are in contact with intermediate insulating pieces which separate the metallic pieces E, E'. When the current in the coil is in one direction (say from E to E), r is in contact with E, and r' with E'. When the current in the coil is in the opposite direction (E' to E), r is in contact with E', and r' with E; thus in each case r is the positive and r' the negative spring, and the current will be from r to r' in an external connecting wire. 00, O'O', are metallic pieces insulated from each other, and connected with the springs rr respectively. Binding-screws are provided for attaching wires through which the current is to be passed.

With this machine water can be decomposed, wire heated to redness, or soft iron magnetized; but these effects are usually on a small scale on account of the small dimensions of the machine.

For giving shocks, two wires furnished with metallic handles are attached to the binding-screws, and a third spring is employed which puts the terminals E E' in direct connection with each other twice in each revolution, by making contact with two plates q. When these contacts cease, the current is greatly diminished by having to pass through the body of the person holding the handles, and the extracurrent thus induced gives the shock. To obtain the strongest effect, the hands should be moistened with acidulated water before grasping the handles.

622. Magneto-electric Machines for Lighthouses.-Very powerful effects can be obtained from magneto-electric machines of large size driven rapidly. Such machines were first suggested by Professor Nollet of Brussels; and they have been constructed by Holmes of London and the Compagnie l'Alliance of Paris. It is by means of these machines that the electric light is maintained in lighthouses; they have also been employed to some extent in electro-metallurgy. Fig. 555 represents the pattern adopted by the French company. It has eight rows of compound horse-shoe magnets fixed symmetrically round a cast-iron frame. They are so arranged that opposite poles always succeed each other, both in each row and in each circular set. There are seven of these circular sets, with of course six intervening spaces. Six bronze wheels, mounted on one central axis, revolve in these intervals, the axis being driven by steam-power transmitted by a pulley and belt. The speed of rotation is usually about 350 revolutions of the axis per minute. Each of the six bronze wheels carries at its circumference sixteen coils, corresponding to the number of poles in each circular set. The core of each coil is

a cleft tube of soft iron, this form having been found peculiarly favourable to rapid demagnetization.

Each core has its magnetism reversed sixteen times in each revolu

tion, 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.

623. 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 repre

sented in Fig. 556. 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. 557, 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 AB being of iron, and MN 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. 559, and is rotated by means of a driving-band passing over the pulley shown at the lower end of Fig. 556.

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. 556, 558. Siemens' machines are much more powerful than Clarke's when of the same size.

Fig. 556. Siemens' Ar

mature.

624. Accumulation by Successive Action: Wilde's Machine.—Dy

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 electromagnet, we can obtain a current of much greater power than the

primary current. This is the principle of Wilde's machine, which is represented in Fig. 559. 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

SIEMENS' AND WHEATSTONE'S MACHINE.

773

reduced to a uniform direction by a commutator, flow to the binding-screws p, q. These are the terminals of the coil of the large electro-magnet 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.

By

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. means of the current from this triple machine, driven by 15 horsepower, the electric light was maintained between two carbons as thick as a man's finger, and 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, but always with the expenditure of a proportionately large amount of energy in driving it. In no magneto-electric machine can the electrical energy obtained exceed the mechanical energy expended in producing it.

625. Accumulation by Mutual Action: Siemens' and Wheatstone's Machine. 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 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. In the machine as exhibited by Siemens in 1867, the