They are also called continuous-current machines, because this current is never interrupted. All such machines can be used as electro-motors, that is to say, they can be driven by sending a current through them from an external source; and if we want to drive them forwards we must send this current in the backward direction; for (§ 804) the current generated by the machine, or a current in the same direction as this derived from an external source, tends to stop the machine and turn it backwards.

Another important class of machines produce alternating currents, that is to say, currents whose direction is alternately in one direction and the opposite, the reversals succeeding each other usually some hundreds or thousands of times in a second. For this purpose no commutator is required, inasmuch as the current in the armature itself is alternating in all machines; but the two collecting springs rub without interruption on the surfaces of two revolving cylinders, to which the ends of the armature-coil or of its several sections are connected. Each cylinder gives off positive and negative electricity alternately, and when the one is giving off positive the other is giving off negative. This is the favourite plan for lighthouses, because the alternating currents make the two carbons of the electric lamp burn away equally, and thus facilitate the keeping of the light in the focus of the optical apparatus. The field-magnets of an alternate-current machine, if they are electro-magnets, must be excited by a current distinct from that of the machine itself; as alternating currents will not serve for this purpose.

829. Wheatstone's Telegraphic Currents.-In Wheatstone's Universal Telegraph (more fully described in § 841) the magneto-electric currents which give the signals are produced by causing a small flat bar of soft iron to rotate rapidly before the poles of a steel horse-shoe magnet, which has two connected coils of wire wound upon it in the same manner as upon electro-magnets. It is in these coils that the currents are generated, the iron bar being a temporary magnet, and thus influencing the coils, nearly in the same manner as if it were a permanent magnet. A current is induced in one direction as it approaches the poles, and in the opposite direction as it recedes from them, so that altogether four currents are generated in each complete revolution. On account of the lightness of the bar, it can be rotated with great rapidity.

830. Arago's Rotations.-Faraday successfully applied his discovery of magneto-electric induction to account for a phenomenon

first observed by Arago in 1824, and subsequently investigated by Babbage and Sir John Herschel. A horizontal disc of copper bb (Fig. 556), placed in the interior of a box, is set in rapid rotation by

turning a handle. Just over the copper disc, but above the thin glass plate which forms the top of the box, a magnetized needle aa is balanced horizontally. When the disc is made to rotate, the needle is observed to deviate from the meridian in the direction of the rotation. When the speed of rotation exceeds a certain limit, the needle is not only deflected, but carried round in continuous rotation in the same direction as the disc. The explanation is to be found in the currents which are induced in the disc by its motion in the vicinity of the magnetized needle. The forces between these currents and the needle are (by Lenz's law) such as to urge the disc backwards; and, from the universal relation which subsists between action and reaction, they must be such as to urge the needle forwards, hence the motion. The direction of the induced current through the centre of the disc at any instant is along that diameter which is directly under the needle, the circuit being completed through the lateral portions of the disc; and it is evident that a current thus flowing parallel to the needle underneath it tends to produce deflection. If the continuity of the disc is interrupted by radial slits, the observed effect is considerably weakened inasmuch as the return circuit is broken. Faraday succeeded in directly demonstrating the existence of currents in a disc rotating near a fixed magnet, by exploring its surface with the amalgamated ends of two wires connected with a galvanometer.

The experiment performed by Arago may be reversed by setting the magnet in rotation, and observing the effect produced on the disc. The latter, if delicately suspended, will be found to rotate in the same direction as the magnet. This experiment was first performed by Babbage and Herschel. Its explanation is identical with that just given. In both cases the induced rotation must be slower than that of the body turned by hand, as the existence of the induced currents depends upon the motion of the one body relative to the other.

When an iron disc is used instead of a copper one, magnetism is induced in the portions which pass under the poles of the magnet; and as this requires a sensible time for its disappearance, there is always attraction between the poles of the needle and the portions of the disc which have just moved past. The needle is thus drawn forwards by magnetic attraction, and the observed effect is similar to that obtained with the copper disc, though the cause1 is altogether different.

