played by the induction coil, has no doubt been remarked. This induction coil, or armature, or electro-magnet, consists of a length of copper wire wound spirally, generally in a transverse direction, round a bar or tube of soft iron. In 1854 Dr. Werner Siemens, of Berlin, introduced a very important modification in the arrangement of the armature, by coiling the wire lengthways upon the body of a H shaped core of soft iron. Owing to the extremely small space required for its rotation, this armature may be kept within a region of strong magnetic force; and a much more powerful current developed than would be the case with an armature with the wire coiled transversely round the cylinder. By taking the current induced into a Siemens armature from small steel magnets, and passing it through a soft-iron electro-magnet to another Siemens coil, Wilde in 1865 obtained an ultimate current of much greater intensity than was the primary one. In some cases, to augment it still further, he has added a second, and even a third, electro-magnet and coil.

But electro-magnets were soon destined to play a still more important part in the future construction of electric machines.

In 1867 Dr. Werner Siemens, Sir Charles Wheatstone, and Mr. S. A. Varley made almost simultaneously the following important discovery; and which Sir Charles Wheatstone and Dr. C. Wm. Siemens each announced, independently and without the previous cognizance of the other, to the Royal Society on the same evening.

If an induction coil be made to revolve in front of a soft-iron electro-magnet, instead of before a permanent magnet, as in the earlier machines, the small amount of residual magnetism always latent in the iron, especially if it has been once magnetized, causes feeble currents to be

induced in the coil; and, if these currents, or a portion of them, are sent round the iron bar, i. e. into the wire surrounding it, the magnetization of the iron is increased. This again produces a proportionate increase in the induced currents in the coil; and thus, by a series of successive mutual actions, intense magnetization and very powerful currents are produced. Machines constructed upon this principle of reaction (i. e. in which electro-magnets only are used) have been termed "dynamo-electric" machines, as indicating that dynamical force is converted into electricity; and which is gradually augmented in intensity. While for the older form, where permanent steel magnets are employed, the term "magneto-electric" machine has been retained; their power being limited to the strength of the magnetization of the permanent magnets made use of. This discovery, by adding considerably to the power of electric machines, led to a corresponding diminution in their bulk and also in their costliness. Almost all of the more recent machines are "dynamo-electric" in construction, and are based upon the principle of " mutual reaction."

Ladd, who was present on the occasion of the announcement just referred to, took advantage of the principle of "mutual reaction" in a machine which he was then at work upon, and which he exhibited at the Paris Exhibition of that year. It consisted of two flat-bar coils, placed horizontally one over the other, and so arranged as to form but one electro-magnet, and with a Siemens armature revolving at each end of the coils; the first coil serving to make the electro-magnet by its reaction, while the current from the second coil was passed off to produce the light.

This principle of mutual reaction soon became acknowledged as a great step in advance. Siemens adopted

electro-magnets instead of the permanent steel magnets he had previously made use of with his armature. In 1871, Gramme, who had also previously constructed a magnetomachine, brought out his earliest form of dynamo-machine, making use of two coils, in the same way that Ladd had done. The coils, however, were different in form; consisting of an annular soft-iron core, with the wire wound transversely around it, and divided around the ring into the requisite number of segments.

Pacinotti of Pisa, seems to claim priority, as far back as 1860, for this form of annular coil used by him in his machine; but whether the two forms are quite alike appears doubtful.

Gramme has since abandoned the use of two coils, and uses but one in the improved form in which this singlelight machine is now ordinarily constructed (A, Fig. 2, Plate II.).

Among the machines which followed, are two well known in the United States, the Wallace-Farmer, and the Brush; and which, together with the Siemens and the lastnamed Gramme, are apparently the best known, and the most extensively applied of the single-light dynamo-electric machines.

A distinguishing feature of the machines belonging to this second stage is, that they produce currents of con→ siderable intensity, all of which are collected upon the circumference of a circular disc termed a commutator; the currents circulating upon one half of its circumference being opposite in magnetic direction to those upon the other half; a separate contact brush serving to carry off the current from each half. The currents from these machines thus pass off in one continuous direction, and are not subject to a constant series of reversals in direction as

with the earlier machines. The currents are generally supplied for the production of a single electric light. The rate of rotation required for them in general is very high, from 700 to 1000, and even more, revolutions per minute, according to size. This rapid motion often causes trouble with the commutator brushes, which heat and wear in consequence.

About 1874 the attention of electricians began effectually to be turned to the production of several lights from one electric machine. Lontin in this year produced a compound machine (Fig. 1, Plate II., and described farther on), together with a regulator sufficiently delicate for the purpose. Then in 1876, Jablochkoff devised his now wellknown "candle" as a simplification of the arrangement of the carbons, or electrodes, between which to produce the light; he found himself obliged to make use of the now ancient Alliance magneto-electric machine, as the only one available which could supply the alternatedirection currents necessary for his regulator. Later on Gramme devised his latest form of dynamo-machine (B, Fig. 2, Plate II.), to perform this duty more efficiently than did the Alliance machine; and it is this last that is now almost always employed where the Jablochkoff candles are made use of as the regulator. Still more recently, 1878, De Meritens has devised a magnetomachine (Plate I., Fig. 1), a modification and improvement upon the Alliance one, much reduced in size and weight; and which hopes to compete with the small and compact dynamo-electric machines of the present day.

In all these machines serving for the production of several lights from each, and which may therefore be termed "many-light" machines, it has been found most convenient to construct them so as to give off currents

alternating in direction; which in practice seem more suitable for the steady continuance of several lights, than do those currents which are continuous in direction.

Electric lighting, it will readily be seen, has only attained to its present development by certain marked stages of progress; each period represented by clearly defined and distinctive characteristics, both in the apparatus employed and in the use to which they were applied.

These stages may be briefly described as

1st. That of lighthouse illumination by the Holmes and by the Alliance magneto-electric machines, giving off currents alternating in direction, and producing a single very powerful light.

2nd. That of dynamo-electric machines producing a single light each, for industrial purposes generally, in addition to lighthouse illumination. The Siemens, and the earlier Gramme, are the best known in Europe of these machines; while the Wallace-Farmer and the Brush claim. most attention in the United States.

3rd. That of machines both of magneto and of dynamo construction, and generally (though not necessarily) giving off alternate-direction currents; and which afford facilities, if provided with suitable lamps, for the production of a number of lights from each machine. Besides the alreadynamed Holmes and Alliance machines, and their recent and more compact successor, the De Meritens (all being of magneto construction), there are the Lontin (double), and the Gramme (double), which also effect this much-desired object. In all these machines the current given off is alternating in direction. The two last-named machines, though dynamo in construction and in principle, are by some persons classed as magneto-machines in their opera