1. Experiments with moving electrostatic and magnetic fields show that Gymnarchus niloticus is sensitive to a potential gradient of about 0.03 µ V./cm.

2. Alternative explanations of some previous experiments are given in terms of this high d.c. sensitivity.

3. An explanation in similar terms is given of experiments in which Gymnotus carapo is trained to detect a stationary magnet.

4. The mechanisms available for the location of objects by electric fish are reviewed. It is concluded from the results of a critical experiment (described in a succeeding section) that Gymnarchus niloticus can detect objects by the disturbance of its own electric field in the water.

5. The approximate theory of this method of object location is derived. The effect on the receptors of the perturbing field due to an object depends on the electrical properties of the receptors: in the extreme cases the stimulation of the receptors is proportional either to the potential or to its second derivative. Graphs are given showing the effect of an object on the potential and on its second derivative around the surface of the fish.

6. Experiments are described using Gymnarchus niloticus which (a) confirm that the mechanism of object location employs electric field distortion; (b) indicate the limits of the sensitivity of the fish.

7. The second derivative mode appears to be the most probable one operating in Gymnarchus. The experimentally determined limits of detection are discussed in relation to the random noise in the receptor circuit: it is concluded that both spatial and temporal integration are likely to be employed.

8. The thresholds for object location and for response to direct currents are compared: it is concluded that the same receptors are probably operating in both cases.

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