The Einstein–Szilard or Einstein refrigerator is an absorption refrigerator which has no moving parts, operates at constant pressure, and requires only a heat source to operate. It was jointly invented in 1926 by Albert Einstein and his former student Leó Szilárd, who patented it in the U.S. on November 11, 1930. The three working fluids in this design are water, ammonia, and butane. The Einstein refrigerator was actually a modification of an earlier three-fluid absorption design created by Swedish engineers Baltzar von Platen and Carl Munters, which was later commercialized by Electrolux.