The strip, A, is first put under a slight pressure, deflecting the needle a few degrees from zero. 弗兰克·刘易斯·戴尔.爱迪生的生平和发明.
Schweigger of Halle discovered that this deflecting force was increased when the wire was wound several times round the needle, and thus he invented the magnetising helix. 威廉·亨利·杜利特.世纪发明.