3 3 39 of the tube and sealed therewith opposite ends of the housing 4 are sealed by closure plates 5 5 39 on which are mounted two positive poles 7 7 39 the negative battery terminal is the steel housing fig5 illustrates another embodiment of the invention in which the capillary tubes are close packed into a cylindrical bundle 1 in the center of a cylindrical housing 4 the quantity of sodium which is displaced in a full discharge of the battery is located in the annular gap 14 between the capillary bundle and the housing the capillary force within the fiber bundle is greater than that in the annular gap so that even when the battery is discharged the sodium remains in contact with the full length of the fibers while the level of sodium within the annular outer volume drops fig6 illustrates an embodiment in which the capillary tubes 1 are arranged in an annular gap 14 defined between the interior wall of the cylindrical housing 4 and the aluminum housing 16 of a cylindrical sodium reservoir 15 the bottom plate of the sodium reservoir has a narrowly dimensioned hole 17 so as to limit the flow of sodium in the case of damage to the housing and it is raised above the base of the housing 4 by means of bridges or ribs or a carbon felt 18 the annular closure plate 5 is formed and sealed with respect to the housing 4 in the same manner as previously described with respect to the embodiment of fig3 an insulating material 19.