tube with a circular cross section and a stop device in the form of a counter support bolt 63 of socket head cap screw type can be screwed from below through a threaded hole 64 in the counter support the counter support bolt can be locked in its position with a nut 67 a damping device 65 in the form of a plane washer of lead is arranged at the upper end of the counter support bolt in the figure the capsule tube 4 with its internal envelope surface machined to correspond to the shape of a frustum of a cone is placed in a chamfered portion 66 at the upper end of the counter support at that part of the counter support which in the figure is shown as the upper part facing the capsule tube the counter support is formed with three slots 68 of which only one is shown in fig2 a such that part of the counter support which makes contact with the capsule tube will exhibit three fingers 69 which grip the capsule tube by a suitable dimensioning of the fingers the advantages are obtained that the counter support by the fingers being resilient in the radial direction allows the capsule tube upon heating to expand in the radial direction and that the heat quantities which by conduction can flow between the capsule tube and the counter support when the former is heated and cooled are minimized the capsule tube is oriented such that that end which during machining into conical shape has been given the largest diameter is placed upwards thereafter a guide tube 7 with circular cross section is placed.