The mechanical interrupter consisted of a 

 motor -driven cam operating a spring -loaded 

 contact point. The cam consisted of eight iden- 

 tical lamina s o that the pitch of the cam and 

 hence the on -fraction was controlled by the ro- 

 tations of the lamina relative to each other. 

 When the lamina were all lined up, the 

 on-fraction was 0.02. The pulse recur- 

 rence frequency was controlled by a variable 

 speed drive between the motor and cam. 



Since the electrodes completely covered one 

 end of the tank it was expected that the current 

 density and hence the electric field would be 

 uniform throughout the cross -section of the tank. 

 Measurements confirmed that this was so. 



The wave shape was checked by means of an 

 oscilloscope connected across the electrodes and 

 was found to deviate slightly from a square wave . 

 The top plateau sloped slightly downward as 

 shown in figure 2A. 



Procedure and Results 



positive for 5 seconds and then the polarity was 

 reversed for another 5 seconds. Observations 

 of the fish's behavior were then recorded along 

 with the lengths of the fish, the total peak 

 current producing the shock, the pulse frequency, 

 the on-fraction, and the water temperature. 

 The total peak current was the total current 

 flowing through the tank when the interrupter 

 contacts were closed. 



It was difficult to classify the fish behavior 

 into a fixed group of categories, but the following 

 classification was used. A "perfect" response 

 was one in which the fish immediately oriented 

 itself and swam directly and rapidly towards the 

 positive electrode when the current was turned 

 on. When arriving at the protective barrier, the 

 fish continued to swim against the barrier until 

 the current was turned off. Upon reversal of 

 the current, the fish immediately turned about 

 and swam towards the opposite end as before. 

 The fish, in a "perfect" response condition, 

 easily reached the opposite end of the tank in 

 less than 5 seconds. 



During the first series of tests two fish 

 were in the tank during each test. In following 

 series, four fish were used. The fish, when not 

 being shocked, tended to stay away from the end 

 of the tank where the reversing switch was 

 operated. To perform a test, the end of the tank 

 farthest from the location of the fish was made 



(A) 



37%RC 



T=RC 



t , sec. 



Figure 2. --Pulse shapes. A. Nearly square 

 wave used in experiments with aholehole. 

 B. Wave of capacitor discharge used in 

 experiments with tuna and other large 

 pelagic fish; and equivalent square wave 

 (for explanation see text). 



Such perfect response Avas not frequently 

 encountered. Very frequently the fish would 

 behave erratically either at the instant the switch 

 was closed or when it reached the barrier screen. 

 This consisted of swimming very rapidly in a 

 circle once or twice. Otherwise the directional 

 swimming was as good as in the "perfect" case. 

 This response was called "good. " Also classi- 

 fied as "good" was the response in which there 

 was no erratic swimming but the directional 

 swimming was slow and the fish barely reached 

 or fell short of reaching the opposite end of the 

 tank in 5 seconds. 



If the fish swam only half the length of the 

 tank or 1 e s s in 5 seconds, the response was 

 called "fair." Or, if the erratic swimming was 

 considerable but with a tendency to swim to- 

 wards the positive electrode, the behavior was 

 also considered "fair." 



Finally, no swimming at all or only very 

 erratic swimming was called "poor." Since 

 there were two or more fish in a test it was not 

 always possible to classify the behavior of each 

 individual fish, but rather the "average" behav- 

 ior of the group was classified according to the 

 above scheme. 



The first series of tests (table 1), each 

 employing two fish, were exploratory in nature 

 but were designed chiefly to determine the opti- 

 mum frequency of interruption over a range of 

 o n -fractions . Disregarding on-fraction, a 

 "satisfactory" response (either "perfect" or 



