Experiments on the magnetic field of nerve action potentials

Records of the magnetic field associated with the propagating action potential in a frog sciatic nerve have been obtained. Peak fields of about 60 picotesla are measured by a system utilizing a SQUID magnetometer, signal averaging, and a novel ferrite-core toroidal transformer. Design considerations for the latter are given and, for example, indicate that it need not have a large number of turns. A method is presented to obtain the net electric current threading the toroid by deconvolution of the magnetometer signal. This routine allows detailed magnetic or current profiles to be obtained of compound neural events.