Published online by Cambridge University Press: 11 May 2009
In recent years attention has been given to the influence of electrical currents as fish deflectors or screens, and experiments have been made to determine the magnitude of the currents which are sufficient to paralyse or act as deterrents to fish which may enter the electrical field, but in such strong fields the perception of the mechanism by which fish respond to electric currents is masked by the paralysis produced. It seemed possible that experiments with very weak currents might give fuller information concerning the mechanism by which fish respond to electrical stimulus than those made in strong fields. There is also the possibility that weak electric currents may occur in nature to which fish respond, but before exploring that field of phenomena it is necessary to observe the behaviour of fish under the influence of very weak fields. McMillan found that the field (volts per inch) required to paralyse young salmon varied with the resistivity of the water; thus when the latter was 10,000 ohms per inch cube the paralysing voltage was about 1·5, so that a current density of ampere per square inch was sufficient to cause paralysis, whereas when the resistivity was twelve ohms per inch cube the current required was ·03 ampere. In the former case the conductivity of the fish was greater than that of the water, in the latter case less.
page 416 note * McMillan, F. O., “Electric Fish Screen,” Bulletin of U.S. Bureau of Fisheries in Washington, 1929, Vol. XLIV, 1928, p. 97, with current alternating at 60 periods a second.Google Scholar
page 417 note * Note by Professor W. M. Thornton: If the current i is taken as spreading out from small cylindrical electrodes the density midway between them is approximately i/πDt, where D is their distance apart and t the depth of the liquid. With i=0·2 ampere, D = 250 cm., t=44 cm., this is equal to 5·8 microamperes. If the current is taken as spreading from strip elements of two flat plates 30 cm. wide and the effect is integrated over their width, the density midway is 19 microamperes. A more complete treatment, assuming the electrodes cylindrical and relatively large (see Pidduck, “Electricity and Magnetism,” §46. See also McMillan, F. O., “Electric Fish Screen,” Bulletin of the U.S. Bureau of Fisheries, Vol. XLIV, 1928, Fig. 12, p. 117), gives the estimated values 15 to 45 microamperes persq. cm.Google Scholar
page 420 note * Vide “Electrical Perception by Deep Sea Fish,” Thornton, W. M.. University of Durham Philosophical Society. Proc, Vol. 8, Part 4.Google Scholar
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