€6 HYDROGRAPHIC MANUAL PaGE 618 



before a sono-radio buoy is put in the water, to determine if the-hydrophone is operating 

 normally. 



Before the hydrophone is attached to the sono-radio buoy, a continuity test should 

 be made of the windings of the electroacoustic unit, and leakage measurements should 

 be made between each wire of the electric cable and the hydrophone case, neither 

 wire being grounded. Next, connect a low-voltage high-impedance rectifier-type 

 voltmeter across the hydrophone wires and tap the hydrophone gently, noting the volt- 

 meter peak reading. The normal peak voltage should be determined by experiment for 

 each type of hydrophone. The choice of meter depends on the impedance of the elec- 

 troacoustic unit. Even units of low mipedance will show a reading on a sufficiently sen- 

 sitive commercial type of high-impedance voltmeter. 



The most frequent cause of hydrophone failure is leakage, usually due to improper 

 assembly of the hydrophone. If the outside diaphragm of the Dorsey hydrophone is 

 removed, a new rubbber gasket should be used and fitted carefully into the annular chan- 

 nel when the diaphragm is replaced. The 16 stud bolts are then tightened evenly until 

 the outer edges of the diaphragm and the base plate meet. "WTien the rubber plug is 

 replaced in a hydrophone that uses such a plug, the compression bolts should be tight- 

 ened, but they should be given a final tightening just before the sono-radio buoy is 

 put in the water. 



If water leaks into a hydrophone, disassemble it completely and wash all parts 

 thoroughly in a solution of hot water and soda, after which they must be rinsed in fresh 

 water and dried. 



A sharp mechanical blow, or the explosion of a bomb too close to the hydrophone, 

 may force the armature of the electromagnetic unit against one of the pole pieces. To 

 rectify this, the unit must be removed and the armature rebalanced either by carefully 

 bending it back into place or by means of an adjustment screw if one is provided. 



66. R.A.R. BOMBS AND EXPLOSIVES 



In Radio Acoustic Ranging (R.A.R.) a sound must be created that will travel in 

 the water in all directions so that it may reach a receiving unit located in any direction 

 from the source. An explosive type of bomb has been used for this purpose from the 

 beginning, and it still seems best adapted. A sound of great intensity is required, and 

 no electric or mechanical means has yet been found to produce a subaqueous sound of 

 intensity sufficient for the distances required. 



Although explosives are a hazard on board ship, their use is necessary in R.A.R, 

 pending the development of another method of producing the required subaqueous sound. 

 But if explosives are properly stowed and thoroughly understood by the persomiel 

 detailed to handle them, the hazard may be minimized and the explosives may be 

 carried and handled on board with little risk of accident. Only the explosives and 

 accessories generally used in R.A.R. are described, together with the required methods 

 of stowage and the necessary precautions to be exercised in the handling of explosives 

 on board ship. 



661. Types of Bombs 



Any type of explosive suitable for use under water may be used for R.A.R. bombs. 

 Dynamite has been used with satisfactory results, but it is unstable and more dangerous 

 than TNT and should be used only as a substitute when TNT is not obtainable. 



Trinitrotoluene (TNT) is in general use for R.A.R. bombs. It is stable, offers no 

 difficult stowage problems, and is well adapted for the construction of bombs on board 



