[Ill.u.s.tration: FIG. 41--A STORAGE-BATTERY PLATE MADE FROM A SHEET OF LEAD]

The storage battery does not store up electricity. It produces a current in exactly the same way as any other battery--by the action of the acid on the plates. When this action ceases it is no longer a battery, though it may be made one again by pa.s.sing a current through it in the opposite direction from that which it gives out. In this it differs from the voltaic battery, for when such a battery is run down it can be restored only by adding new solution or new plates. The storage battery is especially useful for "sparking" in gas or gasolene motors.

Edison has invented a storage battery that will do as much work as a lead battery of twice its weight. Edison's battery is intended especially for use in electric automobiles. By reducing the weight of the battery which the machine must carry the weight of the truck may also be reduced. In the Edison battery the positive plates are made of a grid of nickel-plated steel containing tubes filled with pure nickel.

The negative plate consists of a nickel-plated steel grid containing an oxide of iron similar to common iron-rust.

After working a number of years on this battery and making nine thousand experiments, Edison thought he had it perfected, and indeed it was a great improvement over the storage batteries that had been used--much lighter and cheaper, and more successful in operation. Two hundred and fifty automobiles were equipped with it, and it proved superior to lead batteries for this purpose. But it was not to Edison's liking. He threw the machinery, worth thousands of dollars, on the sc.r.a.p-heap, and worked on for six years. He had then produced a battery as much better than the first as the first was better than the lead battery, and he was content to have the new battery placed on the market.

The Dynamo

For the purpose of lighting and power the electric battery proved too costly. Davy produced an arc light with a battery of four thousand cells. The arc was about four inches in length and yielded a brilliant light, but as the cost was six dollars a minute it was not thought practical. Attempts were made early in the century to use a battery current for power, but they failed because of the cost and the fact that no good working motor had been invented.

Light and power were needed. Electricity could supply both. But how overcome the difficulty of cost, and produce an electric current from burning coal or falling water? For answer man looked to the great discovery of Faraday and his "new electrical machine." Inventors in Germany, France, England, Italy, and America made improvements until from the disk dynamo of Faraday there had evolved the modern dynamo.

Electroplating and the telegraph are the only applications of the electric current that became factors in the world's industry before the dynamo, yet in long-distance telegraphy and in electroplating to-day the dynamo is used. Without the dynamo, electric lighting, electric power, and electric traction as developed in the nineteenth century would have been impossible; in fact, the dynamo with the electric motor (which, as we shall see, is only a dynamo reversed) is master of the field.

The way had been prepared for the application of Faraday's discovery by William Sturgeon, an Englishman, and Joseph Henry, an American. Sturgeon discovered that soft iron is more quickly magnetized than steel, and found that the strength of an electromagnet can be greatly increased by making the core of a soft-iron rod and bending the rod into the form of a horseshoe (Fig. 42). The iron rod was coated with sealing-wax and wound with a single layer of copper wire, the turns of wire not touching. This was in 1825, before Faraday discovered the principle of the dynamo.

[Ill.u.s.tration: FIG. 42--STURGEON'S ELECTROMAGNET]

Professor Henry still further increased the strength of the electromagnet by covering the wire with silk, which made it possible to wind several layers of wire on the iron core, and many times the length of wire that had been used by Sturgeon. Fig. 43 shows such a magnet. One of Henry's magnets weighed fifty-nine and a half pounds, and would hold up a ton of iron. Sturgeon said: "Professor Henry has produced a magnetic force which completely eclipses every other in the whole annals of magnetism." With Professor Henry's invention the electromagnet was ready for use in the dynamo. Fig. 44 shows a strong electromagnet.

