The instrument has a handling device, a gas and a high frequency current connection, an enveloping tube (8) and an electrode (5) mounted on an electrode holder (11) in the enveloping tube. The handling device consists of an axial arrangement of a handle (1) and a rotatable grip (2) with an internal thread (2a). The tube has an outer thread (3) corresponding to the inner thread of the rotary grip at the handle end. As the rotary grip is turned, the tube can move axially forwards or backwards. The electrode holder (11) is connected to the handle.
