ABSTRACT
The gas tube was unstable and unreliable since its X-ray production depended on its gas content and this was a very variable factor. In the earliest production design of Coolidge tube the cathode was a small flat spiral of tungsten, surrounded by and electrically connected to a small molybdenum tube which served to focus the electrons onto the tungsten target. When the cathode is heated to a particular temperature and the high tension is applied to the X-ray tube, the current passing through the tube rises rapidly to a maximum. Thus if the Coolidge tube is operated at saturation current, the penetrating power of the tube can be varied by the tube voltage and the intensity of the X-rays by varying the cathode filament temperature. With the advent of the Coolidge tube, studies on the performance of X-ray tubes were put on a better scientific basis than during the earlier years of the gas tube era.
