  We have made the first absolute, reference-free measurement of the 1s2s 3 S1
$\rightarrow$ 1s 2 1 S0 relativistic magnetic dipole transition in He-like
sulfur. The highly-charged S ions were provided by an electron-cyclotron
resonance ion source, and the x rays were analysed with a high-precision double
crystal spectrometer. A transition energy of 2430.3685(97) eV was obtained, and
is compared to most advanced bound state quantum electrodynamics calculations,
providing an important test of two-electron QED effects and precision atomic
structure methods in medium-Z species. Thanks to the extremely narrow natural
linewidth of this transition, and to the large dispersion of the spectrometer
at this energy, a complementary study was also performed evaluating the impact
of different silicon crystal atomic form factor models in the transition energy
analysis. We find no significant dependence on the model used to determine the
transition energy.
