5243 HYDROGKAPHIC MANUAL PaGE 476 



ometer, and the rear switch is for starting and stopping the auxiUary starting motor, 

 whose purpose is to bring the synchronous driving motor up to synchronous speed. 

 On the lower right-hand edge of the cabinet is a push button by which the starting 

 motor and the synchronous motor are engaged. The parts included in the cabinet 

 are: the synchronous motor, starting motor, stroboscope starting lamp, circular neon 

 tube, and the index disk. 



The synchronous motor is of the reluctance type. The rotor has 100 teeth around 

 its periphery, their slots being parallel to the axis of rotation. An annular channel, 

 cut in the central disk of the rotor, is filled with mercury, its purpose being to dis- 

 courage hunting, or the minute variations in speed of rotation due to acceleration and 

 deceleration caused by electromechanical instability. 



The stator consists of three U-shaped pole pieces, spaced 90° apart around the 

 rotor. The teeth of the stator are identical in size to those of the rotor. An energiz- 

 ing coil is wound around each pole piece in such a manner as to permit the maximum 

 flux to pass between the stator and rotor teeth. The relation of the speed of rotation 



to the frequency of the energizing current is given by N=^ (for no polarizing cur- 

 rent) ; in which A^=the number of revolutions per second, i''=the frequency of the 

 energizing current in cycles per second, and jr=the number of teeth in the rotor. In 

 the case of the Dorsey Fathometer No. 1, i^=1025 cycles per second, and T=100 

 teeth; therefore, N=20.5 revolutions per second. 



A small universal motor is used to bring the non-selfstarting synchronous re- 

 luctance motor to speed. The two motors are engaged by friction coupling. A neon 

 stroboscope aids in synchronizing this motor. The neon lamp of the stroboscope is 

 operated by current from the tuning fork which drives the synchronous motor — this is 

 1025-cycle current. Light from the neon lamp illuminates the teeth of the rotor so 

 that at synchronous speed these teeth appear to be stationary. 



The thin aluminum index disk attached to the synchronous motor is Sji inches in 

 diameter. An index, shaped like the ace of spades, is cut in. this disk, with the pointed 

 end adjacent to the depth scale. The disk rotates between the back of the depth 

 scale and a circular neon tube which is concentric with the depth scale. From a view- 

 point in front of the indicator, the entire neon tube is hidden by the disk, except that 

 part exposed by the index slot. When the neon tube flashes at the instant an echo 

 is received, the illumination of this tube appears through the index ^ot alongside the 

 depth scale at a point corresponding to the depth. 



A photoelectric tube, a source of light, and a rotating mirror form part of the 

 keying circuit. The photoelectric tube and a 6- to 8-volt incandescent lamp are 

 located on the top of the indicator cabinet, protected from stray light by metal shields. 

 The light from the lamp passes through a hole in the top of this cabinet and is focused 

 on a rotating concave mirror attached to the synchronous driving motor. This light 

 is then reflected back to the photoelectric tube through a second hole in the top of 

 the cabinet. Once each revolution of the driving motor the light reflected 

 from the mirror falls on the photoelectric tube. At the point in the cycle where the 

 emission of the acoustic signal is desired, usually at a point on the depth scale corre- 

 sponding to the ship's draft, the mirror must be in a position to reflect the light beam 

 onto the photoelectric tube. The voltage output of the photoelectric tube is then 

 amplified to actuate a type 884 gas-filled discharge tube. One of the characteristics 

 of this latter tube is that it delivers a relatively large amount of current in a very 



