If there is any nonuniformity of the magnetic field over the sample of metal used in a de Haas–van Alphen experiment, then the structure in g(E) will reflect this variation. Different regions will have maxima in g(E) at different field strengths, and the susceptibility, which sums contributions from all regions, may lose its oscillatory structure. To avoid this, any spatial variation δH in the field must lead to a variation δEν that is small compared with Eν+1−Eν for the extremal orbits. Using the fact that ∂A(E,kz)/∂kz vanishes for the extremal orbits, calculate ∂Eν(kz)/∂H from (14.13) for an extremal orbit. Deduce from this that to preserve the oscillatory structure the field inhomogeneity must satisfy
HδH<AΔA,(14.25)
where ΔA is given in (14.12).