The work presented in this thesis is the result of an investigation into the closed loop velocity control of a d.c. arc, between parallel electrodes, subject to a transverse magnetic field. The velocity of the arc is in the order of 0.005 m/sec and has particular application to the processes of seam welding ,profile cutting and zone refining.

Previous workers in this field have, over the past thirty years amassed a great deal of literature on the high velocity behaviour of electric arcs in magnetic fields. This has been used together with original experiments carried out by the author, to produce a mathematical model for a low velocity low power arc.

A control system has been developed using an original arc velocity instrumentation technique the controller uses a hybrid form of both a digital computer and analogue components. By constantly monitoring the gain of the system, the controller is able to adapt itself to long-term changes in electrode surface conditions and to changes in electrode materials.

Although it is not possible to control rapid variations in arc velocity, the feasibility of closed loop controlling the average velocity of electric arcs is demonstrated by output graphs of arc position versus time. These graphs indicate that average velocity can be controlled to within 10% of the input command indicating a suitability to the applications discussed above.