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The principle of controlling a high-current vacuum arc by radial magnetic fields (RMF) forcing the constricted arc to move has been utilized for long time in the design of vacuum interrupters. Detailed electrical and optical measurements in conjunction with finite-element method (FEM)-calculations have provided a better physical understanding of the function of RMF contacts. By balancing the processes of surface heating and momentum gain in the moving arc column, an expression for the speed of the arc and arc voltage is obtained. The speed varies as the 5/6 power of the short-circuit current. This result is then used to describe the number of rotations of the arc on the contact and to explain the linear scaling law of contact diameter with current. The investigations are mainly concentrated on spiral-type contact designs.