Bache's Bar of Invariable Length

Bache's Bar of Invariable Length. . In the 1840s, the U.S. Coast Survey finally began the first-ever accurate mapping of our coasts. The agency's superintendent, Professor Alexander D. Bache, great grandson of Benjamin Franklin and West Point roommate of Jefferson Davis, was a brilliant scientist. His innovations produced remarkably precise results., Measuring tools had always expanded and contracted with temperature changes, but Bache's invention, the "bar of invariable length,” eliminated that longstanding obstacle to accuracy. He placed an iron rod beside a brass rod within a tin tube. Connecting the rods at one end with a compensating lever caused their different expansion rates to cancel each other out, yielding reliable measurements., (caption) Prof. A. D. Bache, inventor of the measuring device called the "bar of invariable length,” later helped establish the Smithsonian Institution and the National Academy of Science. National Oceanic and Atmospheric Administration/Department of Commerce