Two coaxial plane coils, each of n turns of radius, a are separated by a distance a. Calculate the magnetic field on the axis at the point midway between them when a current I flows in the same sense through each coil. Electrons in a colour television tube are accelerated through a potential difference of 25kV and then deflected by 45o in the magnetic field between the two coils described above. If a is 100mm and the maximum current available for the coils is 2 A, estimate the number of turns which the coils must have.