determine the effectiveness of the trap shielding by measurement of 

 the voltage gradients within,, and at the mouth of the trap,, The max- 

 imum voltage gradient allowable at these points for the effective 

 operation of such traps has been determined to be in the order of 0.2 

 volt per inch. 



Bottom resistance measurements . —At the time when these 

 experiments were begun,, it was reasoned that the resistivity of the 

 stream bed, if somewhat lower than that of the stream water,, could 

 cause a considerable dissipation of energy with resultant weakening 

 and distortion of the electrical fields. In order to determine the 

 relative values of stream water and stream bed resistances at the ex- 

 perimental sites, a resistance probe was constructed. This device con- 

 sisted simply of two pointed steel rods, 12 inches long and 7/32-inch 

 in diameter,, which were mounted on a piece of waterproofed hardwood, 

 parallel to each other and spaced 12 inches apart. The upper halves of 

 the rods were insulated with rubber tubing and household cement so that 

 only the lower 6-inch section of each rod was left exposed (Fig. 16). 

 A suitable handle affixed to the top of this assembly permitted it to 

 be forced into the stream bottom to a depth of 6 inches or more. Two 

 insulated wires, one to each rod, were connected to the 2k volt wind- 

 ing of a 110 to 2k volt s 20 watt, isolation transformer. With 2k volts 

 AC applied to the probe, a relative measure of the resistance of the 

 water or stream bed could be determined by dividing the voltage across 

 the probe by the current flowing through the probe (R = E/l) . The small 

 AC currents occurring in these measurements were best determined by read- 

 ing the voltages across a 10 ohm resistor connected in series with one 

 of the probe leads . This voltage, when divided by ten gave the current 

 in amperes in the probe Precise measurements of the voltage used in 

 determining relative resistance were made with a General Radio, Type 727-A 

 vacuum tube voltmeter. 



In making resistance measurements at the experimental sites it 

 was found generally that readings taken at three or four points across 

 the stream did not differ widely. Resistance measurements were first 

 taken in the water with the rods submerged but held clear of the bottom. 

 Where the nature of the bottom materials permitted, the probe was then 

 forced into the bottom to the full length (12 inches) of the metal rods, 

 and another measurement of resistance was taken. Where this procedure 

 was not possible due to concentrations of large rocks, penetration of the 

 bottom to a depth of only 6 inches, the length of the exposed metal rod, 

 provided indicative readings. 



Electrode effectiveness . °°A series of tests were conducted in the 

 laboratory for the purpose of determining the most desirable metal and 

 material for use as electrodes in electromechanical sea lamprey weirs where 

 relatively large numbers of such elements must be employed. The tests 



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