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machine in which were applied and well utilised most of the principles, and even some of the arrangements of detail, which eight or ten years afterwards caused the success of other machines. The attention of the public was not yet attracted in this direction, and Pacinotti's invention was little known till the Electrical Exhibition of 1881 brought it to light, and caused the inventor's merit to be recognised and justly rewarded with a diploma of honour. Pacinotti's machine is given in Fig. 44; it may be seen that he had discovered the ring arrangement of the conducting-wire in which the current is developed, and had thus constructed the movable bobbin now called a ring-armature. Two electro-magnets E E' formed the inductor, and we would again refer the reader to the remarkable sentence quoted at page 80, with which he ends his paper describing the apparatus.

This is the enunciation of the principle of reversibility, and it is the earliest known; therefore the honour of having first clearly perceived this scientific fact belongs to the learned Italian. This historical incident is interesting, from the importance of the principle which it involves. It is not indeed peculiar to electric machines, it is a very general property, as was seen at once as soon as attention was drawn in this direction. Nearly all phenomena are theoretically reversible; in giving out heat we produce movement, as for instance in steamengines. But everyone knows that by movement we get heat for example, if we rub two bodies

together they get hot, and the work used in the movement is represented by the heat produced; the phenomena are so closely allied, that it is possible to determine what quantity of heat corresponds to a given amount of work, and this is called the mechanical equivalent of heat. By giving out heat certain chemical decompositions may be effected, but if it is possible to reunite the bodies of which the elements have thus been separated, the same amount of heat will be generated in the process of reunion as was necessary to separate them. Reversibility is thus a very common property; it cannot always be utilised, but electric phenomena are among those in which it is most easily shown, and where it is generally susceptible of application. It is because induction machines possess it in the highest degree, that they have become the means for the production and transport of mechanical work by electricity.

One important remark must be made here: originally electric motors really produced force, they were set in motion by the current of an electric battery, zinc was expended in the battery, labour was gained from the motor. It was real development of mechanical energy. With induction machines it could doubtless be the same; however, it is not thus we work. The current of the battery being dear, and difficult to produce, we want to get electricity from the machines themselves, and they produce this by expending labour. We thus absorb labour in order to recover it. It may be asked, what is the benefit of such an operation? It lies in the

human power.

transport. We can imagine many cases in which transport is of the first importance. How many waterfalls, for instance, now useless because of their situation, would be priceless if they could be transported near to a populous centre! The amount of force thus neglected is immense, and electric transport furnishes the means of adding this to Let us consider too, the enormous number of cases where power might be applied to scattered machinery or tools. Imagine, for instance, the cranes along a quay: in the ordinary way a special engine is necessary for each; with electricity a single engine might easily be used to work them all. A still more important consideration is that of the sub-division of force. As we shall see further on, the hope is just now established that by means of electricity it will be possible, not only to transport power, but also to divide and distribute it. We will not here dwell upon the importance of this, which must be considered separately; we only wish to point out to what consequences electric transport of power may lead. Besides, the reader may take note of its importance, seeing in the following chapters the successive development of the applications to which it has already led.

CHAPTER II.

GENERAL REMARKS ON MODERN MOTORS.

WITHOUT inquiring further into the working of the machines we are considering, we may see at once why these apparatus have achieved the success till then vainly sought for, and whence comes their superiority to those which have been described in the first part of this book. We have already touched upon this question in the first part, but it will not be amiss to revert to it. In the old machines movement was produced by magnetising one or more masses of iron and making them attract movable armatures, whose displacement produced mechanical work; it was of course necessary that the magnetised masses should cease their action as soon as the attraction was complete, in order to recommence it again so as to give a fresh impulse. This system has three grave defects: 1st, magnetic actions are rapidly weakened by distance, so that, the attractions of a magnet only exercising force in a very small radius, the impulses obtained can be strong only in a very small part of the movement; 2ndly, the motion thus obtained is the result, not of continuous action, but of a succession of jerks, which is always a defective mechanical means of obtaining labour;

3rdly, and this is the greatest disadvantage, the magnetisation and demagnetisation of masses of iron of any size cannot be effected instantaneously: they require a time, very short indeed, but still appreciable; besides, these alternations do not take place without a considerable loss of force. It will be easy to give one experiment in proof of this. An electromagnet being rapidly and frequently magnetised and demagnetised, the core is heated perceptibly; this heat represents so much work, it is the force lost in the successive magnetic movements given to the mass of iron. Yet this expression is erroneous, for force is never lost, only transformed; only here a part of the force employed, instead of furnishing the magnetisation required, produced heat not required of it, which means a loss of power in the useful work in hand.

These three disadvantages are avoided, as may easily be understood, in induction machines. In this sort of apparatus: 1st, the distance of action is reduced to a minimum, the armature turning at a very short distance from 'the magnetic poles; 2ndly, the action, though not theoretically continuous, is composed of such a rapid succession of impulses that it is practically so; and 3rdly, the magnet producing the magnetic field which gives rise to the induction remains always in the same state and goes through no alternations, which allows it to be magnetised to a far greater degree of intensity than in the old machines. There are also other advantages possessed by these machines.

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