MARCONI ECHO METER TYPE 421 & 424

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ABOVE ON THE TOP LEFT IS THE MARCONI ECHOMETER TYPE 421 ( ON ITS RIGHT IS THE MARCONI ECHOMETER TYPE 424 ) THE TOP PORTION IS KNOWN AS THE 421A AND THE LOWER PORTION KNOWN AS THE 421B.THE LOWER PART HOUSES THE AMPLIFIER. FOR INDICATING THE  DEPTH EITHER OR BOTH OF THE FOLLOWING MAY BE USED.
1. OSCILLOGRAPHIC INDICATOR WHICH INDICATES THE DEPTH BY A " PEAK" OF LIGHT ON A SCALE.
2. AN ELECTROLYTIC RECORDER WHICH INDICATES THE DEPTH ON SENSITISED PAPER.
30 TRANSMISSIONS PER MINUTE ARE USED WHEN USING ELECTROLYTIC RECORDER METHOD.60 TIMES IN 70 SECONDS WHEN THE "PEAK" INDICATOR METHOD IS USED.
THE 421 CAN RECORD READINGS FROM 0-360 FATHOMS OR FROM 0-160 FATHOMS. BUT IT IS POSSIBLE TO EXTEND THE RANGE TO 1000 FATHOMS.
THE PROJECTOR ON THE SHIPS HULL USED WITH THE 421 WAS NORMALLY THE TYPE S.4Ter.
THE 424 ECHOMETER WAS VERY SIMILAR TO THE 421 .THE MAIN DIFFERENCES BEING IN THE OPTICAL SECTION I.E FOR THE OSCILLOGRAPHIC INDICATOR.
THE 424 WAS ALSO SMALLER AND HAD  A DIFFERENT TRANSMISSION RATE