So I understand what the B field is in magnetism. This is the fundamental field. We can also define a H field, H = B/u - M, to take into account that we cannot actually measure magnetization currents (cf. solenoid). Now, if we have a B field applied to a material, call it Ba, the field inside is given by B = u(Hi + M), where Hi is the internal H field, defined as Hi = Ha - NM. Its this last part that confuses me. What is this demagnetizing field, NM???