how does a lie catching machine work?

A lie- catching machine is called polygraph. It was invented by john lorsson, a medical student at the University of California in 1921 and from 19 24 it is being used in police interrogation.

In it man seated on chair and many pipes and strings are planted on certain parts of its body observe an activities.

Then he gets asked a number of question. It is believed that when a person lies, some changes in the body occur, the change in the person's breathing, blood pressure, pulse and sweat throught polygraph is noted and later polygraph data is analysed.