Stack height control apparatus for a sheet feeder

Claim 1

A VARIABLE AMPLITUDE VOLTAGE GENERATING DEVICE FOR MEASURING THE DIMINUTION OF A STACK OF DOCUMENTS BEING INDIVIDUALLY FED BY A DOCUMENT FEEDER, SAID DEVICE COMPRISING A ROTATABLE CYLINDER OF MAGNETICALLY IMPERMEABLE MATERIAL, A PLURALITY OF MAGNETICALLY PERMEABLE ELEMENTS CARRIED BY AND SPACED APART CIRCUMFERENTIALLY OF SAID CYLINDER IN MAGNETICALLY SEPARATED RELATION FROM EACH OTHER, SAID ELEMENTS BEING MOUNTED AT A COMMON INNER RADIUS AND TERMINATING AT THE OUTER CYLINDRICAL SURFACE OF SAID CYLINDER, A SENSING MEMBER, MECHANICALLY DISPLACEABLE BY THE STACK OF DOCUMENTS, A PERMANENT BAR MAGNET MOUNTED ON SAID SENSING MEMBER FOR MOVEMENT THEREWITH IN A DIRECTION INTERCEPTING THE PATH OF ROTATION OF SAID CYLINDER WITH ONE END OF THE MAGNET SPACED FROM AND FACING THE CYLINDRICAL SURFACE OF THE CYLINDER, AND AN INDUCTION COIL SURROUNDING SAID BAR MAGNET TO INDUCE PULSE OF INCREASING VOLTAGE AMPLITUDE THEREIN AS THE BAR MAGNET APPROACHES THE CYLINDER.