Page 483 echo sounding , 5263 



mitted signal (this latter is known as the 1,000-fathom cutout); a milliammeter to show the current 

 to the carbon-button hydrophone; a jack into which headphones may be plugged when listening for 

 the echo; and a pilot lamp to show whether the Fathometer is in operation. 



There are three circular depth scales whose ranges are to 20, to 100, and to 1,000 fathoms. 

 The 20-fathom scale is 8 inches in diameter, and contains two scales, one divided into foot inter- 

 vals with every fifth division numbered, and the other into fathom intervals with every division 

 numbered; the 100-fathom scale is 3^4 inches in diameter, divided into fathom intervals, every tenth 

 division being numbered from 10 to 100; and the 1,000-fathom scale is SYi inches in diameter, and 

 divided into 10-fathom intervals, every tenth division being numbered from 100 to 1,000. In a 

 separate compartment in the indicator directly behind the depth scales are three index disks and 

 three neon tubes associated with the depth scales. The functions of the index disks and neon tubes 

 are describefl in 5243. 



The principal parts and circuits in the indicator cabinet are the driving motor, starting motor, 

 starting stroboscope, keying photoelectric tubes and exciter lamps, keying circuits operated by the 

 photoelectric tubes, gas-discharge tubes and circuits for flashing the neon tubes, and a plate-voltage 

 and bias-voltage rectifier and filter circuit to furnish power to the various vacuum-tube circuits in 

 the calnnet. 



The synchronous driving motor is of the reluctance type, run by 1025-cycle current furnished 

 from a tuning-fork circuit. Its rotor has 100 teeth cut in the periphery, parallel to the axis of rotation; 

 and the stator, concentric with the rotor, has teeth that match in size those of the rotor; coils are 

 arranged on the stator so as to cause the flux to pass between the stator and rotor teeth. At synchro- 

 nism the motor speed is 20.5 r.p.s. On the same shaft with the driving motor is a small 60-cycle 

 starting motor used to bring the former to synchronous speed. A stroboscope, described in 5243, 

 aids in synchronizing this motor. 



The 20-fathom index disk is direct-connected to the driving motor and rotates at 20.5 r.p.s., 

 while the 100- and 1,000-fathom index disks rotate at 4.1 and 0.41 r.p.s. respectively. The two 

 latter are coupled to the synchronous driving motor through reduction gears. 



The keying circuit contains five photoelectric tubes, any one of which may arbitrarily be switched 

 into operation, depending on the depth. Each of the three depth scales has a photoelectric-tube 

 circuit that performs the necessary keying functions associated with that scafe. 



The 20-fathom scale has two photoelectric tubes. One of these is actuated 20.5 times i)er second 

 by the reflected light from a mirror rotating at that speed, or once each revolution of the index dial. 

 The other photoelectric tube is actuated by a second mirror rotating at one-half the speed of the 

 former, thereby transmitting a signal only every alternate revolution, to prevent interference with the 

 receipt of the echo from depths near 20 fathoms. 



The 100-fathom scale also has two photoelectric tubes whose performance is similar to those 

 associated with the 20-fathom scale, except that they are actuated by light shining through openings 

 in rotating disks which take the place of the rotating mirrors. 



The 1,000-fathom scale has one photoelectric tube actuated by light shining through an opening 

 in a rotating disk. When this scale is used the emission of the signal can be prevented manually by a 

 push button on the front of the indicator, thus eliminating interference between the transmitted signal 

 and echoes arriving near the 1,000-fathom mark on the dial. 



The appropriate depth scale of the instrument is chosen by means of the depth-selector switch 

 on the front panel. This switch connects the proper photoelectric tube and its associated 6- to 8-volt 

 exciter lamp so they perform the necessary keying functions related to the depth scale selected. There 

 is an index adjustment provided for each photoelectric tube and its exciter lamp, by which the position 

 where keying starts may be changed relative to the depth scale. The locations of these five adjust- 

 ments in the indicator are shown in figure 111. (See also 5542.) 



At the same time that the depth-selector switch connects the proper photoelectric tubes, it also 

 connects the circuits that flash the neon tubes behind the proper index disks. There are three such 

 circuits, one for each depth scale, each incorporating a type 885 gas-discharge tube. 



For the first two positions of this depth-selector switch the 20-fathom dial neon tube operates. 

 For the next two positions of this switch both the 20- and 100-fathom dial neon tubes operate, and for 

 the fifth position both the 100- and the 1,000-fathom dial neon tubes operate. The advantage of two 

 neon tubes operating simultaneously is that the shoaler scale may then be read as a vernier with 

 reference to the deeper scale. 



The three neon tubes behind the three index disks are caused to flash in the following manner: 

 A type 885 tube discharges a condenser in series with the low-impedance winding of a step-up trans- 



