I'm recording a physiological signal with an electrode. What is the point of the ground wire?

Question

If the soldering connection to the ground is not perfect, I have awful noise. That much, I know. But why?

Answer 1

Ground is the voltage reference. It's that against which the positive connection is measured. This is basic electricity: you need a ground wire in order to complete the circuit, ANY circuit. In some applications you can use the earth as a return, although for signal circuit this does introduce noise.

Hope this helps.