corresponding current density (J) at a resistivity 

 (p) of 18. 68 ohm -cm. is J = E/p = 0. 0032 

 amp. /cm. . For a tjink of cross section 11x4 

 feet, the total current (JA) will then be 130 amp. 

 This is, of course, the total peak current during 

 an on-period. Assuming the electrodes will be 

 spaced a distance (/ ) of 33 feet, the voltage be- 

 tween electrodes will be E 1! =60 volts. As the 

 experiments were to be conducted on tuna and 

 other fish greater than 30 cm. in length, a 

 current of about 130 amperes at a potential of 

 about 60 volts was considered the maximum 

 requirement. As this current will be "on" for 

 only 6 percent of the time, the power requirement 

 is a modest 470 watts. 



Unfortunately a current as large as 130 

 annperes cannot be handled as simply as that of 

 the 18-ampere maximum in the preliminary 

 experiments. A major problem is arcing at the 

 contacts on breaking the circuit; a less serious 

 problem is obtaining a current source of this 

 magnitude. These difficulties can be overcome 

 by charging and discharging capacitors, a prin- 

 ciple employed by Kreutzer and Peglow (Cattley 

 1955b). 



The charge and discharge cycle of the 

 capacitor may next be considered. With a pulse 

 frequency of 10 c.p. s., there is 0.1 second 

 available for the entire cycle of charge and dis- 

 charge corresponding to a single pulse. Now 

 the time-constant, T, of this circuit is defined 

 as the amount of time necessary for the current 

 to decrease to 1 /e of the original value, where 

 e ■= 2. 72 (the base of the natural logarithm). 

 Thus, if the original current is 130 amperes, 

 cLfter a time T it will be 130/e =48 amperes, 

 still too large a current to be broken by simple 

 contacts. After a time 2T the current will be 

 48 /e = 18 amperes, and after a time 3T it will 

 be 18/e = 6.5 amperes. This current is suffi- 

 ciently small to break by simple contacts with- 

 out excessive arcing. So we see that, during 

 the discharge, contacter No. 2 must stay closed 

 for a period of time equal to two to three times 

 the tinne-constant. The time-constant with the 

 previously calculated values of R and C is 0.016 

 seconds. Three times'this is 0.048 seconds, or 

 slightly less than half of the cycle available for 

 charging the capacitor. We have, then, 0.05 

 seconds to charge a capacity of 35, 000 micro- 

 farads to a potential of 60 volts. 



A schematic diagram of a system utilizing 

 capacitor discharge is shown in figure 4A. 

 Initially contactor No. 1 is closed and No. 2 

 open, thus charging the capacitor to a voltage 

 approaching that of the source. Then contactor 

 No. 1 opens eind No. 2 closes, discharging the 

 capacitor through the tank. 



The discharge pulse is not, of course, a 

 square wave, but rather an exponential decay 

 as shown in figure 2B. McMillan et al. (1937) 

 have shown that this exponential-decay pulse is 

 equivalent in its physiological stimulus value 

 to a square pulse of the same amplitude pro- 

 vided the duration of the square pulse is 37 per- 

 cent of the tinne-constant of the decay-pulse. 

 (The time-constant, T, of an exponential decay 

 is the product RC, where R is the resistance 

 of the discharge circuit in ohms and C is the 

 capacity of the capacitor in farads.) Assuming 

 that this relationship is applicable to the present 

 situation, we can obtain an exponential-decay 

 pulse equivalent to a 6-millisecond square 

 pulse by letting . 37RC = .006, where R is 

 the resistance of the column of water between 

 the electrodes, and C is then the required 

 capacity. As R = p//A = 0.46 ohms, C may 

 be calculated at 0.035 farads, or 35,000 

 microfarads. This is an extremely large 

 capacity, but in view of the low working voltage 

 of about 60 volts, it is not a difficult capacity to 

 obtain by means of banking small capacitors in 

 parallel. 



The resistance of the charging circuit, 

 which includes the internal resistance of the 

 batteries, nnust be sufficiently low so that the 

 capacitor may become very nearly fully charged 

 in this time. To satisfy this condition the 

 batteries should be connected in series-parallel 

 as shown in figure 4A and heavy connecting 

 wires and terminals used. 



An apparatus was constructed to 

 approximate the requirements, as deduced 

 above, to effect electrotajcis in tuna, 30 cm. or 

 more in length, when confined in the large tank. 

 The power source consisted of twenty 6-volt 

 automobile storage batteries connected in series- 

 parallel (two banks of 10 each). The power 

 source (using any number up to 10 pairs of 

 batteries) was used to charge a bank of fifty-five 

 1,000 mfd. (150 volt) capacitors connected in 

 parallel to give a total capacity of 55, 000 mfd. 

 A motor-driven, variable-speed mechanical in- 

 terrupter was constructed with two cams, 180* 

 out of phase, operating two spring-loaded 

 contact points (fig. 5). 



The electric apparatus was connected by 

 heavy electric cables to two plane electrodes 

 (11x4 feet) fitted to each end of the tank, with a 

 reversing switch included in the circuit. The 

 electrodes, initially separated by a distance of 

 33 feet, consisted of a series of vertical copper 

 wires soldered at intervals of 4 inches along a 

 horizontal brass strip. The system was 



