528 HYDROGRAPHIC MANUAL PaGE 498 



The reverse takes place in the receiving unit. The transmitting and receiving 

 magnetostrictive units are identical, except that the driving coil of the transmitting 

 unit is divided, the two halves being connected in parallel, but in the receiving unit 

 this is a single undivided coil whose inductance is approximately eight times that of the 

 transmitting unit. The nickel of the receiving unit is permanently magnetized by 

 direct current being occasionally passed through the coil for a fraction of a second. 



The oscillation period of these magnetostrictive units is determined by their 

 physical properties, the frequency being inversely proportional to the mean annular 

 diameter and directly proportional to the velocity of sound in the magnetostrictive 

 material. Discharge from the condenser into the coil of the transmitting unit sets up 

 a field that causes the magnetostrictive cylinder to oscillate at its natural period. After 

 excitation ceases, the oscillations die away after only a few cycles, because of loss of 

 energy to the surrounding medium (water). The receipt of an echo signal causes a 

 small change in the radial dimension of the receiving magnetostrictive unit, which in 

 turn induces an electromotive force in the toroidal coil. It is this electromotive force, 

 after amplification, that produces the record of the echo on the fathogram. 



528. Hughes MS 12 D 



The depth recorder MS 12 D, manufactured by Henry Hughes & Son, is a semi- 

 portable graphic-recording instrument designed for hydrographic surveying in shoal to 

 moderate depths from a small vessel. It is used by the Coast and Geodetic Survey 

 for launch hydrographic surveys. Its depth range is from 3 feet to 100 fathoms. 



This instrument is composed of three units; a recording device, the transmitting 

 and receiving units, and the batteries to furnish the necessary operating energy. 



The recording device is housed in a cast-metal cabinet containing the echo amplifier, 

 recording mechanism, equipment for producing and timing the transmitted signal, and 

 the necessary operational controls. The record is made on a predamped, starch-iodide 

 paper that moves under the stylus at a constant rate. The stylus is carried at the end 

 of a radial arm that revolves at constant speed. When the stylus is passing over the 

 paper, the echo is recorded by the passage of an electric current from the stylus through 

 the paper, causing a chemical change that leaves a dark stain on the surface of the paper. 



The instrument may be used to record in either feet or fathoms by the operation of 

 a control that changes the speed of the stylus and paper by a factor of six, the depth 

 range across the paper being respectively 60 feet, or 60 fathoms. Furthermore, the 

 range may be increased by 40 by means of a control that advances the keying position 

 by that amount. 



The speed of the operating motor is controlled by means of a centrifugal governor. 

 A low d-c to high d-c motor generator furnishes the energy for the magnetostrictive 

 transmitting unit. This energy is released from a charged condenser to the transmitting 

 unit by means of a cam that closes a contact once each revolution of the stylus arm. 

 Two pentode-type thermionic tubes are employed in the echo amplifier, whose only 

 selective circuit is the input transformer which is tuned to the operating frequency 

 (about 16 kc). Output voltages of the amplifier are converted to direct current by 

 means of a copper-oxide rectifier, and it is this current that passes through the stylus 

 and paper to make the record. 



Transmitting and receiving magnetostrictive units are installed in a streamlined 

 stainless steel housing. The entire unit is secured to the launch in such a way as to 

 offer the least resistance to the water, resulting in the least turbulence and aeration. 



