  Within the framework of an idealized theoretical model, we study the effect
of external static homogeneous exchange and magnetic field on the spin part of
the singlet wave function of two electrons. We begin by revising the
traditional (text- book) approach to the spin singlet. Basing our own approach
solely on the property of invariance under rotations of the coordinate system
and using the theory of spinor in- variants, we derive a generalized
representation of the spin singlet whose main feature is that the spins are in
mutually time-reversed states. We show that exactly this fea- ture
predetermines the actual form of the Hamiltonian of interaction with the
external field and stipulates time evolution of the singlet. Some applications
of these results to the theory of superconductivity and spin chemistry are
presented. In particular, it is shown that the case of ferromagnetic
superconductors constitutes a good illustration of the validity of our
quantum-mechanical consideration.
