie detector, how often do you get to turn the tables when performing a lie detector test?
“The hardware (besides the PC itself) consists of four parts: the electrodes, two little amplifiers, a delphi/C++/etc programmable USB-Interface and last but not least a black and more or less evil looking monkey. After experimenting a bit with other materials I decided to make my own silver/silverchloride (Ag/AgCl) electrodes. Nonchlorided electrodes on the skin can polarize after a while which means that they themselves work a bit like a battery and emit current.”