The lie detector is an assembly of three different instruments. Their outputs are fed separately to the lie detector and recorded as separate traces on a graph.
One instrument, the pneumogram, records breathing patterns. A rubber tube is strapped across the chest, and instruments measure fluctuations in the volume of air inside the tube, which are brought about by variations in breathing.
The second instrument, the cardiosphygnometer, detects variations in the blood pressure and pulse rate. The information is picked up by a bladder and cuff placed over the upper arm, in the way that doctors check blood pressure.
The third instrument is the galvanometer, which monitors the flow of a tiny electric current through the skin. The skin conducts electricity better when it is moist with perspiration. Electrodes are usually taped to the hand.
Picture Credit : Google