**leaves the electric field invariant. The transformations and are examples of what mathematicians call gauge transformations. The choice of a particular function or a particular constant c is referred to as a choice of the gauge. We are free to fix the gauge to be whatever we like. The most sensible choice is the one which makes our equations as simple as possible. The usual gauge for the scalar potential is such that at infinity. The usual gauge for **

**is such that .**