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ly high value of the resistors connected between the contact points. The film of 

 water left on the instrument by the receding water tends to short the resistors 

 connected between the contact points. Because of this, the use of the series- 

 type resistance gage is restricted to measurement in fresh water, where cur- 

 rent leakage caused by the water film is negligible. 



b. Parallel-type step resistance gage: In this gage, one end of each 

 resistor is connected to a spark plug, the other end to the gage voltage supply. 

 The sea provides a current path between the contact points and a ground rod. 

 The ground rod is then connected to the other side of the voltage source. As the 

 spark plugs are submerged, the resistors are added in parallel. The values of 

 these resistors are so selected that the current flowing in the gage is proportion- 

 al to the number of contact points submerged. 



The parallel-type step resistance gage is not affected as seriously by the 

 accumulation of water film as is the series type, because its resistance values 

 are small compared to the water film resistance. The gage should be used only 

 in salt water because the resistance path between the contact points and the 

 ground rod must be smsill compared to the resistor value. 



c. Portable-type step resistance gage: The size and weight of the step- 

 resistance gages used for permanent installations make temporary installations 

 impractical. A light-weight step resistance gage, 6 feet in length, therefore, 

 has been designed and built by the Beach Erosion Board to fill this need. 



Woods Hole Shore Wave Recorder (Klebba, 1945) - Developed by the Woods 

 Hole Oceanographic Institution, the Woods Hole JRecorder has a transducer con- 

 sisting of a coil and magnet (component part of a radio speaker) arranged so that 

 the total magnetic flux linkage of the coil varies as the water pressure fluctuates. 

 The coil is attached to a metal bellows, the length of which varies with the pres- 

 sure fluctuations. The coil is thereby moved in and out of the magnet, changing 

 the flux linkage. 



Recordings of the flux linkage of the coil can be made with a General 

 Electric photo-electric recorder (a direct writing servo-mechanism type record- 

 er) or with the Woods Hole photographic recorder. The Woods Hole recorder 

 provides a record that can be used directly with the frequency analyzer develr 

 loped at Woods Hole. 



The sea pressure acts upon an exposed metal bellows which is comj- 

 pressed according to the total pressure. A second bellows with a small cross 

 section and a light spring constant is coupled through a tube to the exposed bel- 

 lows; this system is essentially a hydraulic amplifier. Slight movements of 

 the exposed bellows cause large nnovement of the light bellows to which the flux 

 linkage transducer is coupled. A slow leak around the light bellows prevents 

 the static pressure of the depth and of the tide pressure variations from affecting 

 the position of the light bellows. 



Mark IX Shore Wave Recorder (University of California) - The Mark IX system 

 (Snodgrass , 1951) is designed as a general purpose instrument for permanent 

 installations. Its principal component is the Bourns differential pressure poten- 

 tiometer which is used as the unit transducer. The movement of the pressure- 

 sensitive brass bellows is magnified by a potentiometer-contact lever which (in 

 normal position of zero differential pressure) divides the resistance of the po- 

 tentiometer windings equally. Variations of differential pressure cause the po- 

 tentiometer contact arm to move across the potentiometer windings. The posi- 

 tion variation of the potentiomieter arm is converted to a proportional current by 

 the bridge circuit and is recorded by the recording milliameter. 



