Looking inside the CDX-2 tube. Note the line filament and the reduced target angle compared to the first CDX tube.

Schematic view of the first CDX tube introduced by G.E. in 1921. Note the 45 degree target angle, the cup-shaped cathode structure, and the screw-type cathode end connections.  (E.R.N.Grigg – The Trail of the Invisible Light - p. 78 – Charles C. Thomas, 1965)

            The “CDX” (Coolidge Dental X-Ray Unit) introduced by G.E. in 1921, was the first totally protected (electrically and radiation-wise) dental unit, where the small Coolidge tube and all the electric transformers and circuits were oil-immersed in a compact hermetically closed metal housing.

            The Coolidge tube in the first CDX was described as “surprisingly small” compared to x-ray tubes in general use, and particularly when compared to the dental right angle tube, and I remember reading somebody describing it as the “cute x-ray tube”. It was 4” (10cms) long, and was rated 45 kV and 10 mA.

            This CDX-2 tube, is the 1930’s version of the first CDX tube, of the same size, totally built of lead-glass with the exception of the output port, and where the line-focus principle has been adopted allowing improved ratings (60kV and 10mA).

The picture to the right is that of the modern Philips Oralix x-ray tube, 2” (5cms) high, the internal structure of which is, like many present time dental  tubes,  highly similar to that of the CDX-2. It is  rated 65kV and 7.5mA.