The commutator used in the first phase of the experiments was 

 constructed to allow flexibility in the choice of duty cycle and repe- 

 tition rate.il/ This unit, which provided pulsed or interrupted DC, con- 

 sisted of a single switch driven by a controllable-speed motor through 

 a reduction gear and cam shaft. Four interchangeable cams were used to 

 provide duty cycles of 0.025, 0.10, 0.25, and 0,66. A variable trans- 

 former, supplying power to the motor, was used to adjust the cam shaft 

 speed from 1 through 5 revolutions per second with each revolution of 

 the cam shaft producing one pulse. Thus it was possible to check the 

 reactions of the test animals at 20 combinations of repetition rate and 

 duty cycle for each voltage level investigated. 



The results of the initial tests to determine the combination 

 of variables most effective in leading fish indicated the choice of 

 square-wave pulses at a duty cycle of 0.66 and a repetition rate of 3 per 

 second. In the second series of tests, conducted to determine the volt- 

 age gradients required to lead adult rainbow trout ( Salmo gairdneri ) of 

 different sizes, the repetition rate and duty cycle were held at the 

 above values, while the pulse voltage level was varied. In order to 

 facilitate the conduct of these and other tests, the coramutatoi' unit was 

 modified to produce pulses at a fixed duty cycle of 0,66, with provision 

 for easy selection of any repetition rate by means of a plug and jack 

 board, without the necessity of changing cams or motor speed. This se- 

 lection of repetition rate was accomplished by the use of cam and switch 

 assemblies, driven by a controllable-speed motor through a reduction 

 gear. These 5 cams produced 1 through 5 pulses per revolution of the 

 shaft. A revolution counter coupled to the cam shaft permitted the motor 

 speed to be adjusted to a value that resulted in one revolution of the 

 shaft per second. 



Electrode voltage and current could be measured and adjusted 

 in the steady-state condition before test specimens were placed in the 

 tank. Adjustment of the electrode voltage, (i.e., the peak voltage under 

 pulsed operation) was accomplished by means of the 220 volt AC variable 

 transformer. A switch in the output of the DC power supply and commutator 

 unit permitted the polarity to be reversed at will, thus reducing the 

 likelihood of conditioned response in the test animals. A neon bulb con- 

 nected directly across the electrodes provided a convenient visual check 

 of the operation of the power supply and the commutator and of the polar- 

 ity. Two red warning lights, energized by the contactor which supplied 

 power to the 220 VAC variable transformer, served as a constant reminder 

 of the existence of high voltages in the experimental area. A diagram of 

 the electrical equipment is shown in Figure 1. 



h/ The term duty cycle may be defined as the ratio of "on" time to total 

 Time between leading edges of successive pulses. This ratio is determined 

 by the cam configuration. Repetition rate (the number of pulses per se- 

 cond) is controlled by the speed of cam rotation. 



