Figure 1. The experimental set up.

(a) The electric circuit that delivered a low electric current into the experimental tank via electrodes. The strength of the current was varied with the two variable resistors (b) Schematic drawing of the transparent rubber mat on which dipoles were mounted that delivered electric fields on the bottom of the tank. Double layers of rubber are indicated with dashed lines. Cables were protected by these layers to prevent entanglement of the animals. The two plastic tubes of one dipole were oriented approx. perpendicular to the centre of the dipole to avoid the build up of electric fields along the tubes.