Prototype of one of the first efficient electric motor/generators. The ring armature consists of a toothed wheel comprising a very large number of iron laminations. Between its teeth are wound the coils, in 16 separate sections. The coils are activated in turn by a commutator mounted on the vertical spindle above the armature. The ring lay in the magnetic field of the two upright fixed electromagnets and the current passed through the 16 coils around the ring. The apparatus was heavily restored after the 1966 flood.

Antonio Pacinotti described his continuous-current motor in 1863, which was then taken up and series-produced by Zénobe-Théophile Gramme c. 1870. By replacing the field electromagnets with permanent ones, Pacinotti found that his engine became a generator (dynamo). A copper plaque carries the words: "Macchinetta elettromagnetica / Donata da Antonio Pacinotti" ["Small electromagnetic machine / Donated by Antonio Pacinotti"].