Of the three types of dynamos mentioned, series machines are little used except for the lighting of arc lamps, themselves in series, - that is to say, the current from one terminal of the dynamo traverses every lamp in succession, and then returns to the other terminal. For transmission of power, and for charging accumulators, shunt dynamos are principally used; also for the generation of current in electric-light stations or works, as they readily allow of being joined in " parallel". that is to say, two or more such machines can be connected to one external circuit, by connecting all their terminals on one side to one wire of the external circuit, and all their terminals on the other side to the other wire. The dynamos would require, in such an arrangement, to be all of similar potential or voltage, and supposing the potential of each were 100, the total potential would be 100. The current, however, from the combined machines would be the sum of all the currents; if there are five machines, each generating 100 amperes, the total will be 500 amperes.

I have throughout spoken of the current having one terminal of a dynamo, flowing through the external circuit, and returning through the other, and I may here point out that the terminal at which it leaves is called "positive and that at which it returns is called "negative", the former being often designated by the sign + , and the latter by the sign - . The question as to which terminal will he positive, on a dynamo being taken to the testing-room to be run for the first time, is decided by the way its ironwork has become magnetized, and so, unless precautions are taken, it is a mere chance which is positive; should the right limb of the magnets become the "North" pole, and the left the " South" pole, the machine could be "reversed", as it is termed, by reversing the magnetism of the poles.This could be done by reversing the current through the shunt-winding of the magnets.

In speaking above of shunt dynamos being connected in parallel, it should be understood that the "positive" of each machine would constitute the one side mentioned, and the "negative".the other side.

Since, as I have shown, the polarity of a dynamo can be reversed by reversing the current through the magnet-winding, care must be taken to prevent the polarity of one of a number of machines working in parallel becoming reversed accidentally, l>ecause in such an event it would not be like the remainder, sending current in one common direction, but would be acting in just the contrary manner; the reversed machine would be absorbing current given out by the others for the circuit Fortunately a shunt machine cannot be so reversed, since, if the positive brushes of two machines are joined together, the shunt-windings of their magnets are both receiving positive current, and if one machine stopped, the other would still be supplying positive current at that point into its magnet-winding. Whereas, with series machines, if one machine stopped, the current from the other would immediately traverse its magnet-coils in the opposite direction to that in which the current from its own armature had, a moment before, been passing, and the magnets would become reversed, necessitating the reconnecting of the terminals. It is for this reason of immunity from reversing, that shunt dynamos are used in works for generating electricity.

Compound machines, being partly series-wound, can not be joined in parallel without precautions being taken too complicated to mention here.

Either series, shunt, or compound dynamos can be joined "in series", which means that the current from one machine is made to traverse the other beefore arriving at the external circuit, As I mentioned just now, machines in parallel give a total E.M.F of any one of them, and a current of all their currents added together. Machines in series, however, give a current of any one of them, and a total E.M.F of all their E.M.F.'s added together.

Machines in parallel must all be of the same E.M.F. to prevent one "running back" on another; they can, however, give varying quantities of current. Machines in series must all give the same current; otherwise one would be subjected to a greater current than that for which it was built, since the current from each machine passes through the remainder.

Compound dynamos are chiefly used for private electric-light installations, having, as 1 have described, the important feature of maintaining, with a constant speed, a constant E.M.F. The value of a constant pressure is recognized by all, since the fluctuations in the light given by incandescent lamps is chiefly due to slight variations.

As 1 have mentioned transmission of power above, and as electric power is now being more and more largely used, not only in workshops and factories, but also in houses and vehicles, a few words may be added to describe its theory. If a dynamo be driven by an engine, and give (say) 100 volts, and two wires therefrom be taken to a similar machine (called the "motor"), and placed in its terminals, the motor will revolve its armature at such a speed as to give a counter E.M.F. of nearly equal quantity, and will give off at its pulley a power nearly equal to that expended upon the dynamo.

Power-transmission on this principle has been largely adopted of late. In many instances where manufacturers have l>oilers supplying a large number of engines throughout their works, it has been proved most conclusively that, by substituting one main engine driving a dynamo, by which to supply with current motors in the place of the engines previously used, great economy is effected, owing to the avoidance of loss by radiation, and the absence of small steam-engines, which are notoriously not economical. The principle is of course without rival in the case of water-power being utilised to turn a turbine, which is coupled to the dynamo. In some instances the current is conveyed to a motor many miles away.