[Ill.u.s.tration: FIG. 43--AN ELECTROMAGNET WITH MANY TURNS OF INSULATED WIRE]

[Ill.u.s.tration: FIG. 44--AN ELECTROMAGNET LIFTING TWELVE TONS OF IRON]

A moving magnet causes a current to flow in a coil, but a magnet at rest has no effect. A moving magnet is equal to a battery. In Faraday's experiments a current was induced in a coil of wire by moving a magnet in the coil or by making and breaking the circuit in another coil wound on the same iron core. A current was induced in a metal disk by revolving it between the poles of a magnet. In every case there was motion in a magnetic field, or the field itself was changed. A changing magnetic field is equal to a moving magnet. What is needed to induce a current in a coil, whether it be in a dynamo, an induction-coil, or a transformer, is a changing magnetic field about the coil or motion of the coil in the magnetic field.

If fine iron filings are sprinkled over the poles of a magnet the filings arrange themselves in definite lines. This is a simple experiment which any boy can try for himself. Faraday called the lines marked out by the iron filings "lines of force" (the lines of force of a horseshoe magnet are shown in Fig. 36), because they indicate the direction in which the magnet pulls a piece of iron--that is, the direction of the magnetic force. Now, if a current is to be induced in a wire, the wire must move across the lines of force. If the wire moves along the lines marked out by the iron filings, there will be no current. When a coil rotates between the poles of a magnet, the wire moves across the lines of force and a current is induced in the coil if the circuit is closed. This is the way a current is produced in a dynamo.

Faraday produced a current by rotating a coil between the poles of a steel magnet. He made a number of such machines, and used them with some success in producing lights for lighthouses, but the defects of these machines were so great that the lighting of a city or the development of power on a large scale was impractical. The electromagnet was needed to solve the problem.

Siemens' Dynamo

The war of 1866 between Austria and Prussia and the certainty of a coming struggle with France turned the attention of German inventors to the use of electricity in warfare. Werner von Siemens, an artillery officer, was improving an exploding device for mines. An electric current was needed to produce a spark or heat a wire to redness in the powder. Faraday had used a coil of wire turning between the poles of a steel magnet to produce a current. In England a coil turning between the poles of an electromagnet had been used, but the electromagnet received its current from another machine in which a steel magnet was used.

Siemens found that the steel magnet could be dispensed with, and that a coil turning between the poles of an electromagnet could furnish the current for the electromagnet. Two things are needed, then, to make a dynamo: an electromagnet and a coil to turn between the poles of that magnet. The rotating coil, which usually contains a soft-iron core, is called the "armature." The coil will furnish current for the magnet and some to spare; in fact, only a small part of the current induced in the coil is needed to keep the magnet up to its full strength, and the greater part of the current may be used for lighting or power. The new machine was named by its inventor "the dynamo-electric machine." The name has since been shortened to "dynamo." The first practical problem which the dynamo solved was the construction of an electric exploding apparatus without the use of steel magnets or batteries. A dynamo with Siemens' armature is shown in Fig. 45.

[Ill.u.s.tration: FIG. 45--A DYNAMO WITH SIEMENS' ARMATURE]

In his first enthusiasm the inventor dreamed of great things for the new machine, among others an electric street railway in Berlin. But the dynamo was not yet ready. The difficulty was the heating of the iron core of the armature, caused by the action of induced currents. There are induced currents in the iron core as well as in the coil, and, for the same reason, the coil and the iron core within it are both moving in a magnetic field. These little currents circling round and round in the iron core produce heat. The rapid changing of the magnetism of the iron also heats the iron.

It remained for Gramme, in France, to apply the proper remedy. This remedy was an armature in which the coil was wound on an iron ring, invented by an Italian, Pacinotti. Gramme applied the principle discovered by Siemens to Pacinotti's ring, and produced the first practical dynamo for strong currents. This was in 1868. A ring armature is shown in Fig. 46. The first dynamo patented in the United States is shown in Fig. 47. This dynamo is only a curiosity.

[Ill.u.s.tration: FIG. 46--RING ARMATURE]

[Ill.u.s.tration: FIG. 47--FIRST DYNAMO PATENTED IN THE UNITED STATES Intended to be used for killing whales.

