Each core has its magnetism reversed sixteen times in each revolution, by the influence of the sixteen successive pairs of poles between which it passes, and the same number of currents in alternately

opposite directions are generated in the coils. The coils can be connected in different ways, according as great electro-motive force or small resistance is required. The positive ends are connected with the axis of the machine, which thus serves as the positive electrode, and a concentric cylinder, well insulated from it, is employed as the negative electrode.

When the machine is employed for the production of the electric light, the currents may be transmitted to the carbon points in alternate directions, as they are produced. For electro-metallurgical purposes they are brought into one constant direction by a commutator, as in Clarke's machine above described. The driving-power required for lighthouse purposes is about three horse-power.

Machines of this class are never constructed now, as the same power is obtained with only a fifth of the weight in the machines of Siemens and Gramme.

823. Siemens' Armature.-An important improvement in Clarke's machine was introduced by Siemens of Berlin in 1854. It consists

in the adoption of a peculiar form of electro-magnet, which is represented in Fig. 545. The iron portion is a cylinder with a very deep and wide groove cut along a pair of opposite sides, and continued round the ends. The coil is wound in this groove like thread upon a shuttle. Regarded as an electro-magnet, the poles are not the ends of the cylinder, but are the two cylindrical faces which have not been cut away. In Fig. 546, a b is a section of the armature with the coil wound upon it. ABMN is a socket within which the armature revolves, the portions A B being of iron, and M N of brass.

The advantage of Siemens' armature is that, on account of the small space required for its rotation, it can be kept in a region of very intense magnetic force by the use of comparatively small magnets. Its form is also eminently favourable to rapid rotation. It is placed between the opposite poles of a row of horse-shoe magnets which bestride it along the whole of its length, as shown at the top of Fig. 548, and is rotated by means of a driving-band passing over the pulley shown at the lower end of Fig. 545.

Fig. 545. Siemens' Armature.

The polarity of the electro-magnet is reversed at each half-revolution as in Clarke's arrangement, and the alternately opposite currents generated are reduced to a common direction by a commutator nearly identical with Clarke's, and represented in Figs. 545, 547. Siemens' machines are

much more powerful than Clarke's when of the same size.

824. Accumulation by Successive Action: Wilde's Machine.-By

employing the current from a Siemens' machine to magnetize soft iron, we can obtain an electro-magnet of much greater power than the steel magnets from whose induction the current was derived. By causing a second coil to rotate between the poles of this electro

magnet, we can cbtain a current of much greater power than the primary current. This is the principle of Wilde's machine, which is represented in Fig. 548. It consists of two Siemens' machines, one above the other. The upper machine derives its inductive action from a row of steel magnets M, whose poles rest on the soft-iron masses m, n, forming the sides of the socket within which a Siemens'

armaturer rotates. The currents generated in the coil, after being reduced to a uniform direction by a commutator, flow to the bindingscrews p, q. These are the terminals of the coil of the large electromagnet AB, through which accordingly the current circulates. The core of this electro-magnet consists of two large plates of iron, connected above by another iron plate, which supports the primary machine. Its lower extremities rest, like those of the primary magnets, on two iron masses T, T, separated by a mass of brass i; and a second Siemens' armature F, of large size, revolving within. this system, furnishes the currents which are utilized externally.

Wilde's machine produces calorific and luminous effects of remarkable intensity; but the speed of rotation required is very great, being sometimes 1500 revolutions a minute for the large, and 2000 for the small armature. This great speed involves serious inconveniences; and the machine does not appear to have been used for lighthouses, or other practical purposes.

Wilde's principle can be carried further. The current of the second armature can be employed to animate a second electro-magnet of greater power than the first, with a third Siemens' armature revolving between its poles. This has actually been done by Wilde. By means of the current from this triple machinė, driven by 15 horsepower, a bar of platinum 2 feet long and a quarter of an inch in diameter was quickly melted. This system of accumulation could probably be carried several steps further if desired.

825. Accumulation by Mutual Action; Dynamo-electric Machines.Siemens and Wheatstone nearly simultaneously proposed the construction of a magneto-electric machine in which the induced currents are made to circulate round the soft-iron magnet which produced them. Iron has usually some traces of permanent magnetism, especially if it has once been strongly magnetized. This magnetism serves to induce very feeble currents in a revolving armature. These currents are sent round the iron magnet, thus increasing its magnetization. This again produces a proportionate increase in the induced currents; and thus, by a successive alternation of mutual actions, very intense magnetization and very powerful currents are speedily obtained. Machines constructed on this principle are called dynamoelectric. In the machine as exhibited by Siemens in 1867, the current was diverted into an external circuit, at regular intervals, by an automatic arrangement.

826. Ladd's Machine.-Ladd in 1867 constructed a dynamo-electric

machine having two revolving armatures, one for augmenting and sustaining the power of the electro-magnet, and the other for giving an external current.

B, B' (Fig. 549) are two plates of iron surrounded by coils which are connected together at the right-hand end. The other ends are attached to two binding-screws connected with the ends of the coil of a Siemens' armature a'. The direction of winding of the two large coils BB' is the same as for a horse-shoe magnet, so that

the two poles at either end are of opposite sign. The ends of the cores are let into masses of soft iron M M, N N, between which two armatures a a' rotate. The coil of the armature a is connected with the external circuit containing, for example, two carbon points for exhibiting the electric light.

On the principle of mutual action, the electro-magnets B, B', which we may suppose to have at first only a trace of magnetism, are soon raised to very intense magnetization by the rapid rotation of the armature a', and as long as the rotation continues, the magnetization is maintained. The rapid rotation of the other armature a between the poles thus strongly excited, produces a very powerful current which can be utilized externally.