A pair of identical coils, each having a radius of 30 cm, are separated by a distance equal to their radii, that is, 30 cm. Called Helmholtz coils, they are coaxial and carry equal currents in directions such that their axial fields are in the same direction. A feature of Helmholtz coils is that the resultant magnetic field in the region between the coils is very uniform. Assume the current in each is 15 A and there are 250 turns for each coil. Using a spreadsheet program, calculate and graph the magnetic field as a function of z, the distance from the center of the coils along the common axis, for -30 cm < z < +30 cm. Over what range of z does the field vary by less than 20%?