By lightly touching the pencil lead to spots of blue on the blade, or to spots of rust, they formed what is called a point contact diode and the rectified signal could be heard on headphones or crystal ear pieces.
By lightly touching the pencil lead to spots of blue on the blade, or to spots of rust, they found a small area on the blade that had the ability to rectify an alternating voltage. Such a contact is called a point contact diode and the rectified signal could be heard on headphones or crystal ear pieces.