[en] The manufacture essentially consists in that a thin-walled cylindrical case, open on one side and made from flexible nonmagnetic material with zero porosity, is filled with a liquid sample. A plug, preferably of the same material as the case, is forced inside while simultaneously forcing out the excess of the sample. Permanent pressure of the inner case wall on the outer plug surface is thus created. The pressure can be exerted using either a tightening ring or a spacer element forced in the plug. The advantages lie in a long-term tight sealing of the liquid sample without air bubbles and the possibility of accurately observing the geometrical shape and dimensions of the coil former. (M.D.)