MATERIALS. The materials entering into the construc tion of the various parts of dynamo- { electric machines are as follows: The magnetic circuit in the field is usually com posed of cast iron or steel. The pole pieces may be of the same material as the field, or they may be composed of soft iron lamina• placed edge on to the shaft of the machine. In small and medium sized machines the base and pillow blocks of the armature shaft are nearly always cast in one piece with the field. The body of the armature upon which the conductors are wound is always built up of iron laminic placed edge on to the shaft. The object of using lamina- here and in the pole pieces is to prevent electric currents from flowing in the iron of the armature itself. Such currents would absorb power and needlessly heat the armature. The conductors themselves in both the field and the armature windings are copper wires or bars carefully insulated from the iron parts and from one another by cotton thread, or cloth, paper, cardboard, mica, etc. The commutator of direct-current machines is built up of copper segments separated by strips of mica. The brushes that bear upon the com mutator in most modern machines take the form of blocks of carbon. In alternating-current machines there is no commutator, it being re placed by copper or brass rings.
HisronteAt. DEVELOPMENT. In the preceding paragraphs the several parts of dynamo-electric machines and the functions which they exercise have been described, and the attempt has been made to elucidate briefly and non-technically the principles at the basis of their operation. An account will now be given of their development, and afterwards the several types of machine in eommercial use will lie described.
The principle of electro-magnet le induction, upon jell the operation of the dynamo-electric machine is based, was discovered by Michael Faraday in 1S31. In Ids first experiments Fara day produced a current in a coil of wire by starting or stopping a current in a neighboring coil. He then generated currents in a coil by moving it before the poles of a magnet. Ins first electric motor was a homo-polar machine. A disk of copper was so mounted that it could rotate between the poles of a permanent magnet transversely to the direction of the lines of force.
\Then a current was applied at disk and led off at the periphery the disk rotated continnonsly. Faraday constructed a number of machines upon this principle, one of which eon sisted of a copper hollow cylinder closed at one end and hung over and inclosing a bier magnet. The cylinder rested upon a vertical pivot at the closed end and the open end dipped into a cup of mercury. When a current was caused to trav erse t he cylinder lengthwise, the cylinder rotated continuously about the magnet. Fara day's next machine employed a rotating rec tangle of wire like that shown in Fig. 3 above, except that no magnet was used such as is shown in the drawing. When the axis of the rectangle was placed cast and west and the rectangle was rotated, alternating currents could be drawn from slip rings attached to the coil, or, by employing a comniutator, direct current could be obtained. It will be noted that the only magnetic field em ployed was that of the earth. The first machine to give a continuous current was constructed by :Sir Charles Wheatstone, an Englishman, in 1S41. The armature of his machine was wound with six coils, each of which was connected to the brushes only at the instant when it was at its greatest activity. The use of electro-mag nets instead of permanent magnets for the field was patented by Wheatstone and Cooke in 1845. A self-excited machine was constructed by Jacob Brett in 184S. The drum-wound armature was introduced by Werner Siemens at about the mid dle of the last century. At about the same time or a little later a type of armature constructed in an entirely different. fashion was introduced by Gramme. Instead of winding the wires on the outside of a cylinder placed between the magnet poles, he wound them upon an iron ring. The Gramme-ring type of armature was at one time extensively used, but it has almost entirely disappeared from use at the present time. De signs of multipolar machines began to appear about 1880. and it was at about that time also that engineers began to study the principles en tering into the design of dynamo-electric machines for commercial purposes. Since 1880 the greatest development has been in the direc tion of improving details of design and of mechanical construction.