The vector field B is a special field: It has zero divergence. For this reason we call it a divergence-free or divergenceless
field. This type of vector field is also called solenoidal.4 We will not try to explain this term at this point; the source of the
name is rooted in electromagnetic theory. We will eventually understand its meaning, but for now we simply take this as a
name for divergence-free fields.
The foregoing can also be stated mathematically by using the divergence theorem: