Theorem of frozen-in field expounds the law of ideal conductive fluid and magnetic field moving together, namely (1) Kelvin's theorem: the conservation of magnetic induction flux through the area enclosed by any closed curve moving with ideal conductive fluid; (2) Helmholtz Theorem: In an ideal conducting fluid, the fluid element that is initially on a certain magnetic field line always lies on this magnetic field line. These two theorems are similar to the two theorems of the same name for the motion of vortices in fluids.
