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gold and the scratch hardness is the same as for sapphire. It is therefore an ideal material for polishing and also makes a suitable cavity wall. Copper cooling pipes were soldered to the rear of these copper-chromium plates and they were then polished to an optical flat by the optical shop of the Mount Wilson Observatory. The spacing ring for the cavity was made of molybdenum, for that metal has the most advantageous ratio of thermal conductivity to coefficient of expansion. Appendix I. contains a calculation which shows that sufficient cooling is obtained by conduction to negate any changes in length due to expansion.
The cavity is shown assembled in Fig. 9a and disassembled in Fig. 9b Note the holes in the spacing ring through which the R.F. power is coupled to the cavity, and the deflection plates which are mounted in a lucite cylinder.
At either end of the cavity and bolted to the end plates are threaded cylinders which hold the collimating holes. This provides a well aligned structure and insures that the electron beam passes through the center of the cavity. Both of the end plates and the spacing ring are a tight fit in the brass support ring and are pressed together by spring loaded flanges. The aluminum tubes of the envelope are bolted to the brass spacing ring and in this way the entire structure is aligned.
2.3 The Electron Source.
The electron gun design finally employed is shown in Fig. 10. This structure consists simply of a diode with an orifice in the anode. It is mounted in such a way that it can be tilted by adjusting screws
