surfaces on which the ramp surfaces 30 a 30 b bear slidingly by moving the slide 30 perpendicularly to the axis of the valve shaft 10 the axial distance between the pole surfaces of the magnet yokes 20 20 a is changed in fig2 and fig3 anchor plate 12 abuts the pole surface of magnet yoke 20 corresponding to the open position of the gas shuttle valve the lift of the gas shuttle valve according to fig2 is set at the maximum value h 1 corresponding to the maximum distance between the anchor plate 12 and the pole surface of magnet yoke 20 a at the same time return spring 24 is additionally stressed by the slide 34 and has an axial length s 1 corresponding to the distance between a spring plate 10 a on the free end of the valve stem and the opposite buttress of return spring 24 in fig3 by adjustment of the slide 30 the pole surface of magnet yoke 20 a is moved close to anchor plate 12 by the maximum possible value the lift of the gas shuttle valve is reduced to the value h 2 in the extreme case h 2 0 and the gas shuttle valve x remains closed at the same time due to the relief of the return spring 24 by synchronous adjustment movement of the slide 34 the axial length of the return spring 24 is increased to the value.