Input Capacity between I 

 Points I and 2 

 Resonates L 



Output Capacity 

 between Points 3 and 4 

 Resonates L 



Figure 1 - F'unctional Block Diagram of Apparatus 



be described briefly as follows: 



The rise and fall of the water level in which the gaging element has been immersed 

 produces very nearly linear changes of electrical capacitance as measured between the two 

 terminals of the gage; the gage forms a small part of a larger fixed capacitor which is one 

 arm of a resonant a-c bridge. These relatively small capacitance changes produce output 

 voltages from the bridge which are proportional in amplitude and phase, to the degree and 

 sense of the bridge unbalance. These voltages are amplified and demodulated in a linear 

 phase sensitive detector which produces a d-c voltage proportional to the height of the water 

 surface as measured from the water level at which the bridge was first balanced (usually the 

 quiescent water level before waves were generated). This varying d-c voltage is then further 

 amplified and furnishes the driving power required for the direct-writing recorder. Each of 

 the three units so briefly referred to above are described in greater details in the following 

 paragraphs. 



GAGING ELEMENT 



Probably the most interesting part of the system is the gaging element itself. It con- 

 sists simply of a length of No. 28 enamel covered copper magnet wire stretched tightly be- 

 tween two support points, one below the water surface and the other located an equal distance 

 above the surface on a vertical line running through the lower point. The distance between 



