TWO OF FARADAY'S EARLY EXPERIMENTS IN INDUCTION.
He who made the first actual machine to evolve a current in compliance with Faraday's formulated laws was an Italian named Pixü, in 1832. His machine consisted of a horseshoe magnet set on a shaft, and made to revolve in front of two cores of, soft iron wound with wire, and having their ends opposite the legs of the magnet. Shortly after Pixü, the inventors of the times ceased to turn the magnet on a shaft, and turned the iron cores instead, because they were lighter. In like manner, the huge field magnets of a modern dynamo are not whirled round a stationary armature, but the armature is whirled within the legs of the magnet with very great rapidity. The next step was to increase the number of magnets and the number of wire-wound iron cores--bobbins. The magnets were made compound, laminated; a large number of thin horseshoe magnets were laid together, with opposite poles touching. These were all comparatively small machines--what we now, with some reason, regard as having been toys whose present results were rather long in coming.
Then came Siemens, of Berlin, in 1857. He was probably the first to wind the iron core, what we now call the armature, with wire from end to end, lengthwise, instead of round and round as a spool. This resulted, of course, in the shaft of the armature being also placed crosswise to the legs of the magnet, as it is in the modern dynamo. One of the ends of the wire used in this winding was fastened to the axle of the armature, and the other to a ring insulated from the shaft, but turning with it. Two springs, one bearing on the shaft and the other on the ring, carried away the current through wires attached to them. Siemens also originated the mechanical idea of hollowing out the legs of the magnet on the inside for the armature to turn in close to the magnet, almost fitting. It was the first time any of these things had been done, and their author probably had no idea that they would be prominent features of the dynamo of a little later time, in all essentials closely imitated.
It will be guessed from what has been previously said on the subject of induction that the currents from such an electro-magnetic machine would be alternating currents, the impulses succeeding each other in alternate directions. To remedy this and cause the currents to flow always in the same direction, the "commutator" was devised. The ring mentioned above was split, and the two springs both bore upon it, one on each side. The ends of the wires were both fastened to this ring. The springs came to be known as "brushes." The effect was that one of them was in the insulated space between the split halves of the ring while the other was bearing on the metal to which the wire was attached. This action was alternate, and so arranged that the current carried away was always direct. When an armature has a winding of more than one wire, as the practical dynamo always has, the insulated ring is divided into as many pieces as there are wires, and the two brushes act as above for the entire series.
Pacinotti, of Florence, constructed a magneto-electric machine in which the current flows always in one direction without a commutator. It has what is known as a ring armature, and is the mother of all dynamos built upon that principle. It is exceedingly ingenious in construction, and for certain purposes in the arts is extensively used. A description of it is too technical to interest others than those personally interested in the class of dynamo it represents.
Wilde, of Manchester, England, improved the Siemens machine in 1866 by doing that which is the feature that makes possible the huge "field magnet" of the modern dynamo, which is not a magnet at all, strictly speaking. He caused the current, after it had been rectified by the commutator, to return again into coils of wire round the legs of his field magnets, as shown in the diagram. This induced in them a new supply of magnetism, and this of course intensified the current from the armature. It is true he had a separate smaller magneto-electric machine, with which he evolved a current for the coil around the legs of the field magnet of a greatly larger machine upon which he depended for his actual current, and that he did not know, although he was practically doing the same thing, that if he should divert this current made by the larger machine itself back through the coils of its field magnet, he would not need the extra small machine at all, and would have a much more powerful current.