temperature of the sea surface and the water-vapor 

 content of the atmosphere. 



EFFECTIVE BAND WIDTH . The effective band width of 

 a measuring system selectively responsive to energy 

 distributed in a spectrum is given in terms of the 

 band width of a hypothetical system which satisfies 

 two requirements; (1) over its assigned frequency 

 band it has a uniform response equal to the maximum 

 response of the actual system; (2) the width of 

 this uniform response band is such that, if fre- 

 quency is plotted to a linear scale, the areas under 

 the response-frequency characteristics of the hypo- 

 thetical and of the actual systems will be equal. 

 (4) 



EFFECTIVE CENTER . The effective center of a sonar 

 projector is defined as that point at which lines 

 coincident with the direction of propagation, as 

 observed at various points some distance from the 

 projector, appear to intersect. If such a point of 

 intersection exists it will correspond to the source 

 at which acoustic energy, moving along any direction 

 of propagation, appears to originate, as indicated 

 by the variation of intensity with distance. For 

 this reason the effective center is often spoken of 

 as the apparent source. (4) 



EFFECTIVE DIRECTIVITY FACTOR . The effective direc- 

 tivity factor of a hydrophone, at a specified 

 frequency, may be defined as the ratio of the 

 available power per unit band of the electric waves 

 generated in the hydrophone when oriented in a 

 specified manner in a specified location to the 

 available power per unit band of the electric waves 

 which would be generated, at the same location, in 

 a hypothetical nondirectional hydrophone having a 

 receiving response on any bearing equal to the 

 maximum response of a given directional hydrophone. 

 (4) 



EFFECTIVE SOUND PRESSURE . The effective sound 

 pressure at a point is the root-mean-square value 

 of the instantaneous sound pressures, over a time 

 interval at the point under consideration. In the 

 case of periodic sound pressures, the interval must 

 be an integral number of periods or an interval 

 that is long compared to a period. In the case of 

 non-periodic sound pressures, the interval should 

 be long enough to make the value obtained essen- 

 tially independent of small changes in the length 

 of the interval. (2) 



EFFICIENCY . The efficiency of a device with re- 

 spect to a physical quantity which may be stored, 

 transferred, or transformed by the device is the 

 ratio of the useful output of the quantity to its 

 total input, (1) 



EFFICIENCY LOSS . The efficiency loss of a trans- 

 ducer connecting an energy source and an energy 

 load is the transmission loss measured by the ratio 

 of the input power of the transducer to the load 

 power of the load . (4) 



EKG . 1. Electrocardiogram 

 2. Electrocardiograph 



EKMAN CURRENT METER . The Ekman current meter was 

 developed by Dr. V. Walfrid Ekman, a Swedish scien- 

 tist, whose original" design, although modified, 

 remains basically unchanged. The meter was de- 

 signed to give speed and direction of the current 

 at any depth. It consists of an impeller or screw, 

 and shaft connected to a set of dials. The number 

 of shaft revolutions per unit of time is read from 

 the dials on the main body of the meter. A reser- 

 voir of bronze balls is connected by a narrow tube 

 to a compass box containing a compass needle. 

 Below the needle is the compass-ball receptacle 

 which is divided into 36 chambers, each represent- 

 ing 10° of azimuth. As the impeller rotates, the 

 balls fall, one at a time, onto the top of the com- 



pass needle which guides them into one or another 

 of the chambers, depending on the heading of the 

 current meter. This gives the direction toward 

 which the current is flowing. 



The current meter is lowered on either the 

 oceanographic or bathythermograph wire. The im- 

 peller is locked while lowering or hoisting. A 

 messenger is sent down the wire to unlock the 

 impeller and set the meter in operation. A second 

 messenger is sent down to lock the impeller and stop 

 the meter before hoisting. 



Valid measurements cannot be made with an 

 Ekman current meter unless the ship or buoy from 

 which it is suspended is anchored. (35) 



EKMAN SPIRAL . A graphic representation of the way 

 in which the theoretical wind-driven currents in 

 the surface layers of the sea vary with depth. In 

 an ocean which is assumed to be homogeneous, in- 

 finitely deep, unbounded and having a constant eddy 

 viscosity, over which a uniform steady wind blows, 

 Ekman has computed that the current induced in the 

 surface layers by the wind will have the following 

 characteristics: (a) At the very surface the water 

 will move at an angle of 45 CUM SOLE from the v^ind 

 direction. (b) In successively deeper layers the 

 movement will be deflected farther and farther cum 

 sole from the wind direction, and the speed will 

 decrease. (c) A HODOGRAPH of the velocity vectors 

 would form a spiral descending into the water and 

 decreasing in amplitude exponentially with depth. 



The depth at which the vector first points 

 180° from the wind vector is called the depth of 

 frictlonal Influence (or depth of frlctlonal resis- 

 tance) . At this depth the speed is e"'" times that 

 at the surface. The layer from the surface to the 

 depth of frictlonal influence is called the layer 

 of frictlonal Influence. If the velocity vectors 

 from the surface to the depth of frictlonal In- 

 fluence be integrated, the resultant motion is 90 

 cum sole from the wind direction. (12) 



ELASTIC LIMIT . In practice, the elastic limit is 

 determined by subjecting a specimen carrying a 

 strain-measuring device (extensometer) to a series 

 of loading steps in which the maximum load applied 

 is gradually increased, the load being released 

 completely at each step. A load will finally be 

 reached upon release of which the specimen will 

 fail to return to its original length: this load 

 Is the elastic limit. The size of the load incre- 

 ments used and the sensitivity of the extensometer 

 used will, of course, affect the value obtained, 

 and, consequently, this property is not frequently 

 determined . 



ELASTIC LIMIT. APPARENT . An arbitrary approxima- 

 tion of Elastic Limit for a material that does not 

 exhibit a significant Proportional Limit. It is 

 obtained from a Stress-Strain Diagram and is equal 



I -A- 



10 



(/) 

 a> 



■1- 



Strain 



39 



