How does a polygraph machine detect lies and How is a lie detector test conducted?

Police have used polygraphs extensively since 1924.

The machines have no particular magic of second sight which enables them to probe the twisted emotions of a suspected criminal; rather, they measure blood pressure, pulse rate, and respiration simultaneously by means of a pneumograph tube around the subject’s chest and a pulse cuff around the arm.

Impulses are picked up and traced on moving graph paper, which is driven by a synchronous electric motor. The theory is that respiration, blood pressure, and pulse are involuntary actions, not subject to the person’s conscious will, yet they are bound up with the person’s emotional state. Fluctuations from the norm, generally a heightening of those actions, signify emotional tumult and, the police deduce, a lie.

Giving a lie detector test is an involved procedure that requires the expertise and sound judgment of a specially trained administrator. It would, of course, be ludicrous to ask the subject only the key question, such as “Did you murder your wife?”

The subject must also be asked a series of control questions; for example, “What is your name?” or “Did you ever steal anything in your entire life?” If he answers no to the latter, chances are he is lying, and any change in his pulse or breathing can be observed.

In some instances the subject may actually be told to lie so that the administrator can note the degree of curve in the lines appearing on the graph paper. When the key question is asked, the administrator must compare the degree of change in the line with the lines corresponding to other answers, some truthful, some not.

Because the outcome of a lie detector test is so dependent on the abilities of the test giver, polygraphs are absolutely forbidden in courts, nor can test results be used in testimony.

Psychologists are far from convinced of the validity of polygraphs, but the police, who use trained people to give the test, consider them an invaluable aid.