  I extend the well-known photonuclear sum rule that relates the strength of
the photoexcitation of the giant dipole resonance in a nucleus to the number of
elementary scatterers-nucleons to the case of virtual photons. The new sum rule
relates the size of the magnetic polarizability of a nucleus to the slope of
the transverse virtual photoabsorption cross section integrated over the energy
in the nuclear range. I check this sum rule for the deuteron where necessary
data is available, discuss possible applications and connection with other sum
rules postulated in the literature.
