GEODIMETER . The Geodimeter is an instrument which 

 employs an electronic method of measuring distance 

 by measuring the time it takes a modulated light 

 wave to travel from the master unit to a mirror and 

 to return. 



In the Geodimeter a 10 megacycle signal oper- 

 ates a Kerr cell which interrupts the light beam 

 at this frequency. A Kerr cell has the property 

 or rotating the plane of the polarized light beam 

 when under the influence of a strong electrical 

 field. A second polarized lens on the output of 

 the Kerr cell is so adjusted as to pass the light 

 beam after its plane has been rotated. Thus, the 

 Kerr cell has the effect of pulsing (or modulating) 

 the light beam at a rate dependent upon the fre- 

 quency of the applied voltage. Since the applied 

 frequency is 10 megacycles, the Kerr cell will 

 permit the light to pass on the positive alterna- 

 tion of the 10 megacycle signal and also on the 

 negative alternation. 



^The modulated (or pulsed) light beam is 

 directed towards a distant mirror and reflected 

 back to the Geodimeter, which is focused on a 

 phototube (a tube sensitive to light variations) . 

 Also, a portion of the outgoing light is passed 

 to a variable delay line which can delay the light 

 beam over the distance of one phase at 20 megacycle. 

 This distance is 7.5 meters instead of 15 meters 

 as in the Tellurometer and Electrotape. Thus the 

 Geodimeter is capable of being read more closely 

 since the "lanes" are one-half the width of the 

 Tellurometer "lanes". (29) 



GEOGRAPHIC MILE . Same as NAUTICAL MILE. 



GEO ID . The particular geopotential surface which 

 most nearly coincides with the mean level of the 

 oceans of the earth. For mapping purposes it is 

 customary to use an ellipsoid of revolution as an 

 adequate and convenient approximation to the geoid. 

 The dimensions and orientation of the assumed 

 ellipsoid may represent an attempt to find the 

 ellipsoid that most nearly fits the geoid as a 

 whole, or they may represent an attempt to fit 

 only a particular part of the geoid without regard 

 to the remainder of it. When mention is made of 

 the dimensions of the earth, reference is usually 

 made to the dimensions of the ellipsoid most near- 

 ly representing the geoid as a whole. (24) 



GEOLOGIC ERA . The primary and largest division of 

 geologic time. Limits are rather arbitrary, but 

 each begins and ends with a time of major crustal, 

 climatic, and volcanic upheaval in some part of 

 the earth, with a great world-wide withdrawal of 

 the sea from land masses. 



Five geologic eras are recognized: Archeozoic, 

 Proterozoic, Paleozoic, Mesozoic, and Cenozoic . 

 Some authorities regard the Cenozoic as actually 

 two eras, the Tertiary and Quaternary. All eras 

 are divided into at least two geologic periods and 

 a number of geologic epochs. (24) 



GEOLOGIC EPOCH . The third-order division of GEO- 

 LOGIC TIME, delimited by partial withdrawal of the 

 sea from land masses and by gentle crustal distur- 

 bances in localized areas. 



Two or more epochs are required to make up a 

 GEOLOGIC PERIOD; and, in turn, two or more periods 

 are needed to constitute a GEOLOGIC ERA. (12) 



GEOLOGIC ERA . The primary and largest division of 

 GEOLOGIC TIME. Limits are rather arbitrary, but 

 each begins and ends with a time of major crustal, 

 climatic, and volcanic upheaval in some part of 

 the earth, with a great world-wide withdrawal of 

 the sea from land masses. 



Five geologic eras are recognized: Archeozoic, 

 Proterozoic, Paleozoic, Mesozoic, and Cenozoic. 

 Some authorities regard the Cenozoic as actually 

 two eras, the Tertiary and Quaternary. All eras 

 are divided into at least two GEOLOGIC PERIODS and 

 a number of GEOLOGIC EPOCHS. (12) 



GEOLOGIC PERIOD . The secondary division of GEO- 

 LOGIC TIME, delimited by full-scale withdrawal of 

 the sea from land masses and by limited crustal, 

 climatic and volcanic upheaval in a localized area. 

 Two or more periods are required to make up a 

 GEOLOGIC ERA, and each period is comprised of two 

 or more GEOLOGIC EPOCHS. (12) 



GEOLOGIC TIME . Time as considered in terms of the 

 vast geologic past, and basically divided into 

 GEOLOGIC ERAS, PERIODS, and EPOCHS. Seldom is geo- 

 logic time expressed in increments as small as 

 10,000 years, but frequently it is expressed in 

 millions of years. (12) 



GEOMAGNETIC ELECTROKINETOGRAPH (GEK) . The GEK is 

 a shipboard surface-current measuring device de- 

 signed to record the electrical potential developed 

 by the movement of an electrolyte (ocean current) 

 through a magnetic field (the earth's) in depths 

 of more than 100 fathoms. 



The essential physical equipment constituting 

 the instrument is: 



1. A matched pair of electrodes mounted 100 

 meters apart on a 2-conductor cable long enough 

 (ordinarily 3 times the length of the ship) to 

 stream them astern, away from the magnetic in- 

 fluences of the ship. 



2. A recording potentiometer assembly to 

 which the cable is connected. 



3. A gyrocompass repeater, mounted above or 

 close to the recorder assembly. 



With the above equipment, observations of the 

 potential difference between the electrodes along 

 the ship's course and at right angles to it are 

 made underway. These potential differences are 

 due to the motion of the water through the earth's 

 magnetic field. They are rigidly related to the 

 set and drift of the ship and to the electrodes. 

 The potential difference changes sign when currents 

 set the ship to port or starboard. The magnitude 

 of the potential difference depends on the rate of 

 drift normal to the course, the length of cable 

 between electrodes, the local strength of the ver- 

 tical component of the earth's magnetic field, 

 and to some extent on the vertical distribution of 

 water velocities in the vicinity. Through measure- 

 ments of the potential differences on 2 courses 

 nearly at right angles, the drift or component 

 velocities in these 2 directions are known. The 

 vector sum or resultant of these velocities is the 

 surface current vector for that locality. (35) 



GEOMAGNETIC MERIDIAN . See MAGNETIC MERIDIAN. 



GEOMETRIC MEAN DIAMETER . The diameter equivalent 

 of the arithmetic mean of the logarithm frequency 

 distribution. In the analysis of beach sands it 

 is taken as that grain diameter determined graphi- 

 cally by the intersection of a straight line 

 through selected boundary sizes (generally points 

 on the distribution curve where 16 and 84 per cent 

 of the sample by weight is coarser) and a vertical 

 line through the median diameter of the sample. 

 (11) 



GEOMETRIC SHADOW . In wave diffraction theory, the 

 area outlined by drawing straight lines paralleling 

 the direction of wave approach through the ex- 

 tremities of the protective structure. It differs 

 from the actual protecting area to the extent that 

 the diffraction and refraction effects modify the 

 wave pattern. (11) 



GEOMORPHOLOGY . (1) That branch of physical geo- 

 graphy which deals with the form of the Earth, the 

 general configuration of its surface, the distri- 

 bution of the land, water, etc. (2) The investi- 

 gation of the history of geologic changes through 

 the interpretation of topographic forms. (27) 



GEON . Gyro Erected Optical Navigation. A celes- 

 trial navigation system utilizing a meridian gyro 

 compass as the level reference plane. 



51 



