Abstract: A cryogenic thermal switch operates based on the principle of differential coefficients of thermal expansion of differing materials. A small gap is either closed or opened, dependant upon the relative dimensions of two pieces of differing materials. As the temperature of the pieces is raised, the piece having the greater coefficient of thermal expansion (CTE) increases its dimensions at a greater rate, causing a gap to open up. Conversely, when the temperature of the pieces is lowered, the piece having the greater CTE shrinks proportionally faster, thereby closing the gap. The present invention makes use of a reliable flat-faced geometry for the two sides of the gap.