Bouwers Rotating-Anode X-Ray Tube

Coolidge's tube of 1913 solved the electrical problem: a heated filament sets the current, so the operator finally controls the beam instead of negotiating with a bag of gas. What it handed over was a **thermal** problem, and a brutal one. Only about one per cent of the power you put into an X-ray tube leaves as X-rays. The rest becomes heat, delivered into a focal spot a couple of millimetres across. At 100 kV and 400 mA that is roughly 40 kilowatts arriving in an area smaller than a grain of rice - hundreds of times the power density of a hotplate, into tungsten. The spot does not gradually warm. It pits, and then it melts, and a pitted anode makes a blurred and asymmetric beam for the rest of the tube's life. Albert Bouwers' answer at Philips was to stop letting the beam hit the same place twice. **US 1,933,005** "X-ray tube", filed 13. March 1930 and granted 31. October 1933: put the target on a disc and spin it, so the beam traces a ring of fresh metal instead of a point. The patent says why plainly - "since the bombarded surface of the anticathode is materially larger than in tubes of the type having a concentrated focal spot which is formed on a stationary anticathode, such tubes present the advantage that a much higher load may be applied". It sold as the Rotalix from 1929. **The clever part is how it is driven.** You cannot run a shaft through the wall of a vacuum tube and still have a vacuum. So the anode is mounted on a rotor sealed INSIDE the envelope, and the patent claims "a metallic stator carrying magnetic coils for imparting rotative motion to said anode and arranged without the tube" - an induction motor whose stator is outside the glass and whose rotor is inside it, coupled by nothing but a magnetic field. Nothing passes through the wall at all. **That is what you will build.** Not an X-ray tube - this blueprint asks nobody to make ionising radiation - but the actual mechanism Bouwers patented: a stator outside a sealed glass jar spinning a rotor inside it, with no connection between them. It is the same motor, at the same working principle, and you will measure its speed, its slip and its run-up time.