Introduction

The use of magnetic fields as a tool for influencing biological processes, which historically began with attempts to treat human disease, has had a long but checkered record. For many centuries following the discovery of the naturally magnetic material, magnetite (Fe2O3), the purported effects associated with this material were surrounded by superstition. In the first century AD, Pliny the Elder wrote about the apparently magical powers of ‘lodestone’, as magnetite was called then, such as the ability to heal the sick. The credible, scientific study of the biological effects of magnetism, however, has begun only in this century, and only in the 1960s were the first surveys of the laboratory evidence published (Barnothy, 1964, 1969).

Beginning in the 1970s, it was established that several animal species such as pigeon, salmon and honey bee were sensitive to even weak magnetic fields such as that of the Earth (for an overview, see Kobayshi and Kirschvink, 1995). This represents a remarkable sensitivity, since the magnetic flux density (B) of the Earth's magnetic field measured in units of tesla (T) is only about 50 microtesla (μT). For comparison, the magnetic field associated with a small, 1-cm toy magnet would be 1000-fold greater, for example, B≃50 millitesla (mT). In elegantly designed studies, scientists revealed that pigeons, salmon and bees were capable of sensing geomagnetic field lines as a way to orient themselves in their environment. The discovery of small amounts of magnetite in the biological tissue of these animals pointed to the role of magnetite as a potential element of a biological ‘compass’ for magneto-orientation.