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The Contactor Unit BC-608* essentially a clock. It was fitted in the cockpit, either in the instrument panel or an added bracket. This clock had a hand that rotated once every minute. Internally, a switch Operated for a period of 14 seconds, starting at the 12 o’clock position. In turn, the switch actuated the aircraft transmitter so that Direction Finding stations could get a rough idea of the aircraft’s location and determine that it was friendly. The clock was mechanical and hand wound, preferably every 12 hours, although it could run for 24 hours. In operation, the pilot would actuate the clock STOP-RUN switch before take off. Thereafter the aircraft transmitter would be actuated to send out an unmodulated signal for about 14 seconds of every minute, ie. In the 12 o’clock quadrant indicated. The transmitter switching could be disabled by the contactor IN-OUT switch, leaving the clock to continue running. When the contactor was in operation it meant that the pilot could not use his transmitter or receiver during that 14 seconds, or if he was in the middle of transmission when the contactor came to it’s 12 o’clock position, he would be cut off. This system was fitted mostly to fighter aircraft and interconnect to the common SCR-274 (Command) sets in US aircraft and the TR-9 in UK fighters. This early type of I.F.F. system was called “ PIPSQUEAK “.
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