339,715. Dicker, S. G. S., (Naamlooze Vennootschap Philihs' Glocilampenfabriecken) Oct. 7, 1929. R÷ntgen-ray tubes.-An incandescent cathode X-ray tube has a rotating anticathode and a filling of a gas or a mixture of gases such as neon, helium or hydrogen at a pressure above 0.0006 mm., and preferably between 0.01 and 0.001 mm., such that no detrimental ionization is produced. The gas assists the cooling of the anode. The tube consists of vitreous members 1, 2 joined by a metal ring 3 which is provided with a window 4 and may be earthed during operation. The anode 5 is mounted on the rotor' 8 of an induction motor of which the stator 9 is located externally. An eccentrically. located cathode 6 is mounted within a focusing ring 7.