This section maybe divided into 2 parts: (l)the principles and practice of the construction of electro - motors, and (2) their application. The first may be best studied from Prof. Thompson's Cantor Lecture, before alluded to.

Construction

An electro - motor, or electro - magnetic engine, is one which does mechanical work at the expense of electric energy, whether the magnets which form the fired part of the machine be permanent magneta of steel or electro-magnets. Any of the 4 kinds of dynamo can be used as a motor, though soma more appropriately than others. All are electro - magnetic in principle; i.e. there is some part, fixed or moving, which is an electro - magnet, and which attracts and is attracted magnetically. A magnet will attract the opposite pole of another magnet, and pull it round; also, every magnet placed in a magnetic field tends to turn round and set itself along the lines of force. Supposing a small magnetic needle to be confined at right angles to the lines of force in a simple magnetic field, produced between the poles of 2 strong magnets, one on the right, the other on the left, then shortening the lines of force has the effect of rotating the magnetic needle upon its centre, through an angle of 90°. Very soon after the invention of the electromagnet, many perceived that it would be possible to construct an electro - magnetic engine, in which an electro - magnet, placed in a magnetic field, should be pulled round; and further, that the rotation should be kept up continuously by reversing the current at an appropriate moment.

A mere coil of wire, carrying a current, is acted upon when placed in the magnetic field, and is pulled round as a magnet is. On this principle was constructed the earliest electric motor of Ritchie, well known in many forms, but little better in reality than a toy.

Jacobi constructed a multipolar machine for his electric boat. It had 2 strong wooden frames, in each of which 12 electro - magnets were fixed, their poles being set alternately. Between them, upon a wooden disk, were, placed another set of electro - magnets, which, by the alternate attraction and repulsion of the fixed poles, were kept in rotation, the current which traversed the rotating magnets being regularly reversed at the moment of passing the poles of the fixed magnets, by means of a commutator consisting of 4 brass - toothed wheels, having pieces of. ivory or wood let in between the teeth for insulation. Jacobi's motor is, in fact, a very advanced type of dynamo.

An earlier rotating apparatus, and, like Ritchie's motor, a mere toy, was Sturgeon's wheel, described in 1823. This instrument, interesting as being the forerunner of Faraday's disk dynamo, is the representative of an important class of machines, namely, those which have a sliding contact merely, and need no commutator.

A fourth class may be named, wherein the moving part, instead of rotating upon an axis, is caused to oscillate backwards and forwards. Prof. Henry constructed, in 1831, a motor with an oscillating beam, alternately drawn backwards and forwards by the intermittent action of an electro - magnet. Dal Negro's motor of 1833 was of this class; in it a steel rod was caused to oscillate between the poles of an electromagnet, and caused a crank, to which it was geared, to rotate in consequence. A distinct improvement in this type of machine was introduced by Page, who employed hollow coils or bobbins as electro - magnets, which, by their alternate action, sucked down iron cores into the coils, and caused them to oscillate to and fro. Motors of this kind form an admirable illustration of one of the laws of electro - magnetics, to the effect that a circuit acts on a magnetic pole in such a way as to make the number of magnetic lines of force that pass through the circuit a maximum.

Page's suggestion was further developed by Bourbouze, who constructed a motor which looks like an old type of steam - engine. It has a beam, crank, fly - wheel, connecting - rod, eccentric valve - gear, and slide - valve; but for cylinders, 4 hollow electro - magnets; for pistons, iron cores, that are alternately sucked in and repelled out; and for slide valve, a commutator, which, by dragging a pair of platinum - tipped springs over a flat surface made of 3 pieces of brass separated by 2 insulating strips of ivory, reverses at every stroke the direction of the currents in the coils of the electro - magnets. It is a very ingenious machine, but in point of efficiency far behind many other electric motors.

A fifth class of electric motors owes its existence to Froment, who, fixing a series of parallel iron bars upon the periphery of a drum, caused them to be attracted, one after the other, by electro - magnets, and thus procured a continuous rotation.

Last of the various types of motor may be enumerated a class in which the rotating portion is enclosed in an eccen tric frame of iron, so that as it rotates it gradually approaches. Little motors, working on this principle of "oblique approach," were invented by Wheat - stone, and have long been used for spinning Geissler tubes, and other light experimental work. More recently, Trouve and Wiesendanger have sought to embody this principle in motors of more ambitious proportions, but without securing any great advantage.

All the early attempts came to nothing, for 2 reasons: there was no known economical method of generating electric currents, and the great physical law of the conservation of energy was not recognised. While voltaic batteries were the only available sources of electric currents, economical working of electric motors was hopeless; for a voltaic battery, wherein electric currents are generated by dissolving zinc in sulphuric acid, is a very expensive source of power. To say nothing of the cost of the acid, the zinc - the very fuel of the battery - costs more than 20 times as much as coal, and is a far worse fuel; for whilst 1 oz. of zinc will evolve heat to an amount equivalent to 113,000 foot - lb. of work, 1 oz. of coal will furnish the equivalent of 695,000 foot - lb.