Before the days of the electric current, men used the power of falling water. The mill or factory using the water-power was placed beside the fall. The water turned a great wheel, to which was connected the machinery of the mill, It was not until the invention of the dynamo and motor that water-power could be used at a great distance. If a hundred years ago a man had said that the time would come when a waterfall could turn the wheels of a mill a hundred miles away he would have been laughed at. Yet this very thing has come to pa.s.s. Indeed, one waterfall may turn the wheels of many factories, run street-cars, and light cities up to a distance of a hundred miles and even more. The power of the falling water goes out over slender copper wires from a great dynamo near the fall to the motors in the factories and street-cars.

The falling water of Niagara has about five million horse-power. About the hundredth part of this power is now being used. The water, falling in a wheel-pit 141 feet deep, turns a great dynamo weighing 87,000 pounds with a speed of 250 turns per minute. A number of such dynamos are used supplying an alternating current at a pressure of 22,000 volts, the current alternating or changing direction twenty-five times per second. Such a pressure is too high for the motors and electric lights, but the current is carried at high pressure to the place where it is to be used and there transformed to a current of low pressure. In carrying a current over a long line, there is less loss if the current is carried at high pressure. With an alternating current this can be done and the current changed by means of a transformer to a current of low pressure.

A transformer is simply two coils of wire wound on an iron core. The simplest transformer is the form used by Faraday when he discovered electromagnetic induction. If instead of making and breaking a circuit that flows only in one direction as Faraday did, we cause an alternating current to flow through one of the coils, which we may call the primary, each time the current changes direction in the primary the magnetic field is reversed--that is, the end of the coil which was the north pole becomes the south pole. This rapidly changing magnetic field induces a current in the secondary coil. Each time the magnetic field of the primary coil is reversed the current in the secondary changes direction.

Thus an alternating current in the primary induces an alternating current in the secondary. One of these coils is of fine wire, which is wound a great many times around the iron. The other is of coa.r.s.er wire wound only a few times around the iron. Suppose the current is to be changed from high pressure to low pressure. Then the high-pressure current from the line is made to flow through the coil of many turns, and a current of low pressure is given out from the coil of few turns.

By changing the number of turns of wire in the coils we can make the pressure whatever we please. If the pressure or voltage of the secondary coil is less than that of the primary, we have a "step-down"

transformer. On the other hand, if we send the current from the line wire through the coil of few turns, then we get a higher voltage from the secondary coil than that of the line wire, and we have a "step-up"

transformer. The Niagara current is "stepped down" from 22,000 volts to 220 volts for use in motors.

An electric lamp may be lighted though not connected to any battery or dynamo, but connected only to a coil of wire (Fig. 114). More than this, the coil may be insulated so that no current can enter it from any other coil or wire, and yet the lamp can be lighted. This can be done only by means of an alternating current. If the coil to which the lamp is connected is held in the magnetic field of an alternating current, then another alternating current is induced in the coil, and this second current flows through the lamp.

[Ill.u.s.tration: FIG. 114--INCANDESCENT ELECTRIC LAMP LIGHTED THOUGH NOT CONNECTED TO ANY BATTERY OR DYNAMO]

We have already learned that a changing magnetic field induces a current in a coil. Now the coil through which an alternating current is flowing has a changing magnetic field all around it, and if the lamp-coil is brought into this changing magnetic field an alternating current will flow through the coil and the lamp. The insulation on the lamp-coil does not prevent the magnetic field from acting, though it does prevent a current from entering the coil. The current is induced in the coil itself, and does not enter it from any outside source.

The transformer works in the same way, the only difference being that in the transformer the two coils are on the same iron core. But in the transformer the two coils are insulated so that no current can flow from one coil to the other. When an alternating current and transformers are used, the current that lights the lamps in the houses or on the streets is not the current from the dynamo. It is a new current induced in the secondary coil of the transformer by the magnetic field of the primary coil.

A peculiar transformer which produces an alternating current that changes direction millions of times in a second has been made by Nikola Tesla. This current will do many wonderful things which no ordinary current will do. It will light a room or run a motor without connecting wires. It has produced an electric discharge sixty-five feet in length (Figs. 115 and 116). Though this current is caused to flow by a pressure of millions of volts, it may be taken with safety through the human body. Strange as it may seem, the safety of this current is due to the high pressure and the rapidity with which it changes direction. While the current used at Sing Sing in executing criminals has a pressure of about twenty-five hundred volts, a current having a pressure of a million volts and alternating hundreds of thousands or millions of times per second is harmless; With such a current the human body may become a "live wire," and an electric lamp to be lighted held in one hand while the other hand grasps the wire from the transformer.

