It was based on my experimental Ph.D. research studying the effects of mechanical stress on the anomalous eddy-current losses found in iron and in nickel. Essentially, I reasoned that magnetic inductance energy can be stored in the vacuum, a factor which told me that there has to be something in that vacuum that reacts and becomes itself polarized by the presence of a magnetic field. Now do keep in mind here that advanced physics texts dealing with magnetic field energy need to take account of the fact that magnetic field energy has a negative potential. A magnetic state involves negative potential and iron is ferromagnetic below its Curie temperature simply because the magnetic field produced by the collective action of certain of its atomic electrons has a negative magnetic potential that is not outweighed by the accompanying mechanical strain energy set up by electrodynamic interaction forces acting between those particular electrons. The modulus of elasticity, whether for iron, nickel or cobalt, is quite high, meaning that the powerful stresses that accompany the ferromagnetic state involve less strain and so less strain energy, energy which is at a positive potential.
Energy deploys in a physical system in seeking a minimum potential state.
It so happens that, for iron, nickel and cobalt, this is the ferromagnetic state, as I show in my book Physics without Einstein . Now, as to superconductivity, I did not wake up to the link with the supergraviton until many years on from that 1969 book, though gravitons do feature in that book because it describes a unified theory linking gravity and electromagnetism. The supergraviton comes into being only in the presence of heavy molecules and it is the interplay of such molecules with electrons, as they carry a current flow, that brings us into the realm of superconductivity. In that 1969 book I did explain why electrons can avoid shedding energy by radiation as they progress through a conductive material and even pointed to the fact that it was anomalous that uranium 235 changes from superconductive to the normal conductive state at a lower temperature than does uranium 238. That is discussed on p. 16 of the book.