located in a metallic casing in most cases the casing and the bulb 5 are identical the casing is provided with an opening zone which has an opening 8 a closing member 9 is arranged in the opening 8 and held in the opening 8 by a press fit the closing member 9 is in form of a ball or sphere the opening 8 is of cylindrical form i e it has a constant diameter over its length the diameter is slightly smaller than the diameter of the closing member 9 so that the closing member 9 can be pressed into the opening 8 and seals the opening 8 in the absence of other forces the ball has been given a thin layer of tin to ensure tightness furthermore the bulb comprises a connection geometry 10 in form of a hexagon 11 at its circumferential surface to allow a tool to engage the connection geometry the connection geometry furthermore has an inner thread 12 and a smooth section 13 between the inner thread 12 and the opening 8 the section 13 can have a smaller diameter than the inner thread 12 therefore a step 14 is shown between the inner thread 12 and the section 13 the opening 8 is located within the connection geometry 10 the part of the bulb 5 surrounded by the connection geometry 10 is briefly termed u201c opening zone u201d since it is adapted to be opened as will be described below in the form shown in fig2 and 3 the bulb 5 is produced in.