Skin Conductance Response (SCR) is also called Galvanic Skin Response (GSR). The SCR increases whenever there's excitation or emotional response, and the stronger the response the stronger the SCR signal. Being surprised, strong emotions, strong pain and laughing heartily can readily produce strong SCR signals. The SCR signal consistently rises within 1~3 seconds, and relaxed out over longer periods (4~10 seconds). SCR is part of the polygraph test and is commonly used in market research to determine the true emotional response level of the subject. This is very simple measurement: just apply a small DC bias, and measure the voltage across a resistor (to calculate the current level). The key is to keep steady and consistent good contacts with the skin, so it's not easily manipulated by the subject through muscle activity or changes in contact pressure.
Some reference links: Affectiva http://www.affectiva.com/q-sensor/resources/faqs/what-eda-tells/
http://www.scienceofrunning.com/2012/03/measuring-galvanic-skin-response...
http://en.wikipedia.org/wiki/Skin_conductance http://en.wikipedia.org/wiki/E-meter (Scientology Church)