Page 477 echo sounding 5244 



short interval of time. As previously mentioned, an echo-sounding instrument for 

 use in shoal depths should have a transmitted signal of very short duration. This gas 

 tube is one of the features making this possible. The relay action of this tube allows 

 the 17.5-kc vacuum-tube oscillator to oscillate for a period of approximately 0.0007 

 second (0.7 millisecond). 



By means of a control that changes the time constants of the type 884 keying tube 

 circuit, a signal is transmitted only on alternate revolutions. This is to prevent inter- 

 ference between the transmitted signal and the echo in depths slightly greater than 20 

 fathoms. 



The speed of rotation of the synchronous driving motor depends on the frequency 

 of the electric power supplied to it. For utmost accuracy in sounding, the frequency 

 of the supplied power must remain constant. This is accomplished in the Dorsey 

 Fathometer No. 1 by means of a tuning fork (see 51656). The vibration of the tuning- 

 fork is electrically maintained by a vacuum-tube circuit, whose output voltage is ampli- 

 fied sufficiently to drive the motor. 



5244. Power Supply 



The power supply is housed in a metal frame rack. Included with the power 

 supply are such parts as the 17.5-kc oscillator and amplifier, the tuning-fork circuits, 

 the polarizing rectifier and filter, and the high-voltage and bias rectifiers and filters. 



This Fathometer is operated by power derived from the ship's 110-volt d-c supply, 

 which is converted to 110-volt 60-cycle alternating current by means of a 1.5-kva 

 rotary converter. The total power consumed is approximately 1,000 watts. 



5245. Operation 



The procedure in preparing the Dorsey Fathometer No. 1 for sounding is as follows: 

 The front toggle switch on the left corner of the indicator cabinet is moved to the up 

 position ; this action starts the d-c to a-c rotary converter which furnishes the power to 

 all parts. After an elapsed time of less than a minute a 2050-cycle sound will be heard 

 in the indicator. The rear toggle switch on the lower left-hand corner of this cabinet is 

 then thrown to the up position, starting the starting motor. The hinged door on the 

 right side of the cabinet is opened so that the synchronous motor rotor and stroboscope 

 neon lamp may be viewed. The push button on the lower right-hand edge of the cabi- 

 net is then pressed to engage the starting motor and the synclironous motor. These twO' 

 motors are kept engaged until the latter motor has reached synchronous speed, which 

 is 1,230 r.p.m., and which is indicated by the teeth of the rotor, illuminated by the neon 

 tube, appearing stationary. When these teeth first become visible, the push button of 

 the starting motor should be given a series of pushes at the rate of about one per second 

 until synchronization of the motor occurs. 



Caution must be observed not to synchronize the motor at half-synchronous speed. 

 At half speed the teeth of the rotor will appear to be stationary under the stroboscopic 

 illumination, but with much less definition. It will also be much more difficult to syn- 

 chronize the motor at half speed. To guard against this the rotor teeth should be 

 watched from the starting of the motor until it reaches final synchronous speed, noting 

 first when the rotor teeth appear stationary, but keeping the motors engaged until this 

 occurs a second time, when the correct synchronous speed has been reached. Sufficient 

 practice will enable one to start the motor, without the use of the stroboscope, by listen- 



