sensitive element of this system is essentially an inductance which is 

 varied by changes in pressure and is contained in one arm of an a.c. 

 bridge located on shore. The bridge is driven by a 1,000 c.p.s. oscil- 

 lator that has good amplitude and frequency stability. 



An incremental change in pressure applied to the pressure- sensitive 

 element varies the a.c. output of the bridge. This output is combined 

 with an additional output from the balancing network, and after amplifi- 

 cation the combined output is demodulated. The output from the 

 demodulator is filtered and appears as a voltage that varies in accord- 

 ance with the pressure changes on the underwater element. Frequencies 

 below 1.5 c.p.s., after amplification by a d.c. amplifier, are recorded 

 on a milliammeter. This meter also is used to determine the state of 

 balance of the bridge. 



The acoustic system Mark I, Mod 4 can operate to a depth of 200 

 feet and use cable lengths as much as several miles. This system 

 has been used extensively to obtain wave records over long periods 

 of time off several shorebased stations. 



c. Mark IX shore wave recorder 



The Mark IX was designed at the University of California as 

 a general purpose instrument for permanent installation. The principal 

 component is a differential pressure potentiometer, which is used as 

 the transducer. The movement of a pressure- sensitive brass bellows 

 is magnified by a potentiometer contact lever which, in the normal 

 position of zero differential pressure, divides the resistance of the 

 potentiometer windings equally. Variations of differential pressure 

 cause the potentiometer contact arm to move across the windings. 

 The position variation of the potentiometer arm is converted to a 

 proportional current by the bridge circuit and recorded. The low 

 impedance (750 ohms) and high power dissipation (one watt) of the 

 transducer potentiometer enable the pressure head to be used with 

 practically any type of recorder available. In this case, an Esterline- 

 Angus recording milliammeter connected into a 24-volt Wheatstone 

 bridge circuit, of which the pressure head forms two legs, is used as 

 a standard recording system. (See Reference VII- 22.) 



d. Shorebased recorder of low-frequency ocean waves (abstract) 



This instrument has a pressure head on the sea bottom that 

 contains a hydraulic filter. The filter attenuates high-frequency signals 

 caused by ordinary gravity waves and very low-frequency signals 

 caused by tides. The maximum response is for frequencies of about 



VII- 6 



