Chapter XXV: Appendix

It was known two thousand years ago that when a piece of amber was rubbed with a woollen cloth, the amber would attract light objects towards it. Amber was considered to be unique in this respect.

About the year 1600, one of Queen Elizabeth's physicians, Dr. William Gilbert, inquired into this attractive property of amber. He found that many other substances possessed the same property. Indeed it is common to all substances in some degree. We say the amber or other object is "electrified."

It was observed by the early experimenters that there were two kinds of electrification. To one of these they gave the name _positive electricity_, and to the other _negative electricity_.

Every electrified object will attract an object which is not electrified, and two objects which are oppositely electrified will attract one another also. But two objects which are similarly electrified will repel each other.

Man got tired of rubbing objects by hand, so he fitted up simple machines in which glass cylinders or plates were rubbed against leather cushions. The electricity was then collected by little metal points supported on an insulated metal sphere.

The experiment of attempting to store electricity in a glass vessel filled with water was made at the University of Leyden (Netherlands). The water was replaced later by a coating of tin-foil on the inner surface, while a similar metallic coating on the outside took the place of the experimenter's hand. These jars are called _Leyden jars_, after the place in which the discovery was made.

About 1790, Professor Galvani, of Italy, observed that the legs of a freshly killed frog twitched at each discharge of an electrical machine. Later he found that the same twitching occurred when he connected certain parts with a piece of copper and zinc. He believed this to be due to "animal electricity" secreted within the frog.

Professor Volta, also of Italy, proved that Galvani's idea was wrong, and that the electricity resided in the metals rather than in the frog. He showed that when two pieces of dissimilar metal were put in contact with one another, there was a slight transference of electricity between them. He constructed a pile of copper and zinc discs, with a moist cloth between each pair or couple, and by connecting wires from the top copper disc to the lowest zinc disc he was able to show that an appreciable current of electricity was produced. Later he placed a piece of copper and a piece of zinc in a vessel containing acidulated water, whereupon he found that a steady current of electricity was obtained. This was the invention of electric batteries.

The phenomena of _magnetism_ were known to the ancients, but it was not until the nineteenth century that we found any real connection between electricity and magnetism. In 1819, a Danish philosopher, Hans Christian Oersted, discovered that an electric current passing in a wire affected a magnet in its neighbourhood. If the magnet was supported on a pivot, after the manner of a compass needle, it would turn round and take up a position at right angles to the wire carrying the electric current.

The molecular theory of magnetism presumes that every molecule of iron is a tiny magnet, having a north and south pole. In a piece of unmagnetised iron, these tiny magnets are all lying so that they neutralise one another. When they are turned round so that their north poles are all lying in one direction, then the iron is said to be magnetised.

The electron theory of magnetism does not do away with the older molecular theory just referred to. The electron theory goes a step farther, and tells us that these molecules are magnets because of a steady motion of electrons around the atoms of iron.

It was discovered in 1825 that when an electric current was sent through an insulated wire wound around a piece of soft iron, the iron became a magnet; when the current was stopped the magnetism disappeared. Such magnets are called _electro-magnets_. If a piece of hard steel is treated in the same way it becomes a _permanent magnet_. It was this intimate connection between electricity and magnetism, or, in other words, the invention of these electro-magnets, which brought us electric bells, telegraphs, telephones, dynamos, and electric motors.

It should be noted that while iron is attracted by either pole of a magnet, there is such a thing as magnetic repulsion. This, however, takes place only between two magnets, and then only between like poles.

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Some German physicists made a number of electrical experiments with vacuum tubes. When Sir William Crookes (England) was experimenting with similar vacuum tubes he suggested that matter was in a "radiant" state during the electric discharge within the tubes.

In 1880, H. A. Lorentz, of Amsterdam, declared that light was due to the motion of small particles revolving around the atoms of matter.

