been hoped for half a century ago, and which has recently revealed to us effects which one could then hardly have dared to conceive. When we reflect that now we are enabled to transport to almost any distance a force of several horse-power by a wire which could easily be passed through a keyhole without any visible movement or any change in its appearance, imagination itself is stupefied, and we ask ourselves if it is not magic! This, however, is what electromotors can do to-day. Thanks to them, natural forces, hitherto useless, can perform at a distance from the source, work which could not have been made use of on the spot. Position now means nothing, and we may demand a supply of power as we demand a supply of water or gas; the same fluid which gives us power can also give us light. What progress science has thus made in the course of a few years! Then electricity with great difficulty could develop only a few foot pounds of work; and now we work ploughs, enormous pumps, cranes, mechanical saws, planing machines, punching machines, drilling machines, and railways even! The last Exhibition showed these marvels.

The first attempts to obtain motive power from electricity were not successful. Many inventors spent large sums of money only to obtain insignificant results; and it was only when the reversibility of continuous current induction machines was tried, that an advantageous solution of the problem could be looked for. Till then we had no electric currents sufficiently powerful to obtain any appre

ciable work. But when it was shown that with two dynamo machines coupled one to the other, we could receive from the one more than half the motive power expended in the other to produce the electricity, we might imagine that not only were we in possession of a system of transmission of power to a distance which could often be utilised very advantageously even under these conditions, but that the electromotive machines themselves were capable of furnishing a force much greater than was supposed by supplying them with sufficiently powerful currents. Up till then, in fact, we had never been able to produce in this manner a force reaching one horsepower, and the most perfect motors did not give more than one or two kilogrammetres of force, which was ridiculous when compared with the expense which they necessitated. We were also on a wrong track, for it was sought to increase the power by an exorbitant increase of the size of the electro-magnetic apparatus. Since the question has entered the new phase, it has been studied in a more serious manner, and small motors have been made which can now furnish appreciable and useful work. We will devote a chapter to these little motors, of which the best known types are those of Deprez, Trouvé, Griscom, &c.; but we must explain here that power of any magnitude can only be furnished by continuous-current dynamo-electric machines, such as those of Gramme and Siemens.

The causes which led to the failure of the early attempts were principally, that it was only sought

to utilise the direct attractive force, which is, as all know, extremely limited and almost the same for very large electro-magnets as for small ones; that the arrangements of the commutators were very favourable to the development of induced currents in the coils which acted in a contrary direction to the current transmitted; that magnetisation and demagnetisation took place sluggishly in electro-magnets of any size, and therefore only a small part of their magnetism could be utilised, becoming even hurtful when it was not required; that the direct attractions between magnet and armature tended to bend the supports, necessitating too great a separation between the parts for the best of the work to be obtained; and finally, that the commutators were damaged by sparking, especially of the extra currents. We shall have occasion presently to study the various means proposed to modify these different defects, but they were evidently insufficient, since good results were never obtained, and it was only when the new application was discovered that these obstacles were sufficiently surmounted to enable the machines to work smoothly. These considerations have pointed out to us the order that should be followed in this work, which we will divide into two parts, the one treating of the early phase of electromotors since their origin till the time when the action of induction was discovered, and the other treating of this second phase of the question, with everything relating to the researches and applications of practical motors, including the transmission of force and its distribution.

It has, however, seemed necessary, to enable the reader thoroughly to grasp the technical parts of the question, to give some preliminary ideas as to the electric means employed in such motors, and these ideas will be found in the next chapter.

Principles on which the Construction of Electromotors is founded.

All the effects of the electric fluid capable of giving an impulse to a body, or of developing an attractive or repelling force, may be mechanically combined so as to form an electric motor. Thus, the reciprocal effects of electric currents on each other, the action of currents on magnets, of magnets on currents, and of temporary magnets on non-magnetised bodies, may, if the electric force and the size of the parts subjected to the action be sufficiently increased, be utilised as electro-dynamic motors. It may be understood that, possessing in electricity a force which may be cut off at a moment's notice by simply disconnecting the current, very simple mechanism suffices to transform the impulse given by it into a continuous rotary movement. Of all these properties, however, electro-magnetic attractions and repulsions, and those of parallel currents in the same direction, as in solenoids, have been the most utilised, setting aside the reversibility of continuous current dynamoelectric machines, the theory of which has not yet been fully elucidated.

To obtain a rotary movement by electro-magnetic attractions it is sufficient, as is easily understood, to

cause a succession of impulses resulting from these attractions to act upon a movable axis, and to provide this axis with a commutator, which before each electro-magnetic action closes the circuit, and opens it after the effect is produced. This problem may be directly solved by attaching to an axis a series of armatures, arranged like the blades of a paddle-wheel round a non-magnetic circumference, and moving before a like number of electro-magnets fixed round this circumference; or, indirectly, by fitting to this axis a crank and connecting-rod capable of transforming into rotary movement the to-and-fro movement caused by the momentary attraction of one or more armatures to the poles of electro-magnets fixed before them. The effects may also be combined so that the reciprocal actions of the armatures and electro-magnets may give rise to two movable systems acting simultaneously on the same axis, and as this arrangement may be applied to all the electric or electro-magnetic properties of which we have spoken, we see that electromotors of very dif ferent patterns may be constructed, which may be classed under various heads; but before describing them, it will be well to give some details of the best form of the different parts entering into their construction, and we will first speak of electro-magnets and solenoids, which are the most important of these parts.