There are certain theoretical considerations respecting the method of exciting the magnetism of the field in which the armatures are to revolve. The main methods are 4 in number.

Magneto - Dynamos

Magneto - dynamos have the advantage in theory that their electromotive force is very nearly exactly proportional to the velocity of rotation; though, of course, the variable difference of potential between the terminals of the machine depends on the relation of the resistance of the external circuit to the internal resistance of the armature coils. They possess the disadvantage that, since steel cannot be permanently magnetized to the same degree as that which soft iron can temporarily attain, they are not so powerful as other dynamos of equal size.

Separately - Excited Dynamos

The separately - excited dynamo has the same advantage as the magneto - machine, in its electromotive force being independent of accidental changes of resistance in the working circuit, but is more powerful. It has, moreover, the further advantage that the strength of the field is under control, for, by varying either the electromotive force or the resistance in the exciting circuit, the strength of the magnetic field is varied at will. It has the disadvantage of requiring a separate exciting machine.

Series Dynamos

The ordinary or series dynamo is usually a cheaper machine, for equal power, than any other form, as its coils are simpler to make than those of a shunt machine, and it wants no auxiliary exciter. It has the disadvantages of not starting action until a certain speed has been attained, or unless the resistance of the circuit is below a certain minimum. It is also liable to become reversed in polarity, a serious disadvantage when this machine is applied for electroplating or for charging accumulators. From its arrangements, any increase of the resistance in the circuit lessens its power by diminishing the strength of its magnetic field. Hence it is better adapted for use with lamps arranged in parallel arc than for lamps arranged in series. An additional lamp switched in, in series, adds to the resistance of the circuit, and diminishes the power of the machine to supply current; while, on the other hand, an additional lamp in parallel reduces the total resistance offered by the network of the circuit, and adds to the power of the machine to provide the needed current.

It is easy to regulate the currents given by a series dynamo, by introducing a shunt of variable resistance across the field - magnet, thus altering the magnetizing influence of the current.

Shunt Dynamos

The shunt dynamo has several advantages over other forms. It is less liable to reverse its polarity than the series dynamo, and it is commonly considered as providing the magnetizing power to the magnets with less waste of current. Moreover, for a set of lamps in series, its power to supply the needful current increases with the demands of the circuit, since any added resistance sends additional current round the shunt in which the field - magnets are placed, and so makes the magnetic field more intense. On the other hand, there is greater sensitiveness .to inequalities of driving, in consequence of the self - induction in the shunt. The shunt part of the circuit in the present case consists of a fine wire of many turns, wound upon iron cores. It therefore has a much higher co - efficieut of self - induction than the rest of the circuit; and consequently any sudden variations in the speed of driving can but affect the current in the main circuit more than in the shunt. Briefly, the shunt - winding, though it steadies the current against perturbations due to changes of resistance in the circuit, does not steady the current against perturbations due to changes in driving speed.

In the series - wound dynamo, the converse holds good.

Any of these systems may be applied either in direct current or in alternate current machines. Each has its own merits for special cases, but none is perfect. Not one will ensure that, with uniform driving speed, either the electromotive force of the current shall be constant, however the resistances of the circuit are altered.

There is no such thing yet as a best dynamo. One gives steadier currents, another is less liable to heat, a third is more compact, a fourth is cheaper, a fifth is less likely to reverse its currents, a sixth gives a greater volume of current, while a seventh evokes a higher electromotive force.

Combination Methods

A method of rendering a dynamo automatically self - adjusting, so that either its electromotive force or its current (according to circumstances) shall be constant, is due to Deprez. If a dynamo be wound with a double set of coils, one of which can be traversed by an independent current, whilst the other is traversed by the current of the machine itself, there can always be found a certain critical velocity of driving, for which, provided the field - magnets are far from attaining their saturation point, the desired condition is fulfilled. Other combination methods have been suggested, and a summary of them follows.

(1) Series and Separate (for Constant E.M.F.), Deprez. - This method can be applied to any ordinary dynamo, provided the coils are such that a separate current from an independent source can be passed through a part of them, so that there shall be an initial magnetic field, independent of the main - circuit current of the dynamo. When the machine is running, the electromotive force producing the current will depend partly on this independent excitement, partly on the current's own excitement of the field - magnets. If the machine be run at such a speed that the quotient of the part of the electromotive force due to the self - excitement, divided by the strength of the current, is numerically equal to the internal resistance of the machinery, then the electromotive force in the circuit will be constant, however the external resistances are varied. This velocity can be deduced from experiment, and when the critical velocity has once been determined, the machine can be adjusted to work at any desired electromotive force, by varying the strength of the separately - exciting current to the desired degree.

