CRYSTAL CHANNEL FILTERS 



135 



adjustable air condensers appear in each filter section. These are 

 used to secure precise initial adjustment of the filter capacitances. 



To protect the filter elements against moisture, the filters are 

 hermetically sealed in a container made from a rectangular section of 

 seamless brass tubing with closely fitting plates soldered in each end. 

 One end plate carries four metal-glass seal terminals and a nozzle 

 through which dry air is blown after the filter is assembled. The 

 other end plate is provided with a small hole for the escape of the 

 drying air. After the drying operation the hole in the nozzle and the 

 hole in the opposite end of the filter are closed by soldering. The 

 elements that make up the filter are assembled on a chassis which is 

 completely wired and then slid into the container in assembly. Figure 

 10 is a photograph showing this chassis and the arrangement of the 



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Fig. 10 — The filter parts are assembled and wired on a chassis which is slid as a unit 

 inside the filter container. 



elements. The elements are located in such a way as to use very short 

 wiring connections which reduce the magnitude of any stray admit- 

 tances. The wired filter chassis is carefully adjusted by setting the 

 values of the air condenser such that for each filter section the reson- 

 ance frequencies looking into each end of each section occur where 

 they theoretically should. This compensates for the effect of small 

 capacitances between the filter parts. In the design of the filter parts 

 care is taken to use no material which absorbs moisture readily since 

 such moisture would later be released and raise the relative humidity 

 of the air inside the filter. 



If a potential much in excess of about twenty volts is applied across 

 crystal elements at frequencies near resonance, the crystals will break 

 from the mechanical strain of their vibration. The maximum safe 

 voltage across the channel filters at the resonant frequencies of the 



