Page 457 echo sounding 5212 



frequercy of 1050 cycles per second in water. Welded to the inside center of the 

 diaphragm is a group of steel laminations 5 and opposite to these a corresponding 

 group C is welded to the housmg. A coil A is wound around the latter group of lamina- 

 tions to carry the exciting current. The vibrational frequency of the diaphragm is 

 twice that of the exciting current since no polarizing force is used on the diaphragm, 

 and it follows the alternations of the exciting current, regardless of direction of flow, 

 to produce a sound in the water of 1050 C3^cles. The outer face of the diaphragm must 

 be in direct contact with the water, so the oscillator must be mounted in an opening 

 cut in the bottom of the vessel. 



5272, Motor Generator 



Energy to excite the 324 oscillator is supplied by a motor generator, which is 

 operated from the ship's 110-volt d-c power circuit, and which produces 125 volts at 

 525 cycles. It has a rating of 500 volt-amperes. A tuning condenser in series with 

 the generator and oscillator gives unity power factor at 525 cycles, producing maximum 

 energy transfer. The generator speed, and hence the output frequency, tends to vary, 

 but is controlled by a motor-field rheostat. This rheostat and a frequency meter cali- 

 brated from 500 to 600 cycles are located near the depth indicator so that the frequency 

 can conveniently be kept at 525 cycles, or half the fixed frequency of the oscillator. 

 The 525-cycle motor generators of later model 312 Fathometers have been equipped 

 with governors, to keep the no-load and full-load speeds of the machine nearly constant. 

 This keeps the frequency of the generator more nearly in its proper relation with 

 the tuned circuits of the 324 oscillator during the keying period. More sound energy 

 at 1050 cycles is transmitted by this arrangement, and the returning echo will be 

 of greater m tensity. 



This increase in sound energy may cause excessive reverberation in vessels of certain 

 design, resulting in an apparent prolongation of the initial signal. This may seriously 

 interfere with sounding in shoal to moderately deep water, but it may be eftectively 

 damped by placing heavy weights around the oscillator, and sometimes around the 

 hydrophone tanks. On one vessel where reverberation was bothersome 14,000 pounds 

 of pig lead were evenlj^ distributed in the frame spaces surrounding the oscillator, and 

 3,000 pounds were placed around the hydrophone tanks. 



5213. Hydrophone 



The echo receiver, or hydrophone, used to detect the echo and convert its sound 

 energy into electric energy may be one of several types, depending principally on the 

 depths being sounded. Two or more different types of hydrophones are commonly 

 emploj'^ed so that any one can be switched into operation for use in those depths where 

 it gives optimum results. The three types of hydrophones commonly used with the 

 312 Fathometer are: 



(a) Tuned hydrophone No. 314^ is a large brass unit whose diaphragm is tuned to respond best 

 at frequencies of 1050 cycles in water. This hydrophone is generally used for deep-water soundings. 



(6) The K-tube unit, a rubber-cased hydrophone, also known as the rubber rat, employed prin- 

 cipally in shoal and moderately deep water. 



(c) Hydrophone No. 134E is a small brass unit, sometimes known as the watch-case unit, used 

 for shoal and moderately deep soundings. 



Each of these units is installed in a water-filled tank secured to the inside skin of the ship. The 

 electroacoustic element of each of these hydrophones is a carbon button fastened to the inside of 

 the diaphragm. 



