to the stress at which the rate of strain is 507. 

 greater than at zero stress. It is determined as 

 the stress at the point of tangency between the 

 curve and a line having a slope with respect to 

 the stress axis 50% greater than the slope of the 

 curve at the origin. 



ELECTRIC BATTERY. See GALVANIC CELL. 



ELECTROLYTIC CELL . A voltaic cell to which an ex- 

 ternal electromotive force greater than the electro- 

 motive force developed by the voltaic cell is im- 

 pressed across the electrodes. 



ELECTROMECHANICAL TRANSDUCER . An electromechanical 

 transducer is a transducer for receiving waves from 

 an electric system and delivering waves to a me- 

 chanical system, or vice versa. (1) 



ELECTRIC WAVE STAFF . The Electric Wave Staff is an 

 instrument designed to record wave heights and 

 periods at sea. It consists of three 12-foot 

 lengths of water-tight 3-inch aluminum tubing, a 

 3-foot circular steel damping disk, a Brush record- 

 er, a transformer-rectifier circuit, electrical 

 cables, floats, retrieving lines, and balancing 

 weights. The upper section of tubing has 36 con- 

 tact points set 4 inches apart, and is covered with 

 black Waterproofing material. It is called the 

 step-r|esistance gage and has a connection for the 

 electrical cable at the top. The remaining two 

 sections of tubing provide proper buoyancy to the 

 step-resistance gage so that it will float verti- 

 cally in the water with one-half of its length ex- 

 posed. Below the end of the lower tube is suspended 

 the damping disk. Its function also is to provide 

 weight to keep the staff vertical and to damp the 

 tendency of the staff to rise and fall with the 

 passing waves. As these waves pass, the water 

 rises and falls along the step-resistance gage, 

 thereby increasing and decreasing the resistance 

 in the gage as it passes successive contact points. 

 This variation in resistance is transmitted to the 

 Brush recorder aboard ship by the electrical cable 

 and transformer-rectifier circuit. A record of 

 the wave height and period is thus recorded on a 

 strip chart. The usual length of recording time 

 is from 7 to 20 minutes. These observations are 

 made when the ship is lying to and the wave-staff 

 assembly put overboard to windward so that the 

 normal drift of the ship will be away from the 

 staff. (35) 



ELECTRICAL FISHING . Fish respond to electric 

 current by orienting themselves to face the anode 

 and swimming toward that pole in a forced manner. 

 By this means fish may be led into nets, traps or 

 pumps and the method is termed electrical fishing. 

 (34) 



ELECTROACOUSTIC TRANSDUCERS . Hydrophones and pro- 

 jectors, together with microphones, telephone re- 

 ceivers, and loud speakers, are known generically 

 as electroacoustic transducers. An electroacoustic 

 transducer is a transducer for receiving waves from 

 acoustic system, or vice versa. (1) 



ELECTRODE . The conductor by which current enters 

 and leaves an electrolyte when subjected to an 

 externally impressed potential. 



ELECTROLYTE . Any substance which, in solution or 

 fused, exists as electrically charged ions that 

 make the liquid capable of conducting a current. 

 Sea water is an electrolyte. (35) 



ELECTROLYS IS . The process of chemical decomposition 

 of an electrolyte by the action of an electric 

 current . 



ELECTROMOTIVE FORCE SERIES (EMF SERIES) . The 

 elements can be listed according to their standard 

 electrode potentials. The more negative the poten- 

 tial the greater the tendency of the metals to cor- 

 rode but not necessarily at higher rates. This 

 series is useful in studies of thermodynamic proper- 

 ties. A hydrogen gas electrode is the standard 

 reference and is placed equal to zero. All poten- 

 tials are positive or negative with respect to the 

 hydrogen electrode. (35) 



ELECTRONIC LIQUID DENSITY INSTRUMENT . The instru- 

 ment consists of a glass float on the end of a thin 

 rod suspended in the liquid. The float-rod is sup- 

 ported by means of two flat springs so that it is 

 constrained to precise vertical motion. The float- 

 rod assembly carries a coil similar to the voice- 

 coil of a dynamic loud speaker and a differential 

 transformer core. Vertical movement of the float 

 is detected by the electrical response of the dif- 

 ferential transformer. The coil moves in a strong, 

 radial, magnetic field and when the float is buoyed 

 up by the liquid, the reaction force between the 

 coil and the field is used to pull it down. Thus, 

 balance is achieved at a null position by adjusting 

 the coil current while observing the null indicator. 

 (30) 



ELECTRONIC POSITION INDICATOR (EPI) . The Elec- 

 tronic Position Indicator is a pulse, time-measur- 

 ing system similar to SHORAN and LORAN. It is 

 similar to Loran in that it uses the same frequency 

 (about 2 megacycles) and type of signal. It is 

 similar to Shoran in that it is a 2-range circular 

 system instead of a hyperbolic system like Loran. 



The EPI measures the time it takes a radio 

 signal to travel from the ship to the shore station 

 and return. An internal variable delay is in the 

 ship's read-out system. This delay is changed in 

 time until it equals the round-trip time of the 

 radio signal. Then the delay time is equal to the 

 round trip time, and this value is converted into 

 distance. For simplification, the arcs of distances 

 are plotted directly as arcs of time on the hydro- 

 graphic sheet. (29) 



ELECTRONIC SEA-WAVE RECORDER . This instrtjment is 

 a surface type of sea-wave recorder which is based 

 upon the principle that the capacitance between sea 

 water and an insulated wire placed vertically in it 

 varies with changes in the level of the water. 

 This change in capacitance is used to modulate the 

 frequency of an oscillator. An electronic unit is 

 used for recovering from the frequency modulated 

 signal an electrical voltage which is an exact 

 replica of the sea wave. 



The advantages of this recorder are; (1) It is 

 suitable for both laboratory studies and sea-wave 

 recordings; (2) this type of recorder responds well 

 to waves of all frequencies and can follow the rise 

 and fall of the water surface with negligible error; 

 and (3) the performance of the capacitance-wire 

 electrode is fairly reliable and comparatively free 

 from the action of the sea water. (35) 



ELECTROKINETIC TRANSDUCER 



An electrokinetic 



transducer is one that depends for its operation 

 on the dielectric polarization in certain liquids 

 resulting from viscous shearing stress that accom- 

 panies flow through porous materials. (1) 



ELECTRONS . One of the three basic particles of 

 atoms which has a negative charge and carries 

 approximately l/1837th of the weight of a proton. 

 Electrons are the outermost parts of an atom and 

 they revolve or circulate around the relatively 



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