Biofeedback
Measure pulse, blood pressure, respiration rate, brain waves, etc.
Plethysmograph
Record changes in the volume of a part of the body arising from variations in blood supply
Penile Plethysmograph
Record changes in penis blood flow
Polygraph
Lie detective
Unobtrusive measure
Telling a physical trace or record
Ex. amount of whiskey in town measure by counting the number of empty bottles in trash cans