5211 



HYDROGRAPHIC MANUAL 



Page 456 



Early models of the 312 Fathometer were calibrated for a velocity of sound in 

 water of 800 fathoms per second, but later models have been calibrated for a velocity of 

 820 fathoms per second. Most of the instruments now used by the Coast and Geodetic 

 Survey are calibrated for 820 fathoms per second. 



The 312 Fathometer is composed of five separate pieces of equipment: (1) a trans- 

 mitting unit, or oscillator; (2) a d-c to 525-cycle a-c motor generator; (3) an echo 

 receiver, or hydrophone; (4) an echo amplifier; and (5) a visual indicator. All of these 

 units are interconnected so that each performs its important function in relation to 

 the entire operation of the instrument. 



Briefly, the operation of this Fathometer in measuring depths is as follows: An 

 electric contact made by an eccentric on the index shaft, at a predetermined position of 

 the index relative to the depth scale, energizes the oscillator for a few thousandths of 

 a second, causing it to emit a train of sound waves of a given frequency in the water. 

 The echo from the transmitted sound energizes the echo receiver and is amplified by 

 the echo amplifier. The output voltage of the echo amplifier is sufficient to flash a 

 neon tube behind the revolving index slit of the visual indicator. The sounding ig 

 read from the depth scale at the place where the neon tube is seen to flash. 



5211. Oscillator 



The acoustic transmitting unit, or 324 oscillator, which produces the sound in the 

 water, is of the electromagnetic type. An assembled and an unassembled view of this 



Figure! 



-Electromagnetic oscillator type 324. A. Exciting coil. BandC. Steel laminations. DandE. Upper and lower sections 

 of housing. F. Stainless steel diaphragm. G. Hull mounting. H. Hull plate. 



oscillator are shown in figure 99. The diaphragm i^ is 13 inches in diameter and is 

 made of stainless steel. By proper dimensioning, the diaphragm is given a natural 



