Pole Piece : The ferromagnetic portion of a magnetic circuit attached to the core used to shape and direct the magnetic field through the air gaps into the wall of the pipe being inspected. (RP 5A5)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}