portion of the passage 16 the use of the filler material does not affect the efficiency of the boiling cooling apparatus in each of the foregoing embodiments the container 13 is made of a cylindrical tube but any other form can be used for example a tube having a rectangular cross section since the boiling cooling apparatus of the present invention provides at least one passage through the interior of the heat generating assembly through which the liquid coolant flows as a result of ascending bubbles produced by the heat from the heat generating assembly the efficiency of the heat transfer between the container and the liquid coolant is improved so that a reduction in the size of the cooling apparatus may be made fig6 shows an ebullition cooling apparatus similar to that disclosed in fig1 for the purpose of illustrating a problem which arises with respect to the transfer of heat by convection as the liquid coolant traverses the passages 18 in contact with the wall of the container 13 as illustrated in fig6 the width s of the passages 18 between the current control plates 17 and the inner wall of the container 13 is substantially uniform so that the liquid coolant 12 becomes stagnant at the bottom of the container in the cross hatched area a therefore heat dissipation in this area is inhibited it is estimated that this.