Röntgenröhre
In 1913 he invented the Coolidge tube, an X-ray tube with an improved cathode for use in X-ray machines that allowed for more intense visualization of deep-seated anatomy and tumors. The Coolidge tube, which also utilized a tungsten filament, was a major development in the then-nascent medical specialty of radiology (US patent filed in 1913 and granted as US Patent 1,203,495 in 1916). Its basic design is still in use. He also invented the first rotating anode X-ray tube.