This animation shows the magnetic field of a Helmholtz
coil when the current in the top and bottom coils flows
in the same direction (ie. their dipole moments are
aligned). The fields from the two coils add up to create
a net field that is constant in the center of the coils.
Though the coils are fixed, they are attracted to one
another, resulting in a tension in the field between
them. This is illustrated by fieldlines as they stretch
to enclose both coils.