In an apparatus for soldering the windings of electric machines to the commutators therefor having a single-phase magnetic-hydrodynamic pump with metal conduit and a bath with molten solder and heaters, the improvement wherein the magnetic core of the single-phase magnetic-hydrodynamic pump is embraced by a metal conduit disposed underneath the bottom of the bath, and the inlet of the metal conduit is connected to the bottom of the bath, while its outlet end is rigidly connected by means of an elbow with heaters to a casing fastened to the bottom of the bath, the casing being a hollow cylinder with conical bottom and a central hole, to the bottom of the casing there being attached by means of an easily releasable joint a changeable nozzle comprising an external cylindrical component which is a hollow cylinder, the upper end of the inner surface of which is chamfered with respect to the axis of the changeable nozzle unit and ends with a cover covering the case, and an internal cylindrical component of the changeable nozzle unit is chamfered and is parallel to the chamfered upper end of the internal surface of the external cylindrical component of the changeable nozzle unit.