A master-oscillator multiple-detector system for following the movement of adult salmon through a series of underwater orifices is described. The fact that fish exhibit greater conductivity than fresh water has been known for many years, but this is the first known application to American fisheries instrumentation. A sinusoidal audio-frequency oscillator supplies individual conductivity bridges; the unbalance due to fish passage, amplified, operates a relay; the make-break differential of the relay supplies an annunciator action, appropriately switching lights at a central control rack, showing the biologist the orifices chosen by each fish. Reset of the detector advances an electromechanical register.
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