Photo by Claudy.]

The Drum Armature

An improvement in the Siemens armature was made four years later by Von Hefner-Alteneck, an engineer in the employ of Siemens. This improvement consisted in winding on the iron core a number of coils similar to the one coil of the Siemens armature, but wound in different directions.

This is called the "drum armature" (Fig. 48). The heating of the core is prevented by building it up of a number of thin iron plates insulated from one another and by air-s.p.a.ces within the core. The insulation prevents the small currents from flowing around in the core. The air-s.p.a.ces serve for cooling. The drum armature was a great improvement over both the Siemens and the Gramme armatures. With the Siemens one-coil armature there is a point in each revolution at which there is no current. The current, therefore, varies during each revolution of the armature from zero to full strength. In the Gramme armature only half the wire, the part on the outside of the ring, receives the full effect of the magnetic field. The inner half is practically useless, except to carry the current which is generated in the outer half. Both these difficulties are avoided in the drum armature. The dynamos of to-day are modifications of the two kinds invented by Siemens and Gramme. Many special forms have been designed for special kinds of work.

[Ill.u.s.tration: FIG. 48--A DRUM ARMATURE, SHOWING HOW AN ARMATURE OF FOUR COILS IS WOUND]

Edison's Compound-Wound Dynamo

Edison, in his work on the electric light and the electric railway, made some important improvements in the dynamo. The armature of a dynamo is usually turned by a steam-engine. Edison found that much power was wasted in the use of belts to connect the engine and the dynamo. He therefore connected the engine direct to the dynamo, placing the armature of the dynamo on the shaft of the engine. He also used more powerful field-magnets than had been used before. His greatest improvement, however, was in making the dynamo self-regulating, so that the dynamo will send out the strength of current that is needed. Such a dynamo will send out more current when more lights are turned on.

Whether it supplies current for one light or a thousand, it sends out just the current that is needed--no more, no less. It will do this if no human being is near. An attendant is needed only to keep the machinery well oiled and see that each part is in working order. Edison brought about this improvement by his improved method of winding. This method is known as "compound winding."

To understand compound winding we must first understand two other methods of winding. In the series winding (Fig. 49), all the current generated in the armature flows through the coils of the field-magnet.

There is only one circuit. The same current flows through the coils of the magnet and through the outer circuit, which may contain lights or motors. Such a dynamo is commonly used for arc lights. It will not regulate itself. If left to itself it will give less electrical pressure when more pressure is needed. It requires a special regulator.

[Ill.u.s.tration: FIG. 49--A SERIES-WOUND DYNAMO]

In the second form of winding the current is divided into two branches.

One branch goes through the coils of the field-magnet. The other branch goes through the line wire for use in lights or motors. This is called the "shunt winding" (Fig. 50). The shunt-wound dynamo is used for incandescent lights. It also requires a special regulator, for if left to itself it gives less electrical pressure when the pressure should be kept the same.

[Ill.u.s.tration: FIG. 50--A SHUNT-WOUND DYNAMO]

The compound winding (Fig. 51), which was first used by Edison, is a combination of the series and shunt windings.

[Ill.u.s.tration: FIG. 51--A COMPOUND-WOUND DYNAMO]

The current is divided into two branches. One branch goes only through the field-coils. The other branch goes through additional coils which are wound on the field-magnet, and also through the external circuit.

Such a dynamo can be made self-regulating, so that it will give always the same electrical pressure whatever the number of lamps or motors thrown into the circuit. In maintaining always the same pressure it of course supplies more or less current, according to the amount of current that is needed. This is clear if we compare the flow of electric current with the flow of water. Open a water-faucet and notice how fast the water flows. Then open several other faucets connected with the same water-pipe. Probably the water will not flow so fast from the first faucet. That is because the pressure has been lowered by the flow of water from the other faucets. If we could make the water adjust its own pressure and keep the pressure always the same, then the water would always flow at the same rate through a faucet, no matter how many other faucets were opened. This is what happens in the Edison compound-wound dynamo. Turn on one 16-candle-power carbon lamp. It takes about half an ampere of current. Turn on a hundred lamps connected to the same wires, and the dynamo of its own accord keeps the pressure the same, and supplies fifty amperes, or half an ampere for each lamp. With this invention of Edison the dynamo was practically complete, and ready to furnish current for any purpose for which current might be needed.

