I was flying Martin Shupe's Spartan last night and noticed a magnet compass below the direction compass.(Probably wrong term but please bear with me,I am trying to learn).There is an adjustment knob on it but I didn't see it do anything when I moved it right or left.However the AP heading was on and the plane kept changing directions.I finally got it to straighten out by lining it up with the gps.Could someone please explain the function of the magnetic compass gauge?The panel is not the original Spartan panel.It has a radio AP.Thanks,Ron

A screen shot would help, there are no 'pilot' adjustments on a magnetic compass, only on the directional gyro to set it to match the compass .