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Relativistic corrections have been calculated for the Drell-Hearn-Gerasimov sum rule in the framework of the constituent-quark model. These corrections lead to additional absorption strength due to relativistic dipole currents in the case of a hyperfine interaction. The introduction of anomalous magnetic moments requires a subtraction of the dispersion integral at infinity. The additional effects of such anomalous moments, however, are numerically very small within the model.

The helicity structure function of the nucleon has been calculated for the constituent quark model and compared to the prediction of the Drell-Hearn-Gerasimov sum rule. The multipole decomposition of the sum rule shows large cancellations between different resonances. The small isoscalar-isovector contribution is related to the admixture of aD-state (“bag deformation”) in the nucleon's wave function. The calculations indicate a relatively slow saturation of this part of the sum rule with excitation energy.

It is shown that the anomalous magnetic moment of the electron, kappa e, is irrelevant for the experimental determination of the nucleon form factors ('Rosenbluth plot'). The large corrections claimed by previous authors are due to the neglect of the form factor associated with kappa e.

The influence of internal nucleon structure on meson exchange currents has been investigated, in the framework of the nonrelativistic constituent quark model and the MIT bag model with massless quarks. In comparison with the conventional theory of exchange currents, one finds different strengths for the model-dependent higher-order terms and no ambiguity between pseudoscalar and pseudovector pion couplings. Furthermore, the retardation and excitation effects in the many-body system have to be treated consistently, in order to preserve gauge invariance. In this way one obtains 'damping' factors, which differ in structure from the phenomenological form factors.

The relativistic pair current and contributions of nucleon isobars are evaluated in the constituent-quark model. Isovector charge and isoscalar current contributions change substantially compared to previous calculations. The magnetic form factor of the deuteron and the charge form factors of3He and3H are modifed significantly and the previously predicted large differences in the form factors of3He and3H are considerably reduced.

Due to the usual strong truncation of the basis in dynamical quark model calculations, the charge-current operator for the quadrupole (N→ Δ ) transition violates gauge invariance. It is demonstrated explicitly for the nonrelativistic constituent quark model that, in the standard 2ℏ ω -basis, calculations of the quadrupole transition form factor differ by an order of magnitude, depending on whether one uses the charge or current for the evaluation. Since this effect is related to the behaviour of the two operators under hermitean conjugation and time reversal, it is expected to be of importance also for other wavefunctions as well as in relativistic models.