Fig. 28 is a diagram illustrating the connection between the armature-bobbins and the magnet-coils at the time when the commutators are placing them in the same circuit. Re ferring to this diagram, JI JI and J/ M are the two magnets having their similar poles pre sented toward one another on opposite sides of the armature coils A Al. Thus, the coil A is under the influence of a mag netic field produced by the two north poles while at the same time its corresponding bobbin Al is under the influence of the two south poles S A current is therefore induced in the pair of bobbins A Al which is transmitted by wires passing, through the shaft S to the commutators C' whence it is collected by the brushes Bl mid and by them transmitted to the magnet-eoils. which are all connected together in series, and at the same time the other portions of the commutators (which are in connection with the other armature-bobbins) are in contact with the brnshes and by which they are placed in the external circuit of the machine.
One of the most original features of the Brush machine is the commutating apparatus, which collects and distributes the currents from the active armature-coils, and cutting out of circuit the armature-coils one by one as they pass through the neutral regions between the poles. The commutating apparatus consists of two pairs of rings attached to and revolving on the main shaft, and therefore their position is fixed with respect to the revolving armature of the machine. On to the cylindrical circumferences of these rings are placed two pairs of copper-collecting brushes, which run tangentially against the commutator rings, one pair pressing above, and the other pressing below, a line forming the points of contact being a diameter of the ring. The copper brushes are flat strips of elastic copper about 2 in. wide, cut at the ends which press against the rings into 8 tongues, so as somewhat to resemble a grainer's comb. and each comb or brush is wide enough to corer or be in contact with two ttrmature-rings: and in this way, although two of the coils are insulated twice in each revo lution, the main circuit is never interrupted. The disposition of the brushes with respect to the commutators will clearly be understood by comparing Fig. 28.
The Thomson-Hott8ton Are-Light llachine.—This machine. probably the most extensively employed are-dynamo at the present time, is the joint invention of Profs. Elihu Thomson and E. Houston, although many of the details embodied in the recent machines arc due to Prof. Thomson solely. The gen mil appearance of the complete machine is shown in Fig. 29.
The field-magnets consist of•two largo hollow castings. The large flanged portions of the castings are united magnetically by a series of bars of soft iron, and arc firmly held in place by bolting to the side-frame, which also affords feet for the machine and sustains the shaft in its bearings.
The armature, spherical in form (Fig. 30) is nearly inclosed. The commutator and air-blast mechanism, therefore, occupy positions upon that portion of the shaft outside the bearing,. The wires, three in number, from the artnatnre helices are brought out through the hol low shaft and connected to the commutator at the end of the shaft. The armature-core con sists of 1111 iron shell, haring the form of an oblate spheroid, mounted centrally upon the shaft, as seen in Fig. 31. the shaft 1111 passing through the axis of the spheroid. The po lar portions are formed of two thin iron castings, placed, as shown, at G 0, tool keyed firm ly to the shaft. Between these flanges. and supported by them, but insulated therefrom, are a series of cast-iron bridges IA generally 12 in number, and placed at equal distances apart. The bridges are formed with feet that enter corresponding grooves in the internal faces of the flanges. Outside the bridges is wound a quantity of well annealed soft-iron wire I, sealed by heat and shellacked. The depth of the wire varies with the capacity of the machine, and, when all on, completes the form of the spheroidal arma ture. The core is covered with several hirers of insulating paper. and then is wound with in sulated copper wire. To facilitate this winding, hard-wood pins P P are carried by being in serted into openings in the flanges near their periphery. The cure so formed is wound with three helices crossing one another at the polar portions, and tieing divided centrally by the shaft in its passage through the core. To secure mechanical and electrical equality of the three coils or helices, the following procedure is adopted: The first half of the first coil is wound ; the first half of the seeond coil is next wound; the whole of the third coil is then wound ; the second half of the second coil is wound ; finally, the second half of the first coil finishes the winding-, and produces 1111 approximately spherical outline, as shown in Fig. 30. The coils are thoroughly insulated, and are interwoven with tapes. wherever necessary to keep them in place. Finally, a strong binding is applied. consisting of two central lentils GI) and lateral bands d wound around the armatures eirt'umfcreiutiallc.