:::::::: There's a lot of confusion. Is Langmuir responsible for hard vacuum tubes? IIRC, the courts determined that de Forest licensee Federal was making hard vacuum tubes before Langmuir and gave de Forest priority. As I understand it, Federal's goal was to avoid the blue glow of death at high plate potentials. The solution would be pretty obvious to an engineer at the time. Langmuir clarifed the theory. Langmuir's paper described competing theories, he did experiments, and he said that one of the theories was right. Langmuir explained the space charge issue, but I don't know if other researchers were already thinking of that. I'd have to look up Child and Richardson. In any event, Langmuir was contributing to an understanding of how the device worked. The thinking at the time was clouded by experience with cold cathode X-ray tubes. Also, in a sense, the hard vacuum is minor improvement. The early tubes had poor emission, poor geometry, and poor field strengths. Armstrong's biasing would also work with a soft vacuum tube. I don't have the reference handy, but I think a de Forest employee wrote a QST article that explained de Forest sold both soft and hard audions. The soft tubes were used for sensitive detectors.