more uniform and thus make it possible to more accurately determine the emissivity of the surface. To determine the characteristics of the anodized surface in the vacuum bell jar it was necessary to have an adequate heat source at the center of the fin. The magnetron itself can perform this function without the generation of microwave power whose absorption in a vacuum becomes a difficult problem. To employ the magnetron as such a heat source, the permanent magnets are removed from the tube and it is operated as a simple cylindrical diode, in which the current flow between cathode and anode is proportional to the 3/2 power of the applied voltage. However, it was necessary to transfer the heat from the shell of the magnetron to the cooling fin under evaluation. This was accomplished by first press fitting a copper ring to the magnetron shell and then bolting the copper ring to the anodized aluminum fin. The use of the vacuum bell jar for the tests is shown in Figure 11. The pumping system employs a centrifugal pump of good capacity and was able to achieve a vacuum of 10'5 millimeters of mercury during the tests on the anodized aluminum surface. Of prime importance in the evaluation tests were the emissivity of the surface and the stability of the surface and its retention of a high emissivity over a period of time.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==