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IN-WATER ELECTRICAL MEAsummEws 7 electric field is adequately defined by the distribu- tion of the voltage in the water, and these voltages are not difficult to measure and plot on a graph. To better understand the mechanism by which volt- age patterns are created in the water, it is enlight- ening to present a quasi-technical discussion that illustrates the significance of the metal-to-water interface in the creation of any three-dimensional electric field. Quasi-technical Concepts for Electric Fields in Water All electrofishing arrays have two similarities: the electrodes are constructed with high conductiv- ity metals, and the water surrounding the elec- trodes exhibits a much lower electrical conductivity than the metal. This difference in conductivity (con- ductivity of metals is typically 1012, whereas that of fresh water is about 103 p,/cm) necessarily de- scribes an electrical circuit having most of its resis- tance associated with the water, the conductivity of the metal is simply too high to contribute a signifi- cant resistance compared with the water. Addition- ally, the water is a homogeneous material; the electrical characteristics of the water near the elec- trode are the same as the water at some distance from the electrode. These facts imply that, some- how, the geometry at the metal-to-water interface (or interfaces in the case of multiple electrodes) must be responsible for the creation of eleotric fields having different spatial characteristics. It is instructive to consider a long, smooth-metal cylinder and to imagine the volume of water sur- rounding this electrode as being divided into a large number of identical cubes. Since the water is homo- geneous, each cube exhibits the same electrical resistance; there is nothing uniquely different about any particular cube of water. This mental process converts the volume of water into a network of equally valued resistors, and we can now specu- late how the electrical currents must conduct through this maze of resistors and generate an electric field. Due to its high electrical conductivity, the entire length of the metal cylinder is considered energized to the same potential of voltage; there is no differ- ence in voltage along its length. The energy loss within the cylinder may then be assumed negli- gible, and each incremental length of the cylinder is electrically identical to the next. These assump- tions allow us to examine a single, incremental cross section of the cylinder and to direct attention to the flow of the electrical current at the metal-to- water interface. Figure 6a shows a cross-sectional view of an incremental length of the cylinder with the cubes of water aligned around the cylinder in concentric rings. Note that the coaxial (circular) symmetry of the cubes precludes any current flow in a circumferential direction, all the current moves in a radial direction away from the cylinder. Each cube of water may now be replaced by its symbol for electrical resistance to give an indication of the in-water wiring (Fig. 6b). In studying Fig. 6, note that the electrode's sur- face area limits the number of cubes that may actually have a direct metal contact. However, as the distance from the electrode is increased, a greater number of cubes enter into the electrical circuit. Observe how the cubes in a particular Electrical resistance of a cube of water Fig. 6. Diagrams illustrating the metal-to-water interface for a cylindrical electrode: (a) Cross section of the electrode surrounded by squares that represent the circular alignment of cubes of water around the electrode. (b) Electrical resistors are substituted for the individual cubes of water to create a visualization of the in-water electrical circuit. Cubes of water