Society News Archive

It was 100 years ago today that William D. Coolidge first noted in his lab notebook the use of a hot filament as the source of electrons in a high-vacuum x-ray tube.   
 
This was a significant improvement and allowed full development of the technology and expanded use in medicine and industrial applications.

William David Coolidge (1873–1975) was a research scientist and inventor of the modern x-ray tube. Besides Roentgen, with his 1895 discovery and subsequent studies of x rays, perhaps no other individual contributed more to the advancement of x-ray technology than did Coolidge.

Coolidge was born in Hudson, Massachusetts, and received his bachelor of science degree from Massachusetts Institute of Technology (MIT) in 1896. That same year, he went to Europe to study under renowned physicists of the time. Coolidge received his PhD (summa cum laude) from the University of Leipzig in 1899 and soon after joined the staff of MIT. While studying at Leipzig, he met Roentgen. In 1905 he joined the newly established General Electric Research Laboratory in Schenectady, New York. He promptly began fundamental work on the production of ductile tungsten filaments as a replacement for fragile carbon filaments used in incandescent light bulbs. This improved light bulb was brought to market in 1911.

It was application of this work that led Coolidge to his studies in x-ray production. The state-of-the-art x-ray tube at the time was the "gas tube" or "cold cathode" type tube. These x-ray tubes relied on residual gas molecules as a source of electrons for bombardment of low to medium atomic number metal targets. In 1912 Coolidge described the use of tungsten as an improved anode target material for x-ray tubes.

Shortly after, in 1913, he published a paper in Physical Review describing "A Powerful Roentgen Ray Tube With a Pure Electron Discharge." This tube used a tungsten filament as a thermionic source of electrons, with high vacuum, to bombard a tungsten anode target. Great improvements in x-ray tube stability and performance were obtained with the "hot cathode" or "Coolidge tube."

With some variation in filament and target geometry, this 100-year-old invention is the same basic x-ray tube used today. In 1932 Coolidge became director of the GE R&D Laboratory, then in 1940 vice president and director of research. In 1941 he was a member of a small committee, appointed by President Franklin D. Roosevelt, to evaluate the military importance of research on uranium. This committee's report led to the establishment of the Manhattan District for nuclear weapons development during World War II.

Coolidge had 83 patents to his credit and numerous awards and honorary degrees and in 1975 was elected to the National Inventor's Hall of Fame. At the time he was the only inventor to receive this honor in his lifetime.

By David J. Allard, MS, CHP