[Ill.u.s.tration: FIG. 115--AN ELECTRIC DISCHARGE AT A PRESSURE OF 12,000,000 VOLTS, A CURRENT OF 800 AMPERES IN THE SECONDARY COIL]

[Ill.u.s.tration: FIG. 116.--AN ELECTRIC DISCHARGE SIXTY-FIVE FEET IN LENGTH]

X-Rays and Radium

A strange light which pa.s.ses through the human body as readily as sunlight through a window was discovered by Prof. Wilhelm Konrad Roentgen, of the University of Wurzburg. This light, which Professor Roentgen named X-rays, is given out when an electric discharge at high pressure pa.s.ses through a certain kind of gla.s.s tube from which the air has been pumped out until there is a nearly perfect vacuum.

X-rays were discovered by accident. Professor Roentgen was working at his desk with one of the gla.s.s tubes when he was called to lunch. He laid the tube with the electric discharge pa.s.sing through it on a book.

Returning from lunch he took a photographic plate-holder which was under the book and made some outdoor exposures with his camera. On developing the plates a picture of a key appeared on one of them. He was greatly puzzled at first, but after a search for the key found it between the leaves of the book. The strange light from the electric discharge in the gla.s.s tube had pa.s.sed through the book and the hard-rubber slide of the plate-holder and made a shadow-picture of the key on the photographic plate. He tried the strange light in many ways, and found that it would go through many objects. It would even go through the human body, so that shadow-pictures of the bones and organs of the body could be obtained. In Fig. 117 is shown a physician using X-rays. Fig. 118 is an X-ray photograph of the eye.

[Ill.u.s.tration: FIG. 117--A PHYSICIAN EXAMINING THE BONES OF THE ARM BY MEANS OF X-RAYS]

[Ill.u.s.tration: FIG. 118--X-RAY PHOTOGRAPH OF THE EYE The eye is above and to the left of the larger black circle. The smaller black circle is a shot which has lodged back of the eye.]

Not long after the discovery of X-rays it was discovered that light very much like the X-rays is given out by certain minerals. One of the most interesting and the best known of these is radium. Radium gives out a light somewhat like X-rays that will go through copper and other metals.

It does many other strange things. It gives out heat as well as light; so much heat, in fact, that it is always about five degrees warmer than the air around it. It continues to give out heat at such a rate that a pound of radium will melt a pound of ice every hour. It can probably keep this up for at least a thousand years. If this heat could be used in running an engine, a hundred pounds of radium would run a one-horse-power engine without stopping for many hundred years. The power of Niagara might be replaced by the power of radium if an engine that could use this power were invented. Fig. 119 is from a photograph made with radium.

[Ill.u.s.tration: FIG. 119--PHOTOGRAPH MADE WITH RADIUM A purse containing a coin. The strange light from the radium goes through the purse and the slide of the plate-holder and makes a shadow-picture.]

The great inventor of the future may be able to use the heat of radium or some new power now unknown. We have seen how, through the toil of many years and the labors of many men, the great inventions of our age have come into being. It may be that we are now witnessing other great inventions in the making.

APPENDIX

BRIEF NOTES ON IMPORTANT INVENTIONS

Aerial Navigation

First air balloon--Montgolfier Brothers, France, 1783.

First balloon ascension--Rozier, France, 1783.

First gas balloon--Charles, France, 1783.

First crossing of the English Channel in a balloon--Blanchard, 1785.

First successful dirigible balloon--La France, Renard and Krebs, France, 1884.

First successful motor-driven aeroplane--Wright Brothers, United States; date of patent, 1906.

First crossing of the English Channel by an aeroplane--Bleriot, 1909.

First air-ship in regular pa.s.senger service--Count Zeppelin, Germany, 1910.

Agriculture

Plough with cast-iron mold-board and iron shares--James Small, Scotland, 1784.

Grain-threshing machine--Andrew Meikle, England, 1788.

McCormick reaper, first practical grain-harvesting machine--Cyrus H. McCormick, United States, 1831.

Self-raker for harvesters--McCormick, 1845.

Inclined platform and elevator in the reaper, to enable men binding the grain to ride with the machine--J. S. Marsh, United States, 1858.

Barbed-wire fence introduced--United States, 1861.

Self-binder, first automatic grain-binding device for the reaper--Jacob Behel, United States, 1864.

Sulky plough--B. Slusser, United States, 1868.

Twine-binder for harvesters--M. L. Gorham, United States, 1873.

Improved self-binding reaper--Lock and Wood, United States, 1873.

Barbed-wire machine--Glidden and Vaughn, United States, 1874.

Rotary disk cultivator--Mallon, United States, 1878.

Steam-plough--W. Foy, United States, 1879.

Combined harvester and thresher--Matteson, United States, 1886.