William D Coolidge

Dr Daniel J Bell et al.

William D Coolidge (1873-1975) was an American physicist who revolutionised radiology with his groundbreaking x-ray tube, the underlying technology of which remains at the core of every machine more than a century later.

William David Coolidge was born on 23 October 1873 on a small homestead in Hudson, Massachusetts. His father Albert Coolidge was a farmer who was also employed part time at a shoe factory, and his mother, Martha Shattuck, supplemented the family income as a dressmaker. Coolidge was an only child 1,2.

Coolidge was enrolled at the village single-room school and then the local high school. From here, he won a State scholarship to the Massachusetts Institute of Technology (MIT), starting in 1891. He majored in physical chemistry and electrical engineering, graduating with a Bachelor of Science degree in the summer of 1896. Whilst at MIT he started a lifelong friendship with Willis Whitney, who later became a renowned American chemist, and whom Coolidge said had a major influence on his career 1,2.

Whitney encouraged Coolidge to go to Leipzig in Germany, where he himself had previously obtained his PhD. In June 1896 he moved to Leipzig on a scholarship to study under Paul Drude, a German physicist, receiving a PhD in 1899. During his time in Leipzig he met with Wilhelm Roentgen who was being considered for a position at the university. Returning from Germany, he spent the next five years at MIT, researching the ability of heated aqueous solutions to conduct electricity 1,2

In 1905 Coolidge left MIT and moved to Schenectady in New York where he became a member of the recently established General Electric Research Laboratory, of which Willis Whitney was the first director. His initial task was to innovate on light bulb filaments. Theoretically, in view of its very high melting point and durability, tungsten would seem to be the ideal metal for the filament, but it was a brittle metal that was difficult to work. In 1910 Coolidge found that the metal (and also molybdenum) was easier to work at lower temperatures than hitherto were used, and his research was key to developing so-called “ductile tungsten” 1,2,6.

Within a couple of years tungsten became the mainstay material for filaments in incandescent light bulbs. This development led to much more resilient lighting systems which could be used in a much wider range of applications, for example in moving vehicles. 

The invention of ductile tungsten led to a search for other uses of the material. Coolidge felt that tungsten would be superior to platinum as a target in an x-ray tube. At the time, all x-ray tubes were derived from the Crookes gas tube design with some major limitations. Both researchers and practitioners were desperate to improve this rudimentary technology which had only been incrementally improved since Roentgen’s initial discovery of x-rays in 1895.

Although the Crookes tubes were known as vacuum tubes, they actually contained a small amount of gas, which was thought to be essential to their operation. When a voltage was applied between the cathode and anode, electron emission was initiated by positive gas ions striking the cathode and creating an ionization cascade and the formation of a stream of electrons.

These electrons then struck a platinum target to produce the x-rays. Therein lay one of their major flaws, as the electrons collided with it, the platinum target rapidly heated and easily vapourised, absorbing some of the gas, condensing on the inner surface of the glass envelope and impairing the efficiency of the tube. 

Coolidge knew that tungsten vapourised the least of all metals. However, just substituting tungsten for platinum as the target was found to not be change enough, until it became clear that getting rid of the residual gas would be required. The main hurdle being that electron emission needed the positive gas ions as the trigger, thus an alternative source of electrons was required.

Many years earlier, Thomas Edison, the inventor of x-ray fluoroscopy, had discovered that a hot filament produced an electron stream by an effect called thermionic (or Edison) emission. Unfortunately early research suggested that this effect was diminished unless there was some gas remnant in the tube. But it was Coolidge’s GE co-worker, Irving Langmuir, who showed that in actuality, a hard vacuum improved thermionic emission and improved a mercury vapor pump to achieve a harder vacuum than had hitherto been possible.

In his improved tube, tungsten was used for both the filament and the target. The tungsten filament also doubled up as the cathode, whilst the target also acted as the anode. In gas tubes a significant limitation was that a change in the electrical current inevitably led to a change in both the amount and hardness of the x-rays produced. However in the new tube varying the tube current varied the filament temperature and hence the quantity of electrons, and thus x-radiation emitted. However, varying the tube voltage modulated the acceleration of the electrons, allowing control over the penetration of the x-rays.

The Coolidge tube was groundbreaking in many ways, it was 14,15

  1. stable: gas tubes had a tendency to fail without warning

  2. accurate: created precise number and penetration of the x-rays with high fidelity

  3. consistent: uniformly good quality images every time

  4. versatile: it could be switched from soft to hard rays instantaneously

  5. durable: long life of the tube

  6. powerful: incredible x-ray output

  7. much safer: markedly reduced scattered/secondary radiation

Coolidge installed his new tube in the Manhattan office of Louis Gregory Cole, a professor of radiology at Cornell University 6,13. Dr Cole published his initial experience of the new tube in a glowing report in the first ever edition of AJR in 1914 15. The Coolidge tube rapidly replaced all other x-ray tubes on the market, although gas tubes remained in use for some niche applications for many years to come 6.