831. Copper Dampers.-Precisely similar to the above is the explanation of the utility of a copper disc in checking the vibrations of a magnetized needle under which it is fixed. As the needle swings to either side, its motion induces currents in the copper which urge the needle in the opposite direction to that in which it is moving. When it rests for an instant at the extremity of its swing, the currents cease; and as soon as it begins to return, the currents again resist its motion. A copper plate thus used is called a damper, and the vibrations thus resisted and destroyed are said to be damped. The name is applied to any other means for gradually destroying vibrations.

The resistance which induced currents oppose to the motion producing them is well illustrated by Faraday's experiment of the copper cube. A cube of copper is suspended by a thread, and set spinning by twisting the thread and then allowing it to untwist. If, while spinning, it is held between the poles of a powerful magnet, like that represented in Fig. 445, it is instantly brought almost to rest. If the poles are brought very near together, so as to heighten the intensity of the field, and a thin sheet of copper is inserted between them and moved rapidly in its own plane, the operator feels its motion resisted by some invisible influence. The sensation has been compared to that of cutting cheese. Foucault's apparatus for the heating of a copper disc by rotating it between the poles of a magnet (§ 486), is another illustration of the same principle. In all cases where induced currents are generated, and are not called upon to perform external work, they yield their full equivalent of heat.

The advantage of employing copper in experiments of this kind arises from its superior conductivity, to which the induced currents are proportional.

'That is to say, the main cause; for there must be induced currents in the iron as well as in the copper, though inferior in strength, on account of the inferior conductivity of the former metal.

ELECTRO-MEDICAL MACHINES.

805

832. Electro-medical Machines.-The application of electricity is often resorted to for certain nervous affections and local paralyses. Many different forms of apparatus are employed for this purpose. One of the most convenient is represented in Fig. 557. Two small coils connected with each other, and furnished with a vibrating. contact-breaker, are traversed by the current from a miniature battery. The coils are surrounded by hollow cylinders of copper or brass, in which induced currents are generated as often as the current in the coils is established or interrupted. This action diminishes the energy of the extra-currents on which the shock depends, and the operator can accordingly regulate its strength at pleasure by sliding the cylinders on or off. 833. Caution regarding Lines of Force.-After the very extensive use which has been made in this chapter of lines and tubes of force, we think it right to caution the reader against supposing that these conceptions depend upon any doubtful hypothesis. They merely serve, like meridians and parallels of latitude, to map out space in a mode convenient for the statement of physical laws.

Fig. 557.-Electro-medical Machine.

CHAPTER LX.

ELECTRIC TELEGRAPHS.

834. Electric Telegraph: History.-The discovery that electricity could be transmitted instantaneously to great distances, at once suggested the idea of employing it for signalling. Bishop Watson, already referred to in § 636, performed several experiments of this kind in the neighbourhood of London, the most remarkable being the transmission of the discharge of a Leyden-jar through 10,600 feet of wire suspended between wooden poles at Shooter's Hill. This was in 1747. A plan for an alphabetical telegraph to be worked by electricity is minutely described in the Scots Magazine for 1753, but appears to have been never experimentally realized. Lesage, in 1774, erected at Geneva a telegraph line, consisting of twenty-four wires connected with the same number of pith-ball electroscopes, each representing a letter. Reusser, in Germany, proposed, in the same year, to replace the electroscopes by spangled panes exhibiting the letters themselves. The difficulty of managing frictional electricity was, however, sufficient to prevent these and other schemes founded on its employment from yielding any useful results. Volta's discoveries, by supplying electricity of a kind more easily retained on the conducting wires, afforded much greater facilities for transmitting signals to a distance.

Several suggestions were made for receiving-apparatus to exhibit the effects of the currents transmitted from a voltaic battery. Sömmering of Munich in 1811 proposed a telegraph, in which the signals were given by the decomposition of water in thirty-five vessels, each connected with a separate telegraph wire. Ampère, in 1820, proposed to utilize Ersted's discovery, by employing twentyfour needles, to be deflected by currents sent through the same