deflection of this meter obtainable by adjustment of the decade capacitor and resistance bal- 

 ance controls indicates that the bridge is balanced. If difficulty in arriving at balance is en- 

 countered with a high-sensitivity setting of the attenuator, this control may be rotated toward 

 the 25-in. position until a position is reached where the null indicator meter responds to ad- 

 justment of the balance controls. 



After the bridge has been balanced, the operator should select the attenuator position 

 whose marking most nearly approximates the peak-to-peak amplitude of the expected waves 

 to be recorded. At this time, calibration may be performed by raising and lowering the gage 

 element through an accurately measured distance and recording the corresponding deflections 

 of the recording pen. 



PERFORMANCE 



A complete wave-height recording system of the type described in this report was in- 

 stalled in the miniature model basin in late March 1952. After an initial period devoted to 

 calibration, testing, and minor adjustments to suit the selected operating conditions, the per- 

 formance characteristics were verified to be as follows: 



1. Maximum usable sensitivity was 0.6 in. (double peak amplitude) of wave height which 

 produced 3 cm deflection of the recording stylus; (the built-in sensitivity control provides 

 attenuation in 10 fixed steps covering the range from 0.6 to 40 in. of wave height). 



2. Linearity of the gaging element and the electronic system is approximately 1 percent . 

 of full scale on any sensitivity range selected. The recorder itself was found accurate to 

 approximately 2 percent of full scale. 



3. Resolution (on the record) on the most sensitive step of the sensitivity control is of 

 the order of 0.010 in. of change in water level. 



4. Frequency response of the system was measured by mounting the gage and its support- 

 ing bracket on a cam-driven vertical oscillator whose frequency of vibration was adjustable 

 from to about 5 cps. Response was found to be uniform up to the upper frequency limit of 

 the mechanical oscillator. There is no reason to believe that the range of uniform response 

 does not extend considerably above 5 cps, since no measurable amplitude distortion was 

 evident at this frequency. 



Figure 6 is a photographic copy of one continuous record taken in the 140-ft towing 

 basin by personnel of the Hydromechanics Laboratory using the instrumentation system de- 

 scribed. The upper trace on each strip shows the record obtained from a gage placed 38 ft 

 from the wavemaker and the lower trace shows the record from another gage located 22 ft 

 from the wavemaker. 



Early in October 1952, an exact duplicate of this system was constructed and placed 

 in use along with the original pilot model. The performance characteristics of this system 

 were found to be identical to the first one, and no adjustments or modifications were required. 



