ELECTBIC FISH SCREEN 113 



DIRECTION OF THE ELECTRIC FIELD WITH RESPECT TO THE PROTECTED OPENING 



AND THE STREAM FLOW 



The experience with the four electric screens tested in the concrete pool and the 

 tests conducted in the aquarium demonstrated clearly the importance of considering 

 the direction of the Hues of current flow and the equipotential surfaces in the water. 

 The Unes of current flow and the equipotential surfaces for two parallel gratings of 

 opposite polarity and consisting of parallel cylindrical electrodes immersed in water 

 or any electrolyte of uniform resistivity are shown by Figure 10. The Unes of cur- 

 rent flow originate in one electrode and temiinate in the one of opposite polarity. 

 The Unes of current flow in Figure 10 are drawn to include one-thirty-sixth of the 

 current from one electrode between adjacent liiaes of current flow. The equipotential 

 surfaces start as eccentric circular tubes about the electrodes, change to elUptical 

 tubes, then to curved surfaces passing in front of the electrodes along the plane of 

 the grating, gradually straightening out into a plane surface midway between the 

 gratings. Figure 10 is drawn as a plane surface at right angles to these equipo- 

 tential surfaces, hence they appear as Unes in the drawing. These equipotential 

 surfaces are drawn so that one-fortieth of the potential between the gratings is 

 included between adjacent equipotential surfaces. Such a graph is very helpful 

 in the study of the electric field between gratings, because the distance between the 

 lines of current flow is a measure of the current density, and the distance between 

 the equipotential surfaces is a measure of the rate of change of potential or voltage 

 gradient. The nearer the lines of current flow are together the greater the current 

 density, and the nearer the equipotential surfaces are together the higher the voltage 

 gradient. 



It should be noted that the Unes of current flow and the equipotential surfaces 

 always intersect at right angles. The equipotential surfaces, as the name indicates, 

 are surfaces in which there is no change in potential. It is obvious, then, that a 

 fish swimming in electrified water parallel with the equipotential surfaces is subjected 

 to a potential difference only equal to that spanned bj'^ the thickness of his body, and 

 the current that flows through his body is from side to side at right angles with the 

 spinal column and major nerve channel. On the other hand, a fish swimming in 

 electrified wp^ter at right angles with the equipotential surfaces and parallel with the 

 Unes of current flow is subjected to a potential difference equal to that spanned from 

 the tip of his snout to the end of his tail, and the direction of current flow is length- 

 wise through the body in the direction of the spinal column and major nerve channel. 

 The ratio of the length to the thickness of a fish is very large for most species. Then, 

 in consideration of what has been said about the two positions of a fish in an electric 

 field, it is obvious that when at right angles with the equipotential surfaces and parallel 

 ■with the Unes of current flow he wiU be subjected to a potential difference several 

 times that when he is at right angles to this position. Furthermore, in this position 

 the direction of current flow is in the direction of the spinal column and main nerve 

 channel and is probably much more effective in producing a disagreeable sensation. 



In all of the tests when moderate values of potential were used (so the fish 

 were not in too great distress) there was always a decided tendency to Une up with 

 the equipotential surfaces. This is, of course, the most comfortable position if 

 there is no way to escape from the field entirely. 



