While most means of converting AC to DC involved the use of vacuum tubes or semiconductors, for a time, there was a third way: the Snook mechanical arc rectifier, which was used in conjunction with an early X-ray machine. A motor spins an armature at 3600 RPM, and four pairs of metal shoes surround the armature such that at the right time, an arc will form between one of the AC shoes and the armature, and from the other side of the armature to the corresponding DC shoe. Pretty interesting to see in action.