CHAPTER IX.

SOURCES OF ELECTRIC POWER.

THE practical applications of electricity afford rich illustrations of the transformations of energy. The coal which generates the steam which drives the engine was produced by electro-magnetic waves from the sun. In the dim past these waves, in the form of light and heat waves, nourished the great ferns and palm trees, and all that luxuriant vegetation which now constitutes our chief reliance for power. Through the combustion of the coal we are enabled to drive the dynamo which produces the electric energy, which is again transformed into motion.

Hardly a tenth of the electro-magnetic energy stored up in the coal is given out by the steam engine. Our principal source of electricity to-day is the steam engine; yet it can be maintained that electricity is back of the steam engine. It is in the coal. It produced this coal, and could make itself manifest to a large degree if we knew how to transform it. Many attempts have been made to obtain electrical power direct from coal, but they have not been successful. I have already referred to thermo-electricity. We have seen that it is possible to construct a furnace with junctions of suitable metals imbedded in its fire pot so that the combustion of coal will produce electrical currents in these

junctions. Indeed, an electric arc light has been formed by such means. The furnace, however, must be cumbrous, and the junctions expand and contract under the changes of temperature and are soon ruined.

We have seen that the modern dynamo machine is a comparatively simple apparatus, merely a number of coils on a revolving shaft surrounded by fixed pieces of iron, around which the currents formed in the revolving coils are made to circulate. The working of the dynamo machine is entirely dependent upon the steam engine which drives it. In the best dynamo machine only about 15 per cent of the energy supplied by the steam engine is lost. When we consider the friction of the bearings and the resistance encountered, the economy of the transformation of energy from that of steam to that of electricity is very perfect. The modern dynamo seems to have reached its highest development.

The spectacle of the transformation of energy by the dynamo in our great cities is most impressive. At the central station are immense steam engines which are whirling the movable coils of the dynamos on axles which run at about one thousand revolutions a minute. The earlier dynamos could almost be lifted by one man. The dynamos of the central stations weigh tons. Thus the mechanical engineer, taking the principles discovered by Faraday, has adapted it to a very perfect machine, and has made one of the greatest transformations of energy witnessed in the mechanical arts.

Another transformation of energy resulted from the construction of the dynamo machine. We have shown that the current produced by one dynamo can, if led to the wires of a second dynamo, make the movable coils of the latter revolve. Thus the second dynamo becomes a motor, and can be used to turn shafting or set in mo

tion any form of machinery. Every electric car has a dynamo motor connected with its axles, and the current produced by the great dynamo at the central station sent over the trolley wire propels the electric cars. Now, electro-magnetic engines were not unknown in the time of Faraday. They were, however, mere lecturetable models, and were run by the current obtained from batteries. The lecturer of twenty years ago often took such models as a text to show the impossibility of obtaining power in this way, for a short calculation of the amount of zinc consumed in the battery compared with the power produced showed the want of economy in such electric motors. Moreover, the mechanical construction of the early electro-motors was very defective. The idea of reciprocating motion, like the piston of a steam engine, was the ruling one. So machines were made with iron cores suspended from a species of walking beam, like that of a steamboat. These cores were alternately sucked into or repelled from coils of wire through which currents of electricity were circulating. The early investigators were appalled by the quickness with which the magnetic forces of attraction and repulsion decreased with the distance. These forces are proportional to the product of the attracting masses and inversely as the square of the distance. Thus the force at the distance of one inch is only one fourth of that at one half an inch. The play of the reciprocating magnetic engine was therefore very small, and the cost of the zinc which was consumed in producing this small play was great. There seemed, therefore, little hope in the employment of magnetic motors. With the discovery of magnetic induction and with clearer ideas of the magnetic field surrounding magnets and electro-magnets hopes in this form of motor revived and were realized.

Currents of electricity could be produced of almost unlimited strength by the dynamo. Very powerful attracting magnets could therefore be made. The revolving coils of the dynamo cut the lines of force of the stationary electro-magnets very close to the face of the poles of these magnets, where the number of lines of magnetic force are greatest. The distance between the revolving coils and these poles in some cases is less than one eighth of an inch. When, therefore, an electric current is sent through a dynamo, the force of attraction between the revolving coils and the stationary coils is very great, since the distance between them is so small and the commutator changes, as I have said, the poles, so that a continuous rotation of the movable coils is produced.

The mechanical arrangement which we have called the commutator is an important factor in the production of a magnetic motor. A clearer conception, however, of the way magnetic lines of force spread out from magnetic poles has led to the perfection of the magnetic motor. Every one is familiar with the way iron filings arrange themselves near the pole of a magnet. They form radiating lines which spread out from the poles as centres of disturbance, and seem to arch from the negative to the positive pole, forming what may be termed magnetic circuits between the poles. If our magnet is made in the form of a horseshoe, the arching is more pronounced; the filings crowd into the space between the two poles, and fewer extend into space. By making the horseshoe nearer and nearer the form of a ring, and bringing the north pole almost into contact with the south pole, the air gap between them being very small, hardly any lines of magnetic force extend into the space about the poles. They are

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concentrated in the air gap. They form almost a closed magnetic circuit through the ring, and the magnetic field in the air gap can thus be made very intense by the number of lines of magnetic force which crowd into this space. In the modern dynamo and the electric motor the stationary electro-magnets are made as near the ring form as possible. The revolving coils are placed in the air gap, where the magnetic lines crowd from one pole of the stationary electro-magnet to the other. Very few lines of force are suffered to stray out of the air gap into outer space. They are nearly all cut by the revolving coils. In considering this form of construction, we readily see how imperfect the early electric motors were. Very few of the lines of magnetic force were utilized in attraction. Most of them strayed into the air and were lost, so far as useful work was concerned.

With the perfection of the electric motor the steam engine became a more important factor in civilization than ever. Electricity appeared to be ready to usurp its place. It could not be produced, however, economically without steam. The servant could not take the place of the master. A new method of distribution of power, however, has sprung into existence, for the electric current generated by steam can be carried

es from the producing station, and can be transrmed again into motion. The present limit of disance to which steady currents of electricity can be economically carried for conversion into power is about five miles, for the electricity tends to escape from the wires to the earth. Moreover, the resistance offered to its flow by the wires becomes too great. I have said steady currents; we shall see later that the distance to which fluctuating currents can be carried is far greater