An objective account of the action of a Stern-Gerlach apparatus on spin-1/2 particles is given, using the Dirac equation. This generalizes earlier work on a causal interpretation of the Pauli equation to the relativistic domain, leading to a more natural choice for the current in the model.

A. D. Challinor, A. N. Lasenby, S. F. Gull and C. J. L. Doran, **A relativistic, causal account of spin measurement, ***Phys. Lett. A* **218**, 128-138 (1996)