Improvements in the Design and Manufacture of the Coolidge Tube

(Coolidge Physical Investigation, Work in Progress in Tubes and Accessories. The American Journal of Roentgenology. February p. 57, 1917).

After his brilliant invention of the X-ray tube based on thermionic current, giving radiation of constant hardness, Coolidge set himself the goal of constructing a metal tube. A preliminary report gives us the opportunity to form an idea of a number of problems he solved in connection with the principal task of making the tube of metal.

1) Coolidge introduces a number of improvements into the design of the cathode, with the aim of obtaining a more precise focus. As is known, the focus of the X-rays in Coolidge tubes was achieved by inserting a tungsten spiral inside a cylinder serving as the cathode in the tube. The different position of the spiral relative to this cylinder, as well as the shape of the latter, determine the different size of the area on the anticathode that is the place of origin of the X-rays. The accompanying figures 1 and 2 make it possible to judge the best form and the best position of the spiral relative to the cathode (the cathode is represented by a solid line, the spiral by a dotted line, and the character of the focus is depicted, at the place where the anticathode is located, in the form of a circle). It should be noted that the most acceptable form of cathode is a form very close to that of the cathode in the ordinary Roentgen air tube. The last factor influencing the size of the focus is the distance between the cathode and the anticathode. Investigation shows that the most suitable distance will be 1 English inch.

2) Kulidzhi describes a method for obtaining the high vacuum necessary in Kulidzhi’s tube. The best pump of all existing ones is the recently invented condensation pump of Langmuir (Fig. 3). Mercury vapors, having acquired a high velocity after evaporation in the heated vessel \(A\), entrain the air located in the space between the inner tube \(B\) and the middle tube \(C\). In \(E\) water circulates for the condensation of the mercury vapors, after which the mercury flows back again into vessel \(A\). Tube \(D\) leads to the space being evacuated through the vessel shown in the figure, immersed in liquid air for the condensation of mercury vapors. For removing adsorbed gases Kulidzhi uses a powder of metallic thorium. Finally, in order to free oneself entirely from copper oxides, repeated washing of the tube with purified hydrogen was employed while strongly heating it. Langmuir gives comparative data characterizing various kinds of pumps in use. He compares the following four pumps: 1) Gaede’s rotary mercury pump; 2) his molecular pump; 3) his diffusion pump; and 4) Langmuir’s pump—with respect to the speed of evacuation. At pressures of about 0.001 mm, the corresponding speeds will be 120, 1300, 80, and

Fig. 1.

Fig. 2.

Fig. 3.

$4000\ \dfrac{\mathrm{cm}^3}{\mathrm{sec.}}$ for air1. An enormous advantage of Langmuir’s pump in comparison with all the others is the circumstance that, as the pressure decreases, the pumping speed must remain constant, whereas for all Gaede pumps the speed falls sharply with increasing vacuum. With the comparatively high speed possessed by the molecular pump, it is of little use because of the extreme difficulty of giving it the stable position necessary for a tight connection with the space being evacuated. For hydrogen, the maximum pumping speed of condensation pumps reaches up to $7000\ \dfrac{\mathrm{cm}^3}{\mathrm{sec.}}$.

3) Coolidge’s metal tube (see Fig. 4) consists of a copper tube $a$, into the front part of which a platinum mirror $g$, serving as the anticathode, is sealed. This part of the tube is the anode. The cathode of the tube is the spiral $f$, situated inside a cylinder drilled into the inner copper tube $e$. The insulator is the glass $c$, connected with the copper tube $a$ by the platinum plate $b$; $h$ is the wire bringing current to the spiral $f$. Such a tube, when the outer tube $a$ is cooled, withstands a load of 200 milliamperes. Its advantages are the following:

Fig. 4.

1) Small dimensions: length about 30 cm, width in the region of the anticathode 2 cm (in the drawing the length is about $\dfrac{1}{3}$ of the actual length, and the width about half the actual width). Weight $\dfrac{1}{2}$ English pound. 2) The narrowness of the beam of rays emerging from the tube, or the absence of lateral rays.

N. Selyakov.

1. On the basis of experiments made with Langmuir’s pump at the Physical Institute of the Moscow Scientific Institute, evacuation by a small pump from 50 cu. cm of mercury balloon per liter of capacity down to the limiting vacuum requires about fifteen seconds, if the preliminary vacuum reaches thousandths of a cm Hg. ↩