Usually we see lie detectors and polygraph machines in movies and spy shows, but some of the more popular uses of this gadget are rather simple. Some companies use it to question the people they wish to hire.

When a person is taking this test, they are attached to four to six sensors in which the signals are taken and recorded on a graph for the interpreter to see. So how does the polygraph work?

Functions and Parts:

These sensors usually record things like the person’s pulse, blood pressure, perspiration and breathing. The device has four sensors that fix around the finger tips and they measure the pulse. Two sensors are attached to the chest to record the heart rate and monitor the breathing pattern of the person taking the test.

A blood pressure cuff is placed over the arm, much like an actual blood pressure machine and it monitors the BP of the person sitting to take the test. Two rubber tubes are placed across the chest to monitor the breathing pattern and two metal plates are latched onto the fingers to see if the person is sweating or not.

The test starts with a few simple questions so that the examiner can record the normal heart rate, breathing pattern and blood pressure of the person, then compares it to when they assume the person is lying.

What it does:

A computer is connected to the polygraph machine and it records all the information given to it, displaying erratic breathing, rising blood pressure and sweating.

When someone lies, their breathing becomes erratic and their heart rate increases because they are nervous; the machine simply records this.

The machine cannot detect a lie per se, but it can tell the examiner about the differences in blood pressure and sometimes even record movements of the hands and feet. The machine records these readings and shows them to the examiner.  It is the examiner’s job to assume when and if the person is lying or not.

During and after the exam, the examiner can see the recordings and note the changes in the heart rate and pulse, which normally indicates that the person is lying about something.

A trained examiner can easily tell when a person is lying by looking at these readings but it is not a completely accurate result because each person reacts differently to lying. Some are very confident while others get extremely nervous. In each instance, it is the examiner’s job, not the machines, to find out if the person is lying or just plain nervous.

When the test is taken, the examiner and an interpreter often encourage the person taking the exam to be calm and easy. The polygraph is applied to them only when they are calmed down because if a person takes this test when they are nervous, it confuses the machine and affects the results immensely.

However, the results cannot be a hundred percent accurate because it is not the machine that tells if a person is lying; it all depends on the interrogating examiner, not the device.