ept of direct sensitivity was therefore replaced by one of indirect sensitivity to the earth’s magnetic field, and the whole hypothesis was resurrected (Lincoln 89). In the year 1947, Yeagley suggested that the flying bird, which acted as a linear conductor moving through the lines of force field, could detect the earth’s field. Theoretically this would result in a small potential difference being set up between the two ends of the conductor, though at this time had not been demonstrated in practice (90). While the theoretical case against the detection and measurement of the earth’s magnetic field by indirect methods is overwhelming, a good deal has also been done to test the hypothesis from a practical point of view. When dealing with certain biological systems the results of such experiments are always more convincing than physical arguments that may be based on false premises (Carthy 112). Griffin reported two techniques aimed at disturbing an electro-magnetic apparatus in 1940. The first passed electric currents through the heads of Pigeons before the release and the other subjected Leach’s Petrels to an intense electro-magnetic field for a few seconds before the beginning of the outward journey from home (Griffin 61). In both cases no effects on homing were apparent but the techniques were not very critical as it is really required that the bird should be subjected to ‘interference’ during the actual flight (62). Fixing magnets rigidly to the head will not be a satisfactory test since the additional field would be constant which could be taken in to account by the analyzing mechanism. It is therefore essential that the magnets should move relative to the bird’s body. It was imperative to attach small, powerful magnets to the wins of Pigeons, sewing them on through the metacarpal joints (70). The fluctuating e.m.f. induced in the bird’s body when the wings were beating woul...
