The principle behind FSRs is quite old, and was actually employed in the first decent microphone – invented by Alexander Graham Bell for the telephone, and is still used today in older corded telephones. These were called carbon microphones, because of the carbon granules inside of them which had a slight resistance. As a person spoke into the microphone, a diaphragm would compress the granules inside, increasing the amount of contact surface area between granules. With more contact area, there were more efficient routes for electricity to flow, and the resistance dropped accordingly.