Birkhoff's theorem in electromagnetism is a theorem about a particular type of solution of Maxwell's field equations of electromagnetism.
More precisely, it states that any spherically symmetric solution of the source-free Maxwell equations is necessarily static. Pappas (1984) gives two proofs of this theorem.
References
- Richard C Pappas, Proof of "Birkhoff's theorem" in electrodynamics, Am. J. Phys., 52 (3) 1984, 255-256.