A linear stroke reciprocating electric machine provides both high efficiency and linearity. A cylindrical air gap is defined between outer and inner gap defining surfaces, at least one of which has a length equal to the length of an electric coil plus the length of the stroke. A flux focusing ring, having...http://www.google.fr/patents/US4542311?utm_source=gb-gplus-shareBrevet US4542311 - Long linear stroke reciprocating electric machine
