ELECTRIC FISH SCREEN 



101 



terminals of an insulating transformer. The insulating transfoi-mer was used to in- 

 sure against leakage to ground from one plate, due to the grounded neutral of the 

 110-220 volt lighting circuit. A variable voltage from zero to the maximum value 

 required was obtained by the use of a resistance potentiometer supplying the primary 

 of the insulating transformer. A picture of this apparatus as set up is shown in 

 Figure 1. This arrangement of parallel plates supplied from a variable voltage 

 supply makes it possible to obtain a uniform voltage gradient in the aquarium of 

 any desired value from zero to the maximum necessary in these experiments. All 

 of the tests described in this report, unless otherwise specified, were made with 

 60-cycle alternating current. The voltages and the voltage gradients are root mean 

 square values. 



Fia. 5 



The method of determining the paralysis voltage was to place a number of fish 

 selected for uniformity of size in the aquarium between the parallel plates and raise 

 the voltage in small increments, holding each increment one minute. When the 

 first fish became paralyzed the plate voltage was recorded as the minimum paralysis 

 voltage. The increase in voltage was continued until the fish were paralyzed and 

 the plate voltage again recorded, this time as the maximum paralysis voltage. From 

 these voltages and the known plate spacing the minimum and maximum voltage 

 gradients per inch to produce paralysis were calculated. The lengths of the fish 

 used in the experiment were measured carefully then from the tip of the snout to 

 the end of the middle rays of the caudal (tail) fin. These measurements were made 

 in inches. The average length of the test group was calculated and recorded. These 

 tests were repeated on many fish ranging in length from 1.87 to 31.75 inches. A 



