The negative values mean the current flows in the opposite direction of positive current. I think most useful way to think of it is with the solenoid+magnet prac (mod 6 IQ3 i think? or IQ2), when you move the magnet in/out, fast/slow or south/north the needle can deflect positive or negative. Negative deflection just means the current is negative in terms of the wire direction — and vice versa for positive. Hope this helps, not sure if it makes much sense.
