hello, here is my second message.(and my English is still very bad) I don't understand these 3 little experiments(we need Helmholtz coils and a Hall Effect probe) I know I have to draw a graph for each situation but I can't interpret the difference between the theory and the experiment. 1)Measure the magnetic field in the middle of a circular coil crossed by a current I(I max=3 A) 2)Draw a graph to explain the intensity of the magnetic field made by the two cicular coils (radius=a),on the axis of this assembly and for different distances L between the coils.Try to find experimentaly the Helmholtz condition(2 b=a) when the field has a maximal homogenity. 3)Using some more little currents (2.A),measure the magnetic field at the end of a solenoid (square coil 500 spires) with an iron bar (the framework of the processing).How is the magnetic changed -with or without the iron "nucleus" -if we take the half-number of spires I have sent 3 images concerning the 2nd question: 1°:2b=4 2°:2 b=8 3°:2b=16 Can you tell me how to comment on those graphs because I must find the Helmholtz curve. For the 3 experiments,what is the theorical aspect?(cause I obtain some strange graphs...).I don't know if I have to use some formula's too. Thank you for your help!!