used in a continuous flow, in others it was interrupted by a 

 specially-desxgned interrupter consisting essentially of 

 revolving disks with different proportions of brass and 

 bakelite, the current being "on" when two brushes were in 

 contact with the brass sector j and "off" when the two brushes 

 were in contact with the bakelite sectoro Series of experi- 

 ments were conducted with various currents and with vaiaous 

 frequencies of interruption at each currento In other series 

 of experiments the "on -off" ratio was changed^, using disks 

 which allowed the current to flow for Oo75s Oo50, and Oo25 

 of one comjjlete revolution. To maintain a source voltage of 

 220-230 volts, he found it necessary to design a special type 

 of electrode— a carbon rod enclosed in a plastic tube with 

 open ends and with holes bored through the sideSo By adjust- 

 ing the exposure of the carbon rods to the seawaterj, it was 

 possible to vary the current in the system^, but at the same 

 time to maintain the voltage at the soui'ceo 



Morgan's results showed (1) that frequency of interrupt- 

 ion was not critical for the species and size-range of fish 

 used — -about the same response was evoked for frequencies 

 between l5 and 2$ ropoSo, and (2) that, within the limits of 

 his experiments, the shorter the pulse duration the smaller 

 the average (interrupted) current required to give the 

 desired response— decisive and rapid movement from the center 

 of the tank to the positive poleo To illustrate, positive 

 response (an entire time period of 60 seconds spent by. all 

 fish tested in the half of the tank adjacent to the positive 

 pole) was obtained v/ith an average current of 8 amps and an 

 on-fraction of Oo75ii with an average current of 5 amps and an 

 on -fraction of Oo^O, and with an average current of 3 to U 

 anps and an on-fraction of Oo25o The corresponding current 

 densities may be calculated at OoOOii?? Oo0029, and Oo002i;- 

 O0OOI8 amps/cm^, respectivelyo Ihus^ as the on-fraction of 

 a pulse was reduced, there was a decrease in the average 

 current require., to attract the fish and a corresponding sav- 

 ing in power o 



In extending Morgan's workj two lines of investigation 

 suggested themselves, (1) to determine the relationship betwee:: 

 source voltage and electrode size^ and (2) to further decrea^:e 

 the on-fraction to determine the minimal current which would 

 evoke positive response o 



SOURCE V0LIAG3 AND ELSGTRODE SIZE 



The wiring hook-up of the apparatus is shown in figure' 

 18 The source of power is an Onanj, ^-cylinder^ gas-driven, 

 air-cooled, direct current generator unit with a maximum out- 

 put of 5000 watts C2I08 amps at 230 volts) o The source 



70 



