I did an experiment with Indium antimonide (InSb) in Liquid Nitrogen. I turned the sample one way in the B field and took a reading of the voltage (positive) and turned it 180 degrees and took a reading of voltage (positive, and a greater value). I use V_h = v1 - v2 / 2 and I get negative values for the Hall voltage and a negative gradient when I plot hall voltage against current. i get a negative value for the Hall coefficient. But my tutor says the charge carriers are holes. Can someone explain this to me?