mechanism. A magnetic pen recorder is used which has a high fre- 

 quency response and can record waves with periods as small as one 

 second. 



The values of the resistors connected to the contact points of the 

 gauge are adjusted so that the variation in the current is proportional 

 to the submerged length of the gauge. By recording the variation of 

 the gauge current, a record of the rise and fall of the sea surface is 

 obtained which includes tide variations as well as wave action. (See 

 References VII-1 and VII-2.) 



Two types of gauges have been built and used by the Beach Erosion 

 Board. One is a series-type step-resistance gauge and the other a 

 parallel- type. 



(1) Series-type step-resistance gauge 



In this type of gauge, the resistors are connected in a 

 series circuit, and the junctions between resistors are tied to the 

 contact points. As the sea surface rises, the water shorts all resistors 

 tied to submerged contact points and causes an increase in current 

 proportional to the number of contacts below the surface. 



(2) Parallel-type step-resistance gauge 



In this type of gauge, one end of each resistor is connected 

 to a spark plug, the other end to the gauge voltage supply. The sea 

 provides a current path between the contact points and a ground rod 

 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 circuit 

 is proportional to the number of contact points submerged. The parallel- 

 type step-resistance gauge is not as seriously affected by the accumu- 

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

 values are small compared to the resistance of the water film. 



b. Electronic sea- wave recorder (abstract) 



This instrument is a surface type of sea- wave recorder which 

 is based upon the principle that the capacitance between sea water and 

 an insulated wire placed vertically in it varies with changes in the 

 level of the water. This change in capacitance is used to modulate the 

 frequency of an oscillator. An electronic unit is used for recovering 

 from the frequency modulated signal an electrical voltage which is an 

 exact replica of the sea wave. (See Reference VII-18.) 



VII-13 



