The following description of dynamo - electric machinery is mainly derived from a series of Cantor Lectures by Prof. Silvanus Thompson, delivered before the Society of Arts in December, 1882.

A dynamo - electric machine is an apparatus for converting energy in the form of dynamical power into energy in the form of electric currents, by the operation of setting conductors, usually in the form of copper wire coils, to rotate in a magnetic field. From a consideration of the principles underlying dynamo - electric machines, Prof. Thompson formulates the following summary: -

(1) A part at least of the energy of an electric current exists in the form of magnetic whirls in the space surrounding the conductor.

(2) Currents can be generated in conductors by setting up magnetic whirls round them.

(3) Magnetic whirls can be set up in conductors by moving magnets near them, or moving them near magnets.

(4) To set up and maintain such magnetic whirls requires a continuous expenditure of energy; i.e. consumes power.

(5) To induce currents in a conductor, there must be relative motion between conductor and magnet of such kind as to alter the number of lines of force embraced in the circuit.

(6) Increase in the number of lines of force embraced by the circuit produces a current in the opposite sense to decrease.

(7) Approach induces an electromotive force in the opposite direction to that induced by recession.

(8) The more powerful the magnet - pole or magnetic field, the stronger will be the current generated (other things being equal).

(9) The more rapid the motion, the stronger will be the currents.

(10) The greater the length of the moving conductor thus employed in cutting lines of force (i. e. the longer the bars, or the more numerous the turns of the coil), the stronger will be the currents generated.

(11) The shorter the length of those parts of the conductor not so employed, the stronger will be the current.

(12) Approach being a finite process, the method of approach and recession (of a coil towards and from a magnet pole) must necessarily yield currents alternating in direction.

(13) By using a suitable commutator, all the currents, direct or inverse, produced during recession or approach, can be turned into the same direction in the wire that goes to supply currents to the external circuits, thereby yielding an almost uniform current.

(14) In a circuit where the flow of currents is steady, it makes no difference what kind of magnet is used to procure the requisite magnetic field, whether permanent steel magnets or electromagnets, self - excited or otherwise.

(15) Hence the current of the generator may be itself utilized to excite the magnetism of the field - magnets, by being caused, wholly or partially, to flow round the field - magnet coils.

Many varieties of dynamo - electric machine have been constructed upon the foregoing principles. Prof. Thompson distinguishes 3 main classes: -

I. - Dynamos in which there is rotation of a coil in a uniform field of force, such rotation being effected round an axis in the plane of the coil, or one parallel to such an axis. Examples: Gramme, Siemens (Alteneck), Edison, Lontin, Burgin, Fein, Schuckert, Jur - gensen (Thomson's Mousemill - dynamo), [Brush].

II. - Dynamos in which there is translation of coils to different parts of a complex field of varying strength or of opposite sign. Most machines of this class furnish alternate currents. Examples: Pixii, Clarke, Niaudet, Wallace, Farmer, Wilde (alternate), Siemens (alternate), Hopkinson and Muir - head, Thomson (alternate), Gordon (alternate), Siemens - Alteneck (Disk Dynamo), Edison (Disk Dynamo), De Meritens.

III

Dynamos having a conductor rotating so as to produce a continuous increase in the number of lines of force cut, by the device of sliding one part of the conductor on or round the magnet, or on some other part of the circuit. Examples: Faraday's Disk - machine, Siemens's (" Unipolar " Dynamo), Voice's Dynamo.

One machine does not fall exactly within any of these classes, and that is the' extraordinary tentative dynamo of Edison, in which the coils are waved to and fro at the ends of a gigantic tuning - fork, instead of being rotated on a spindle.

A dynamo of any one of these plans must be constructed upon the following guiding lines: -

(a) The field - magnets as strong as possible, and their poles as near together as possible.

(6) The armature having the greatest possible length of wire upon its coils.

(c) The wire of the armature coils as thick as possible, so as to offer little resistance.

(d) A very powerful steam - engine to turn the armature, because.

(e) The speed. of rotation should be as great as possible.

It is impossible to realize all these conditions at once, as they are incompatible with one another; and there are many additional conditions to be observed. Prof. Thompson deals with the various matters in order, beginning with the speed of the machine.

## Relation Of Speed To Power

Theory shows that, if the intensity of the magnetic field be constant, the electromotive force should be proportional to the speed of the machine. This is true within certain limits, for machines in which the field magnets are independent of the main circuit, i.e. for magneto and separately - excited dynamos. It is not, however, quite exact, unless the resistance of the circuit be increased proportionately to the speed, because the current in the coils itself reacts on the magnetic field, and alters the distribution of the lines of force. The consequence of this reaction is that (1) the position of the " diameter of commutation " is altered, and (2) the effective number of lines of force is reduced. So that, with a constant resistance in circuit, the electromotive force, and therefore the current, are slightly less at high speeds than the proportion of the velocities would lead one to expect. Since the product of current into electromotive force gives a number proportional to the electric work of the machine, it follows that, for " independently excited" machines, the electric work done in given time is nearly proportional to the square of the speed, and the work drawn from the steam - engine will be similarly proportional to the square of the speed.

In self - exciting machines, whether "series " or " shunt" in their arrangements, a wholly different law obtains. If the iron of the field - magnets be not magnetized near to saturation, then, since the increase of current consequent on increase of speed produces a nearly proportional increase in the strength of the magnetic field, this increase will react on the. electromotive force, and cause it to be proportional more nearly to the square of the velocity, which again will cause the current to increase in like proportion. But since the magnetization of the iron is, even when far from saturation point, something less than the magnetizing force, it is in practice found that the electric work of the machine is proportional only to something slightly less than the third power of the speed. As mechanical considerations limit the velocity of the moving parts, it is clear that, at the limiting speed at which it is safe to run any given armature, the greatest amount of work will be done by using the most powerful magnets possible - electro - magnets rather than steel.

Deprez has found that for every dynamo there is a certain " critical" speed, at which, no matter what the current is which circulates in the coils of the field magnets, the electromotive force is proportional to the strength of that current; and he bases upon it 2 methods for obtaining, automatically, either a constant electromotive force or a constant current, at will, in a circuit in which the resistances are varied to any degree. In all these combinations, however, everything depends upon the condition that the driving speed shall be uniform. Gas engines are out of the question as a source of power; even with the best steam - engines a specially sensitive valve is required, and probably such valves will, in future, be operated electrically by self acting electromagnet gearing. Where the driving is at all liable to be uneven, the precaution should be taken of placing a heavy fly - wheel on the axis of the dynamo.