|Tubes of this type were supplied
until about 1925, after which, the hot cathode Coolidge
tube took over. This is a late example of a gas tube, which featured a
tungsten target set in a copper anode. Tungsten has a high atomic
number, and therefore produces more useful radiation than a copper
anode. In addition, the melting point of tungsten is much higher than
that of copper, and so is able to withstand much higher current
densities than copper. On the other hand, tungsten is a poor heat
conductor, and so heat is not able to dissipate easily from the focal
spot area. By using a comparatively small tungsten target set in a
larger copper anode, it is possible to optimize the performance.
The high temperature achieved by the anode has caused a mirror to form on the glass envelope.
The operating voltage of a gas regulated tube is determined by the gas pressure within the tube. As the tube is run, gas is slowly removed from the tube, by combining with the glass envelope. As a result, the operating voltage slowly rises. As the operating voltage rises, the wavelength of the radiation generated gets shorter, and as a result the penetration of the radiation increases. If one is trying to produce a radiograph of a thick section of the body, such as the abdomen, this can be seen as an advantage, however, if one is radiographing a thin section, such as a hand or a foot, it is a disadvantage.
Since the operating voltage of the tube could not be readily adjusted, most X-ray departments had a selection of three or four tubes, each of which would run at a different potential. The operator would then select the tube that was most appropriate for the examination in hand. When the operating voltage of any particular tube had risen to too high a voltage, the tube would be "regulated", by heating up the walls of the tube with a Bunsen Burner, but this was a hazardous procedure.
The rube shown here had a regulator, the "T" shaped structure at the top of the bulbe.