This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
In the part to which we refer, Dr. Pacinotti states that it occurred to him that the value of the apparatus would be greatly increased if it could be altered from an electro-magnetic to a magneto-electric machine, so as to produce a continuous current. Thus, if the electro-magnet, A B (Figs. 3 and 4), be replaced by a permanent magnet, and the annular armature were made to revolve, the apparatus would become a magneto-electric generator, which would produce a continuous induced current always in the same direction, and in analyzing the action of such a machine Dr. Pacinotti observes that, as the position of the magnetic field is fixed, and the iron armature with its coils rotates within it, the action may be regarded as the same as if the iron ring were made up of two fixed semicircular horseshoe magnets with their similar poles joined, and the coils were loose upon it and were caused to rotate over it, and this mode of expressing the phenomenon was exactly what we adopted when describing the Gramme machine, without having at that time seen what Dr. Pacinotti had written fifteen years before.
In explanation of the physical phenomena involved in the induction of the electric currents in the armature when the machine is in action as a generator, Dr. Pacinotti makes the following remarks: Let us trace the action of one of the coils in the various positions that it can assume in one complete revolution; starting from the position marked N, Fig. 2, and moving toward S, an electric current will be developed in it in one direction while moving through the portion of the circle, N a, and after passing the point, a, and while passing through the arc, a S, the induced current will be in the opposite direction, which direction will be maintained until the point, b, is reached, after which the currents will be in the same direction as between N and a; and as all the coils are connected together, all the currents in a given direction will unite and give the combined current a direction indicated by the arrows in Fig. 2, and in order to collect it (so as to transmit it into the external circuit), the most eminent position for the collectors will be at points on the commutator at opposite ends of a diameter which is perpendicular to the magnetic axis of the magnetic field. With reference to Fig. 2, we imagine either that the two arrows to the right of the figure are incorrectly placed by the engraver, or that Dr. Pacinotti intended this diagram to express the direction of the current throughout the whole circuit, as if it started from a, and after traversing the external circuit entered again at b, thus completing the whole cycle made up of the external and internal circuits.
Dr. Pacinotti calls attention to the fact that the direction of the current generated by the machine is reversed by a reversal of the direction of rotation, as well as by a shifting of the position of the collectors from one side to the other of their neutral point, and concludes his most interesting communication by describing experiments made with it in order to convert it into a magneto-electric machine. "I brought," he says, "near to the coiled armature the opposite poles of two permanent magnets, and I also excited by the current from a battery the fixed electro-magnets (see Figs. 3 and 4), and by mechanical means I rotated the annular armature on its axis. By both methods I obtained an induced electric current, which was continuous and always in the same direction, and which, as was indicated by a galvanometer, proved to be of considerable intensity, although it had traversed the sulphate of copper voltameter which was included in the circuit."
Dr. Pacinotti goes on to show that there would be an obvious advantage in constructing electric generating machines upon this principle, for by such a system electric currents can be produced which are continuous and in one direction without the necessity of the inconvenient and more or less inefficient mechanical arrangements for commutating the currents and sorting them, so as to collect and combine those in one direction, separating them from those which are in the opposite; and he also points our the reversibility of the apparatus, showing that as an electro-magnetic engine it is capable of converting a current of electricity into mechanical motion capable of performing work, while as a magneto-electric machine it is made to transform mechanical energy into an electric current, which in other apparatus, forming part of its external circuit, is capable of performing electric, chemical, or mechanical work.
All these statements are matters of everyday familiarity at the present day, but it must be remembered that they are records of experiments made twenty years ago, and as such they entitle their author to a very distinguished place among the pioneers of electric science, and it is somewhat remarkable that they did not lead him straight to the discovery of the "action and reaction" principle of dynamo-electric magnetic induction to which he approached so closely, and it is also a curious fact that so suggestive and remarkable a paper should have been written and published as far back as 1864, and that it should not have produced sooner than it did a revolution in electric science. - Engineering.