ELECTRIC AND MAGNETIC SENSES 



523 



The stingrays were tested in a circular fiberglass pool, surrounded by a 

 light-tight, 12-sided hut devoid of ferromagnetic materials (Figure 9). The 

 pool measured 1.8 m in diameter and rested on Teflon blocks to insulate it 

 from ground. It was filled with natural seawater to a depth of 15 cm. Coarse 

 sand covered the bottom, and two air-stones maintained a slow internal 

 circulation. The seawater was kept at 20° C by regulating the air temperature 

 in the hut. To control the horizontal component of the earth's magnetic 

 field in the tank, two north-south oriented Helmholtz coils, each 5 m in 

 diameter, were erected outside the hut. We chose for our experiments the 

 horizontal induction of the Southern California region from which the 

 animals were taken (0.26 G). 



During the 1- to 2-h training sessions, twelve concealed incandescent lights 

 illuminated the ceiling over the experimental tank to produce an even, 

 low-level light distribution. For the rest of the day, the lights were 

 programed to simulate the sunshift and daily variation in the brightness of 

 the sky in accordance with the direction of the magnetic field (either normal 

 or reversed). Lights, heaters, and coils were d.c. powered from distant 

 voltage and current sources. The wiring was tightly twisted and judiciously 

 installed to keep unwanted electric and magnetic fields from the animals' 

 habitat. Extraneous noise levels were low at our test site in the undisturbed 

 woods of the Quissett campus. 



Figure 9 Magnetic test facility on the Quissett campus of the Woods Hold Oceanographic 

 Institution. The two large, north-south oriented Helmholtz coils control the horizontal 

 component of the magnetic field in the hut. The actual training experiments are con- 

 ducted in the top tank. The lower pool holds extra stingrays in reserve. (After Kalmijn 

 1977b.) 



