Hello everybody and thanks so much for your help, (I know I have to improve my English) here is my big problem concerning 3 formula's : (It's just the beginning) -first at all,we have a magnetic field B created by the circulation of a current I into a spire (with a spoke a) on a point at a distance b of the plane of the spire on his axis -->B(b) = µo I a²/ 2(a²+b²)^3/2 -then,we have an other magnetic field with 2 identical coils separated with a distance 2b just to be in the same axis and to have the same current I circulating in the same direction. -->B(x)=B1(x) + B2(x) = µo I a²/2 ((1/(a²+b+x)²)^3/2) + (1/(a²+(b-x)²)3^/2)). -It's possible to give a Taylor's development of this expression,so we obtain the Helmholtz condition (I don't understand that very well ) and the magnetic field is given by:BH=8µ0NI/5 (square root 5)a. Now I have to find these 3 expressions: aid of the Biot-Savart law,how could you obtain the expression of the magnetic field created by a current I circulating into a spire on a point of the normal axis in the plane of the spire and passing through his centre. >>From this expression,how could you deduce the magnetic field on any point of this axis of an Helmholtz assembly.You have to give the taylor's development in the powers of x around the centre of the assembly and deduce the Helmholtz condition 2b=a which verifyes an homogeneous magnetic field in the assembly.Which is the value of the magnetic field?At which distance of the centre,on the axis of the assembly,is the field different of 1% from his value at the centre? (*concerning the assembly,we use a Hall Effect probe too) Please help me because I don't understand anything.I am desperate .What is the Helmholtz condition?Why is that so important?Could you please help me for the questions (principally from>>>) Thank you very much!!!