Support 



Figure 2 - Sketch of Gage and 

 Support Bracket 



Cable to inttrument these two Support points may be about twice 



the expected double peak amplitude of the 

 largest wave to be measured. This wire is 

 completely insulated from any electrical con- 

 tact with the water. One type of suitable gage 

 support bracket is shown on the sketch, Figure 

 2. The J-shaped support bracket is fabricated 

 from a length of 1/4-in. brass rod. The lower 

 end of the gaging wire is fitted with an insula- 

 tor coupling which plugs into a small bayonet- 

 type socket, an integral part of the lower end 

 of the bent brass rod. The upper end of the 

 wire is fastened, electrically and mechanically, 

 to a 2-pin cable connector mounted on the upper 

 support member. The connecting cable (any 

 reasonable length of high-quality, low-capacity 

 cable such as Amphenol RG-62U) is fitted with a connector in such a manner that the "high" 

 side contacts the conductor in the gage wire and the "low" shield side makes electrical con- 

 tact with the water through the upper support arm and the J-shaped brass rod. A tensile pull 

 of about 3 pounds has been found sufficient to maintain the wire taut and straight under all 

 conditions encountered in tests so far conducted. 



Electrically, the conductor of the gaging element and the water in which it is immersed 

 form two plates of a coaxial capacitor. Measurements made with the General Radio precision 

 capacitance bridge indicate that the capacity of this unique condenser varies linearly with the 

 length of submerged portion of the gage wire. This is not surprising since the wire diameter 

 is very closely controlled by the manufacturer and the thickness of the enamel insulation is 

 very nearly uniform. The power factor of the gage in the water is exceptionally good espe- 

 cially if at least 4 in. of the wire are always wetted. No appreciable difficulty has been ob- 

 served from meniscus or flowback effects, and static peak-to-peak calibrations obtained by 

 physically moving the gage element up and down in the water hold equally well for dynamic 

 conditions. 



The choice of gaging wire size was made by use of experimental methods to determine 

 the degree of error encountered under dynamic conditions; the use of very large wire, rod, or 

 tubing, which might seem desirable from the support problem viewpoint, is precluded by sur- 

 face tension and slow flowback effects when the water recedes. The No. 28 enameled copper 

 wire appeared to be about the optimum since the capacity changes follow wave-height varia- 

 tions up to at least 4 cps with negligible phase error, and yet it can be put under sufficient 

 tension to avoid bending by the advancing waves. 



