  Different models for conserved two- and three-body electromagnetic currents
are constructed from two- and three-nucleon interactions, using either
meson-exchange mechanisms or minimal substitution in the momentum dependence of
these interactions. The connection between these two different schemes is
elucidated. A number of low-energy electronuclear observables, including (i)
$np$ radiative capture at thermal neutron energies and deuteron
photodisintegration at low energies, (ii) $nd$ and $pd$ radiative capture
reactions, and (iii) isoscalar and isovector magnetic form factors of $^3$H and
$^3$He, are calculated in order to make a comparative study of these models for
the current operator. The realistic Argonne $v_{18}$ two-nucleon and Urbana IX
or Tucson-Melbourne three-nucleon interactions are taken as a case study. For
$A$=3 processes, the bound and continuum wave functions, both below and above
deuteron breakup threshold, are obtained with the correlated
hyperspherical-harmonics method. Three-body currents give small but significant
contributions to some of the polarization observables in the
$^2$H($p,\gamma$)$^3$He process and the $^2$H($n,\gamma$)$^3$H cross section at
thermal neutron energies. It is shown that the use of a current which did not
exactly satisfy current conservation with the two- and three-nucleon
interactions in the Hamiltonian was responsible for some of the discrepancies
reported in previous studies between the experimental and theoretical
polarization observables in $pd$ radiative capture.