Fig. 52 shows one of Edison's first dynamos. Fig. 53 shows a dynamo used for lighting a railway coach.

[Ill.u.s.tration: FIG. 52--ONE OF EDISON'S FIRST DYNAMOS Permission of a.s.sociation of Edison Illuminating Companies.]

[Ill.u.s.tration: FIG. 53--A DYNAMO MOUNTED ON THE TRUCK OF A RAILWAY CAR The dynamo furnishes current for the electric lights in the car. When the train is not running the current is furnished by a storage battery.]

Electric Power

It has been said that the nineteenth century was the age of steam, but the twentieth will be the age of electricity. Before the end of the nineteenth century, however, electric power had become a reality, and there remained only development along practical lines.

We must turn to Oersted, Ampere, and Faraday to find the beginning of electric power. In Oersted's experiment, motion of a magnet was produced by an electric current. Ampere found that electric currents attract or repel each other, and this because of their magnetic action. Faraday found that one pole of a magnet will spin round a wire through which a current is flowing. Here was motion produced by an electric current.

These great scientists discovered the principles that were applied later by inventors in the electric motor.

A number of motors were invented in the early years of the century, but they were of no practical use. It was not until after the invention of the Gramme and Siemens dynamos that a practical motor was possible. It was found that one of these dynamos would run as a motor if a current were sent through the coils of the armature and the field-magnet; in fact, the current from one dynamo may be made to run another similar machine as a motor. Thus the dynamo is said to be reversible. If the armature is turned by a steam-engine or some other power, a current is produced. If a current is sent through the coils, the armature turns and does work. If the machine is used to supply an electric current, it is a dynamo. If used to do work--as, for example, to propel a street-car and for that purpose receives a current--it is a motor. The same machine may be used for either purpose. In practice there are some differences in the winding of the coils of dynamos and motors, yet any dynamo can be used as a motor and any motor can be used as a dynamo. This discovery made it possible to transmit power to a distance with little waste as well as to divide the power easily. The current from one large dynamo may light streets and houses, and at the same time run a number of motors in factories or street-cars at great distances apart. A central-station dynamo may run the motors that propel hundreds of street-cars. Dynamos at Niagara furnish current for motors in Buffalo and other cities. One great scientist, who no doubt fore-saw the wonders of electricity which we know so well to-day, said that the greatest discovery of the nineteenth century was that the Gramme machine is reversible.

The First Electric Railway

The electric railway was made possible by the invention of the dynamo and the discovery that the dynamo is reversible. At the Industrial Exposition in Berlin in 1879 there was exhibited the first practical electric locomotive, the invention of Doctor Siemens. The locomotive and its pa.s.senger-coach were absurdly small. The track was circular, and about one thousand feet in length. This diminutive railway was referred to by an American magazine as "Siemens' electrical merry-go-round." But the electrical merry-go-round aroused great interest because of the possibilities it represented (Fig. 54).

[Ill.u.s.tration: FIG. 54--FIRST ELECTRIC LOCOMOTIVE]

The current was generated by a dynamo in Machinery Hall, this dynamo being run by a steam-engine. An exactly similar dynamo mounted on wheels formed the locomotive. The current from the dynamo in Machinery Hall was used to run the other as a motor and so propel the car. The rails served to conduct the current. A third rail in the middle of the track was connected to one pole of the dynamo and the two outer rails to the other pole. A small trolley wheel made contact with the third rail. The rails were not insulated, but it was found that the leakage current was very small, even when the ground was wet.