Abstract: The arc-length control device for TIG welding described in this paper has a closed-loop circuit voltage control and an open loop circuit servo-motor speed control.It is simple in construction and reliable in service.The servo-motor,powered by a transistor bridge,suffers no troudle from relay contactors and therefore makes the life of the device much longer.The amplification factor of this system is over 10⁶ and the accuracy of voltage control is within ±0.114V.The static process of arc-length regulation has been analyzed,thus providing a theoretical basis for the design of apparatus of this kind.The production experience in the past two years has proved that this device is a very useful tool for TIG welding.