Electrophysiology amplifier problems (electrode design help)
When you have two different metals, you are making a battery - also called galvanic cell: http://en.wikipedia.org/wiki/Galvanic_cell

If your electrodes are silver and zinc (assuming alligator clips a zinc covered), we can get the resulting voltage using "standard electrode potential" table. In your case, the amplifier gets 0.7996 - (-0.7618) = almost 1.5 volts at input. Given that it is designed for millivolts, it overloads and shuts down.