A magnetic force acts on the arc, in this system of magnetic field, only when the angle between the direction of the electron stream and magnetic lines of force is not zero. As the arc has a conical shape and the current carrying electrons also move along the surface of the arc, their motion can be resolved in two components, one along the axis of the arc and the other perpen­dicular to it. The component along the arc does not contribute to the magnetic movement. The component perpendicular to the arc exerts a force on the arc causing the arc (molten particles of the metal in the arc) to rotate clockwise or anticlockwise depending upon the direction of magnetic field and polarity used.

The first work on the influence of the external longitudinal magnetic field was reported to have come from Erdman-Jesnitzer and associates, who worked on coated electrodes and for MIG welding of steel. They found that this field influenced the droplet formation and metal transfer. Gvozdetskii and Mechev carried out basic studies on the behaviour of MIG arc in external longitudinal magnetic field. Longitudinal magnetic field has been found by Gupta to increase weld-width, decrease depth of penetration and increase reinforcement height. The bead has been found to deflect in one side in MIG welding while no such effect was found in submerged arc welding. Alternating longitudinal magnetic field has also been found to increase weld width, decrease depth of penetration and increase reinforcement height with increase in the intensity of longitudinal magnetic field.

Regarding the mechanical properties of welds, Erdmann-Jesnitzer et al. in 1959, reported no increase or decrease in HAZ hardness due to the application of magnetic field. Gupta has also reported results which agreed with Erdmann-Jesnitzer. On the basis of Hall and Petch relation it has been postulated that tensile strength of the welds made with high current welding arcs under longitudinal magnetic field superimposition should be higher because of grain refinement.

The first report regarding the effect of external longitudinal magnetic field came from Erdmann-Jesnitzer and associates who studied the effect of such field on metal transfer and welding parameters such as arc-current arc-voltage, rate of metal deposition and arc tempera­ture etc. during welding with coated and uncoated electrodes as well as for gas shielded arc welding. In 1967 they gave a method of modifying, through the action of magnetic field, the phenomena associated with the operation of the electric arc.

The effect of longitudinal magnetic fields on the shape of the transferred metal droplets in gas-shielded-arc welding has also been reported recently.

Erdmann-Jesnitzer and associates have also the credit of introducing, for the first time in the history of welding, the concept of pulse magnetic field similar to the pulse current arc welding. The effect of magnetic field on droplet formation and metal transfer, special possibility of arc control and basic principles of Lorentz force have been considered by them. To study the droplet transfer phenomena during welding Erdmann-Jesnitzer and associates used various methods and Rehfeldt in 1966 developed a wonderful device the “Analyser Hannover” for this purpose.