In my text it is given that the magnetic susceptibility X = I/H but in another it was dI/dH. But second can be used to draw the hysterisis graph so I think the second must be correct. X is dependent on magnetic field applied . As we increase the field the value of I increases until 100 percent or close to that alignment takes place. But after that, as magnetic field increases, the value of I decreases, therefore X becomes negative; why does this happen? ( My guess is that every material has a diamagnetic behaviour which was negligible until this point). And please also tell me: how do we measure the magnetic field inside magnet.