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IN-wATER ELEcmicAL MEASUREMENTS 3 net value of resistance can be associated with every electrode. If two electrodes are constructed physically identical, they will also be electrically similar and exhibit the same value of electrode resistance. The total circuit resistance for two similar electrodes is then twice the value of a single electrode expressed in ohms. Hence, 6. Apply Ohm's Law to calculate the total circuit resistance: R(total) = V(volts) / I(A) ohms. 7. Apply equation 2 to calculate the resistance of a single electrode: R(individual electrode) = R(total) / 2 ohms. R(total) _ (2) 2 x R(individual electrode) ohms. Measurement Procedures The following procedures will determine the resistance for an electrode. These measurements may be conducted at any test site (lake, river, swimming pool, irrigation canal, etc.) judged real- istic for the anticipated operational situation. 1. Construct two identical electrodes or electrode arrays. 2. Immerse the electrodes in water to their in- tended operational depth. Be aware that the depth of water, the separation distance, and the surrounding substrate can alter the results. 3. Connect a power source of alternating current (AC) with appropriate multitesters (high imped- ance, analog or digital volt-ohm-amp meters) to measure the volts (V) and amperes (A) as shown in Fig. 2. If a grounded electrical source is used (e.g., household 120 volts AC), it is advisable to use an isolation transformer for the safety of personnel and the prevention of unwanted leak- age currents. The AC generators used to power electrofishing boats can also be used as an elec- trical supply, and since these generators are normally operated with the neutral electrical connection removed (this isolates the generator frame and boat hull from the power circuit), there is no need for an isolation transformer. 4. With power applied to the electrodes, increase the separation between the electrodes and note if the current changes. A significant variation in current indicates interspatial coupling be- tween the two electrode fields; the electrodes are not isolated and operating independently. It is usually desirable to measure electrodes having isolated electric fields. 5. To ensure isolation, separate the electrodes to a distance where the ammeter readings stabilize before recording the current and volts. It is not necessary to belabor this separation procedure because small variations in the current do not significantly change the results. 8. Record the electrical conductivity of the water. The above procedures specify an AC power source. Actually, other voltage waveforms can be used to measure electrodes if precautions are taken regarding the instrumentation. The resis- tance of an electrode is not changed by the applied waveform: the resistance is the same for AC, direct current (DC), pulsed direct current (PDC), or pulsed alternating current (PAC). The concern when making an electrical measurement is that the common multitesters are only designed to pro- vide correct readings with continuous DC or sinusoidal AC waveforms. Pulsed direct current or nonsinusoidal AC waveforms require special in- struments. Additionally, DC measurements pre- sent an uncommon problem. Conway (1965) de- scribed an unstable electrical phenomenon known as the Helmholtz effect when DC voltages are applied to metal electrodes, but the Helmholtz effect is avoided by using AC. Caution is advised when making resistance measurements in static water. Electrolysis forms gas bubbles around the electrodes, and the surface of an electrode can become partially insulated. This effect is minimized by quickly recording the voltage and current before many bubbles adhere to the surface. Under field conditions, these gas by- Isolation Transformer AC Power Source Air I Voltmeter Water Ammeter Electrode 1 Electrode 2 Fig. 2. Circuit configuration with metering for measuring electrode resistance.