Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 953 



required for shallow echo-sounding. Ultrasonic receivers and transmit- 

 ters of the electromagnetic, piezoelectric, or magnetostrictive type, previ- 

 ously discussed (see pages 946 and 948), are mounted on opposite sides of 

 the ship, usually in water-filled tanks on the inside of the hull. In many 

 installations, only one device is used for both transmitting and receiving. 



In regard to construction and operation, three different kinds of echo- 

 sounders may be distinguished: (1) phase shifters, (2) dial indicators, and 

 (3) automatic recorders. An example in the first group is the British 

 Admiralty depth finder, in which the transmitter is a 2000 cycle diaphragm 

 that is struck a hard blow with an electromagnetic hammer to start the 

 sound impulse. The electrical impulses are transmitted to the oscillator 

 three times a second from a contactor switch rotating at uniform speed. 

 This commutator carries two segments which are connected to the re- 

 ceiving phones and which put a short circuit on the phones except for a 

 brief instant. The brushes making contact with the transmission seg- 

 ments are fixed, whereas the receiving brushes may be revolved with re- 

 spect to the former. The operator adjusts the angle between the two sets 

 of brushes until he can hear the echo distinctly, the angular rotation then 

 being a measure for the echo time and therefore for the depth. In one of 

 the sonic depth finders developed by the U. S. Navy, the above principle 

 is reversed and the time interval between successive impulses is changed 

 until transmitted and received impulses are heard simultaneously. This 

 will be the case if the transmission interval is an integral multiple of the 

 echo time. The tra,nsmission interval is controlled by varying the dis- 

 tance of a friction wheel from the center of a driver disk rotated at con- 

 stant speed. Further details on this method will be found in Stewart 

 and Lindsay.^** 



In the second group of echo-sounding devices, depth readings are taken 

 on rotating dials or pointers that are started by the sound impulse and 

 stopped by the echo. The earliest of these is the "microtimer" invented 

 by Behm."^ It is essentially an electrically operated stop watch and 

 consists of a disk with a steel projection which is held in the starting posi- 

 tion by an electromagnet. When an underwater cartridge is fired, a 

 nearby microphone picks up the sound impulse and by a relay disconnects 

 the starter electromagnet from the circuit. The disk is then set in motion 

 by a leaf spring which engages a projection on the outside of the disk. A 

 second microphone, when struck by the echo, disconnects a second or 

 brake electromagnet, thus releasing its armature which is fastened to a brake 



"* Op. cit., p. 283-286. 



'"Literature will be found in: B. Schulz, Ann. Hydro., 62, 254-271, 289-300 

 (1924). H. Maurer, Ann. Hydro., 62, 75 (1924), 64, 336-340, 391 (1926), 66, 347-352 

 (1928); Zeit. Ges. Erdk. Berlin, 62, 371-377 (1927), 63, 248-249 (1928); Erg. H., Ill, 

 130-218 (1928). Hecht and Fisher, loc. cit. 



