By far the most important of the dynamos of this second class are those usually known as " alternate - current machines." This type of dynamo was originally created by Wilde, in 1867. The field - magnets consist of 2 crowns of fixed coils, with iron cores, arranged so that their free poles are opposite one another, with s apace between them sufficiently wide to admit the armature. The poles taken in order round each crown are alternately ofN. and S. polarity, and opposite a N. pole of one crown faces a S. pole of the other. This applies to the magnets of the alternate - current machines of Wilde and Siemens, the Ferranti machine, and, with certain reservations, to the Lachanssee and the Gordon. The armatures in almost all machines of this type consist of a disk, bearing at its periphery a number of coils, whose axes are parallel to the axis of rotation. The principle is shown in Fig. 8O, which gives a general view of the arrangement. Since the lines of force run in opposite directions between the fixed coils, which are alternately ft - N., N. - S., the moving coils will be traversed by alternating currents; and as the alternate coils of the armature will be traversed by currents In opposite senses, it is needful to connect them up so that they shall not oppose one another's action.

Fig. 80.

In Wilde'a dynamo, the armature coils have iron cores, and the machine is provided with a commutator on the same principle as that used by Jacobi in his motor of 1838, consisting of 2 metallic cylinders, cut like crown wheels. having the teeth of one projecting between those of the other, so that the brushes make contact against them alternately as they rotate. The brushes are tiled so that they do not both touch the same part. This commutator Wilde usually applied to a few, or only one, of the rotating coils, and utilised the current thus obtained to magnetize the field - magnets. The main current was not so commuted, but was led away from a simple collector, consisting of 2 rings. connected to the 2 ends of the armature circuit, each being pressed by one brush.

Siemens prefers to use a separate direct current machine to excite the field - magnets of alternate - current dynamos, In the armature of the latter, the coils are wound, usually without iron, upon wooden cores. In some forms of the machine, the individual coils are enclosed between perforated disks of thin German silver. When currents of great strength are required, but not of great electromotive force, the coils are coupled up in parallel arc, instead of being united in series.

In a dynamo by Lachaussee, which very strikingly resembles the preceding one, there is iron in the cores of the rotating coils. But the main difference is that the rotating coils are the field - magnets, excited by a separate Gramme dynamo, whilst the coils, which are fixed in 2 crowns on either side, act as armature coils in which currents are indured.

Gordon's dynamo is constructed on the name lines as the Lachanssee, but with many important improvements. In the first place, there are twice as many coils in the fixed armatures as in the rotating magnets, there being 32 on each side of the rotating disk, or, in all, 64 moving coils; while there are 64 on each of the fixed circles, or 128 stationary coils in all. The latter are of an elongated shape, wound upon a bit of iron boilerplate, bent up to an acute V form, with cheeks of perforated German silver as flanges. The object of thus arranging the coils, so that the moving ones shall have twice the angular breadth of the fixed ones, is to prevent adjacent coils of the fixed series from acting detrimentally, by induction, upon one another. The alternate coils of the fixed series are united together in parallel arcs, so that there are 2 distinct circuits, in either or both of which lamps can be placed; or they can be coupled up together. Great care appears to have been taken, in the construction of this large machine, to guard against the appearance of Foucault currents, by arranging the cores, frames, and coils, so that all metallic parts of any size shall be slit, or otherwise structurally divided at right angles to the direction of the induced electromotive forces.

Fig. 81.

The wires that bring the exciting current are passed up and down, in a zigzag form, between iron blocks projecting from an iron frame.

Prof. Thompson is not at all convinced that this type of machine, though at present fashionable, is destined to prove of very great value, because he doubts whether any dynamo that yields alternate cunents can compete with continuous - current machines. For the purposes of a general system of distribution, where more than one dynamo must be available, and for the purpose of supplying motors, alternate - current.

Besides the disk armatures described, pole armatures have been employed in alternate - current machines by Gramme, Jablochkoff, and Lontin. Hefner Alte - neck has gone a stage farther, and, by the device of employing a disk armature in which the number of coils differed by 2, or some other even number, from those of the field, and by the employment of a multiple - bar collector with complicated cross connections, has succeeded in converting this type of dyna mo into a continuous-current machine.

Thomson's "Mouse - mill." - This dynamo is shown in Fig. S3 in diagram The armature is a hollow cylinder a, made up of parallel copper bars, ar - ranged like the bars of a mouse - mill (whence the name of the machine). These bars are insulated from each other, but are connected all together at one end. At the other, they serve as collector - bars, and deliver up the currents generated in them to the "brushes," which here are rotating disks of springy copper shown as dotted circles at c. As the armature is a hollow barrel, with fixed electro - magnets within.it cannot be rotated on a spindle, but runs on friction rollers AA', by one or more of which it is driven.