Page 521 



ECHO SOUNDING 



5552 



The procedure is as follows: 



(a) A 1-second break-circuit chronometer, such as is used in Radio Acoustic Ranging, must be 

 used. Connect the contact circuit of the chronometer and a resistance-condenser combination to 

 the points in the indicator as indicated in figure 112. The values of the resistance and capacitance 

 of the combination are not critical. The connection marked X in the figure is made by removing 

 any one of the three left-hand type 885 gas discharge tubes which are mounted on the back hinged 

 panel of the indicator, inserting the end of the wire in the grid hole of the tube socket, and replacing 

 the tube, thus holding the wire in place. Connection Y is made to the variable tap nearest ground 

 on the bias potential divider. The voltage of this tap is about minus 50 volts. If the circuit oper- 

 ation is not satisfactory, connection Y may be moved by a slight amount on the potential divider. 



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Figure 112.— Circuit for verification of tuning-fork frequency by chronometer. 



This divider is located in the left end of the indicator. Make certain that the potential divider is 

 that of the bias supply and not that of the high-voltage supply. 



(b) Run the Fathometer 15 minutes before calibration. Set the depth-selector switch on position 

 1 and the starting-frequency switch on position 4. 



(c) Count the flashes on the 20-fathom dial as a measure of time. If the flashes precess more 

 than 2 fathoms in 12 secpnds the tuning fork should be adjusted as described in 5552. Clockwise 

 precession of the flashes indicates that the fork frequency is too high, and counterclockwise precession 

 indicates that the fork frequency is too low. The flashes will appear alternately 180° apart on the 

 dial, once each second. Every flash must be counted in making these measurements, not just those at 

 one position on the dial. 



55S2. Frequency Adjustment of the Tuning Fork 



The frequency of the tuning fork in the Dorsey Fathometers can be increased or 

 decreased by fihng certain of its parts. If tests indicate that the tuning-fork frequency 

 is too low, it should be increased by filing the vertical edges of the ends of the fork 

 tines. The fork must be removed before filing to prevent filings from getting on the 

 head receiver magnet or on the di'iving-coil pole pieces. A fine file should be used. 

 By beveling the two vertical edges of each tine, one can judge by eye when equal 

 amounts have been taken off each edge. When filing the fork the first time, only a 

 small amount should be filed off before a second test, or until the effect that a given 

 amount of filing on the frequency has been determined. It may be necessary to re- 

 move and file, and replace and test the fork several times before it has the correct 

 frequency. When replacing the fork in its mounting, paper shims should be inserted 

 between the tines of the fork and the pole pieces of the pickup and driving magnets 

 so as to locate the fork properly relative to these poles pieces. 



