You hear about polygraph or lie detector but you think that how a machine knows about a person who speaks truth or lying? In our daily routine, we speak more and more lie intentionally or non intentionally but it don't effect.
When a statement occur that so many thing depend on it that time it effected , like Court hearing, victims hearing etc. that time very important that person is speaking truth or lie.
If you are an expert, it may be that you can understand from his gesture that he speaks the truth or speaks lies and this is not a perfect and it is very difficult. But a machine checks that time so it is possible that the accuracy level is high.
In 1921, John Larson invented polygraph. He was a medical student at the University of California. After 1921, we are continuously daily improve the polygraph test. But this is a questionable thing. Some places is not allows this test and some places allows.
In India, this test is illegal. You can't do this test forcefully but a person want to do this test so he/she can. This test is not strong evidence in court.
How Polygraph works ?
During polygraph test, four or six sensors are attached with human body. A lie detector is machine in which multiple poly signals from the sensors are recorded on a single strip on moving paper graph.
The polygraph records following parameters...
- Blood Pressure
- Breathing Rate
In a polygraph test, First ask three or four easy question and then asked high accuracy question that effect the human body parameters.
There are so many relevant and irrelevant question to knowing about your parameter in the test.
After Polygraph test, the examiner check the graph and generally it changed such as higher blood pressure and heart rate etc.
So many people tell this is accurate and so many people tells it is not accurate and you can bypass this test. A well-trained person can bypass this test controlling his parameters.