(2) Shunt And Separate (For Constant Current), Deprez

When cases arise, as for a set of arc lamps in series, that it is desired to maintain the current in the circuit at one constant strength, the previous arrangement must be modified by combining a shunt - winding with coils for a separately - exciting current. This arrangement is, in fact, that of a shunt - dynamo, with an initial magnetic field independent of the strength of the current in the circuit. Seeing that the only object in providing the coils for separate excitement is to secure an initial and independent magnetic field, it is clear that other means may be employed to bring about a similar result.

(3) Series and Magneto (Constant F.M.F.), Perry. - The initial electromotive force in the circuit, required by Deprez's theory, need not necessarily consist in there being an initial magnetic field of independent origin. It is true that the addition of a permanent magnet, to give an initial partial magnetization to the pole - pieces of the field magnets, would meet the case to a certain extent; but Prof. Perry has adopted the more general solution of introducing into the circuit of a series - dynamo a separate magneto machine, also driven at a uniform speed, such that it produces in the circuit a constant electromotive force equal to that which it is desired should exist between the leading and return mains. This arrangement may be varied by using a shunt - wound dynamo, the magnets being, as before, included in the part of the circuit outside the machines.

(4) Shunt And Magneto (Constant Current), Perry

Perry's arrangement for constant current consists in combining a shunt - dynamo with a magneto machine of independent electromotive force, this magneto machine being inserted either in the armature part or in the magnet - shunt part of the machine. As before, a certain critical speed must be found from experiment and calculation.

(5) Series And Shunt

A dynamo haying its coils wound so that the field - magnets are excited partly by the main current, partly by a current shunted across the brushes of the machine, is not so perfect as either of the preceding, being more limited in operation. If the shunt coils be comparatively few and of high resistance, so that their magnetizing power is small, the machine will give approximately a uniform electromotive force; whereas, if the shunt be relatively a powerful magnet - izer, as compared with the few coils of the main circuit, the machine will be better adapted for giving a constant current; but, as before, each case will correspond to a certain critical speed, depending on the arrangements of the machine.

(6) Series And Long Shunt

Prof. Thompson gives this name to a combination closely resembling the preceding. If the magnets are excited partly in series, but also partly by coils of finer wire, connected as a shunt across the whole external circuit, then the combination should be more applicable than the preceding to the case of a constant electromotive force, since any variation in the resistance of the external circuit will produce a greater effect in the " long shunt " than would be produced if the resistance of the field - magnets were included in the part of the main circuit external to the shunt. Although the last 2 combinations are not such perfect solutions of the problem as those which precede, they are more likely to find immediate application, since they can be put into practice upon any ordinary machine, and do not require, as in the first 4 combinations, the use of separate exciters, or of independent magneto - machines.

All these arrangements presuppose a constant velocity of driving; but they are not the only ones consistent with this condition. An ordinary series dynamo may be made to yield a constant current, by introducing across the field - magnets a shunt of variable resistance, the resistance of the shunt being adjusted automatically by an electro - magnet whose coils form part of the circuit, This is actually done in the automatic regulator attached to Brush dynamos, as used in supplying a series of arc lights. A shunt - dynamo may similarly be controlled, so as to yield a uniform electro - motive force, by introducing a variable resistance into the shunt - magnet circuit as is done in some of Edison's dynamos. To make the arrangement perfect, this variable resistance should be automatically adjusted by an electro - magnet whose coils are an independent shunt across the mains of the external circuit. Yet another way of accomplishing the regulation of dynamos is possible in practice, without the condition of a constant speed of driving.

Let the ordinary centrifugal governor of the steam - engine be - abandoned, and let the supply of steam be regulated, not by the condition of the velocity of driving, but by means of an electric governor, such as an electro - magnet working against an opposing spring. If this electro - magnetic governor is to maintain a constant electromotive force, its coils must be a shunt to the mains of the circuit. If it is to maintain a constant current, its coils must be part of the main circuit. Such a governor ought to be more reliable and rapid than any centrifugal governor intended to secure a uniform speed of driving.