1. A process for controlling an arc welding process comprising the steps of: beginning a weld by providing electrical current to form an arc between a welding electrode and a base metal to thereby form a weld pool of liquid metal on said base metal; measuring the natural frequency of oscillation of said weld pool; determining the position of a first edge of the pool by analyzing light reflected from the surface of said weld pool and the adjacent base metal in a direction parallel to but non-coincident with the axis of the welding electrode for a first local light intensity minimum; determining the position of a second edge of said pool by a similar method on the opposite side of said pool; determining the distance between said first and second edges of said pool to determine a width; adjusting the electrical current of said arc welding process in response to said frequency and width to maintain a constant pool mass and a constant width between said first and second pool edges.