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two filters (Fig. 44). Presence of the ])ins in one of the filters detuned it 

 and therefore prevented ])o\vcr from Uowing through it. Rotation of the 

 pins accordingly produced switching as desired. 



In a later modification of this switch the same general princi})les were 

 used but resonant irises rather than resonant cavities were employed. 



The SJ Submarine Radar was in use at a comi)aralively early date in the 

 war and saw much ser\-ice with the Pacific submarine lleet. Despite some 

 early doubts, submarine commanders were soon convinced of its powers. 



.*<C 



SWITCH UNIT 

 CHAMBERS 



OFTUIMING 

 Pi MS 



Fig. 44— The SJ Tuned Cavity Switch. 



It is believed that in the majority of cases it replaced the periscope as the 

 principle fire control instrument. In addition it served as a valuable and 

 unprecedented aid to navigation. 



It is interesting and relevant to quote from two letters to Laboratories 

 engineers concerning the SJ. One dated October 3, 1943, from the radar 

 officer of a submarine stated that there were twenty "setting sun" fiags 

 painted on the conning tower and asked the engineer to "let your mind dwell 

 on the fact that you helped to put more than 50% of those flags there". 



