The galvanic skin response (GSR) can be used for capturing the autonomic nerve response as a parameter of the sweat gland function. Due to relatively simplicity of measurement, and a quite good repeatability GSR can be considered to be useful and simple method for examining autonomic nervous system function, specifically the peripheral sympathetic system.
A sample GSR signal of 60 seconds (Source: wikipedia).
The simple psycho-galvanometer was one of the earliest tools of psychological research. A psycho-galvanometer measures the resistance of the skin to the passage of a very small electric current. It has been known for decades that the magnitude of this electrical resistance is affected, not only by the subject’s general mood, but also by immediate emotional reactions. Although these facts have been known for over a hundred years and the first paper to be presented on the subject of the psycho-galvanometer was written by Tarchinoff in 1890, it has only been within the last 25 years that the underlying causes of this change in skin resistance have been discovered.
I have found two guys that have developed their own GSR circuits that utilize Arduino for measuring their skin conductance.
Chris from Chris300 has designed a very simple schematic and also provises both Arduino and Processing codes here.
It uses 3 Op Amps. One electrode gets 1.5v (from an external battery) and the other electrode has a 1M resistor and a 1M pot going into ground.