Just as in gravitational fields, you can draw equipotentials on a field diagram. They are points in the field of equal potential. In all cases, the equipotentials are at right angles to the field lines. The separation of the equipotentials tells you about the field. Evenly spaced equipotentials - it's a uniform field. Increasing separation - a weakening field. In fact, the rate of change of spacing between equipotentials is called the ''potential gradient'' and it can be shown that Field strength, E = potential gradient or for the mathematicians amongst you.