Dr Coolidge published the first scientific article “A powerful Roentgen x-ray tube with a pure electronic discharge” on his new tube in December 1913 in the journal Physical Review 11, which was re-published in Archives of the Roentgen Ray (forerunner of the BJR) in January 1914 12

The US patent on his hot cathode tube, for which he had applied in 1913, was finally granted on 31 October 1916. 

During the First World War, General Electric was corralled to be part of a US governmental effort to develop the ability to detect submarines as the Allies were losing a large amount of merchant shipping to German torpedoes. The C tube (for Coolidge) was fitted to British and US submarine ships which greatly aided efforts to clear the Mediterranean of German submarines.

The Coolidge x-ray tube was also modified to be used in field x-ray units. The military tube was more compact and rugged and proved itself as a vital tool in the war theater.

Following the First World War, word reached Coolidge that the Germans were using higher voltage tubes to create more penetrating x-rays for radiation therapy. Thus, Coolidge embarked on development of higher voltage tubes, so-called orthovoltage or deep x-ray therapy, leading to 200 kV tubes in the 1920s 6. Unfortunately, efforts to use higher voltages still were initially unsuccessful due to what became known as the cold cathode effect. When voltages exceeded a certain value it was found that a current flowed from cathode to anode even when the filament was not heated. This could cause a fracture of the tube or even a runaway arc discharge.

It was only in 1926 that Coolidge overcame the problem by using multipart tubes with each segment stepping up the voltage by a further 250-300 kV increment leading to tubes accelerating electrons to ~900 kV. Nevertheless this solution was imperfect as the cold cathode effect remained an issue, maintaining the high vacuum was onerous and focusing the electrons was difficult. The efforts of other researchers and newer technologies were required to overcome these difficulties (see megavoltage therapy) 6.

In the run up to the US joining the Second World War, Coolidge joined President Roosevelt’s Advisory Committee on Uranium. This important body laid the scientific groundwork for the Manhattan project, the secret American effort to develop an atomic bomb 17.

Following Willis Whitney’s retirement in 1932, Coolidge assumed Directorship of the GE Research Laboratory, and became a Vice-President and Director of Research at GE in 1940. Upon retirement in 1945 he became Director Emeritus at the laboratory, an honorific he kept until his death 30 years later.

In 1908 Coolidge married Ethel Woodward who died prematurely in 1915. His second wife was Dorothy MacHaffie, whom he married in 1916, and who passed away in 1969. He had two children from his first marriage, five grandchildren and four great-grandchildren.

On 3 February 1975 Coolidge died aged 101 years old. Clearly his early exposure to x-rays did not shorten his lifespan!

  • eighty-three US patents for medical and electrical inventions 1,2

William Coolidge had many, many accolades bestowed on him throughout his long life and career, this is only a selection!

  • Rumford Medal of the American Academy of Arts and Sciences (1914) for the tungsten filament 7
  • Howard N Potts Medal of the Franklin Institute (1926) for his x‐ray tube 7
  • Edison Medal of the American Institute of Electrical Engineers (1927) 16
  • Louis E Levy Gold Medal of the Franklin Institute (1927)
  • Hughes Medal of the Royal Society (1928) for his x-ray tube 9
  • Gold Medalist of the American College of Radiology – first awarded (1931) 4
  • Washington Award of the Western Society of Engineers (1932) 17
  • Faraday Medal of the Institution of Electrical Engineers (1939) 8
  • Duddel Medal of the Physical Society of the UK (1942) 3
  • National Inventors Hall of Fame located in the U.S. Patent and Trademark Office in Washington, DC 1975 1,7

Many honorary degrees were bestowed on Coolidge over his long life, here is a selection 17:

  • honorary MD degree, awarded by the University of Zurich (1937) in recognition of his contributions in the field of applied physics in medical sciences 7
  • Doctor of Laws, Ursinus College, Pennsylvania (1942)

  • Doctorate, National School of Engineering, University of Brazil (1945)

  • Doctor of Science, Catholic University of Chile (1945)

  • Coolidge tube was the basis for all future x-ray tubes up to the present day
  • incandescent light bulb with tungsten filament is still made today, although increasingly replaced by filament-less LED lights
  • Annual Coolidge Award from the American Association of Physicists in Medicine
History of radiology

Article information

rID: 84503
Synonyms or Alternate Spellings:
  • Dr William David Coolidge
  • Dr William Coolidge
  • Dr. W.D. Coolidge
  • William David Coolidge
  • William Coolidge

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Cases and figures

  • Figure 1: William D Coolidge (photo)
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  • Figure 2: Coolidge tube
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