Distance between the coils can be decreased/increased by tapping on left/right halves of lower half portion.
Two coils of equal radius are mounted coaxially. They carry currents in the same sense. Magnetic field due to these on their common axis is shown as a function of position on the axis. Blue curve shows the variation of magnetic due to the left coil and the red curve is due to the coil on the right. Black curve is the sum of the two magnetic fields. It can be noticed that when distance between the centers of the coils (d) equals the radius (R) of a coil, the peak of the black curve flattens. The magnetic field on the axis remains nearly uniform between the coils.