"a pole piece of ferromagnetic material extending through said coil assembly and having one end disposed adjacent a string of said instrument and the other end extending towards a pole face of said magnetic element, and non-magnetic means adjacent said other end of said pole piece separating said pole piece from said pole face of said magnet element by a predetermined distance and introducing a gap in the magnetic path between the magnet and said pole piece, said distance being selected to establish a magnetic field of desired intensity in the vicinity of said string"