Professor Zeeman, of Holland, produced experimental proof of Lorentz's theory. He showed that the revolving "particles" were influenced by a powerful magnetic field, in the manner explained in the electron's story. This discovery was made in 1896, or sixteen years after Lorentz's declaration. It was Dr. Johnstone Stoney, of Dublin University (Ireland), who christened these particles "electrons."

The X-rays were observed for the first time by Professor Roentgen, of Germany, in 1895. The screens used for viewing the luminous effects produced by the X-rays are coated with very fine crystals of _barium platinocyanide_. These screens were in use for another purpose previous to the discovery of X-rays.

We know now that _chemical affinity_ is merely electrical attraction between the atoms of matter.

The spectroscope consists of a glass prism, or series of prisms, mounted between two metal tubes. One tube is provided at one end with a vertical slit, through which the light that is to be examined is passed. At the other end of the tube is a lens, so that the beam of light from the slit emerges through the lens as a pencil of parallel rays. The pencil of light then falls upon the glass prism, striking it at an angle. In passing through the prism, the light is bent round so that it enters the second tube, which is simply a small telescope. The prism separates the aether waves according to their wave-lengths, and produces the well-known coloured spectrum, which is magnified by the telescope. The reason for the bending of the different waves is explained in the electron's story.

INDEX

Absorption of light, 148

Aether, the, 24

Aether waves, 96, 131, 133, 137, 146, 148, 163

Alpha rays from radium, 190

Alternating electric current, 121

Amber electrified, 32, 34 to 37, 201

Artificial light, 140, 142

Atoms breaking up, 188, 190

Atoms co-operating with electrons, 108, 123

Atom's internal energy, 187, 191

Atoms of matter, 52, 54, 78, 128, 180, 184

Attraction between atoms, 180

Attraction, electrical, 35, 202

Attraction, magnetic, 78, 205

Aurora, 132

Automatic telegraph transmitter, 91

Battery, electric, 70, 203

Beginning of the world, 53

Beta rays from radium, 189

Birth of the moon, 52, 54

Bricks of the universe, 180, 195

Chemical affinity, 206

Chemical combinations, 56, 182

Chemistry of the stars, 52, 55, 144, 153

Chlorine atoms, 56, 182

Cloud formation, 56

Circuit, earth, 72

Coherer, tube, 98

Cohesive force, 183

Colour, 136

Compass needle, 77

Complete electric circuit, 71

Conductors, 37, 68

Connecting link between aether and matter, 118, 127

Corpuscles, 66

Crookes, Sir William, 205

Current of electricity, 68

Dark lines in spectrum, 154

Detachable electrons, 78, 181

Detecting imitation diamonds, 174

Direct electric current, 121

Discharge of electricity, 42

Discharge through a vacuum, 60

Discovery of electrons, 160, 206

Discovery of X-rays, 169

Dynamo, 116, 118

Earth circuit, 72

Electrical discharge, 42

Electricity, positive, 23, 32, 39, 52, 180

Electricity, negative, 23, 32, 39

Electric battery, 70

Electric current, 68, 70

Electric motor, 116, 122

Electric shock, 47

Electrified objects, 37, 38, 201

Electro-magnets, 76, 81, 83, 118, 205

Electrodes, 61

Electrocution, 49

Electron as a go-between, 118

Electron, derivation of the word, 23

Electron, discovery of, 160, 206

Electrons, 25, 32, 66, 78, 138, 162, 195

Energy transmission through the aether, 73, 121

Energy within the atom, 187, 191

Field, magnetic, 68, 76, 118

Fluorescent screen, 169, 206

Galvani's discovery, 202

Gamma rays from radium, 189

Gilbert's discovery, 201

Glass, electrified, 37, 38

Glass prism, 147, 152

Glow-lamp, electric, 140, 141

Glow-worm, 142

Heat, radiant, 126, 131, 133, 142

Helium atoms, 188, 190

Hydrogen atoms, 55, 182

Insulators (non-conductors), 37, 47

Iron atoms, 77

Iron wires discarded, 88

Lamp, electric, 140

Leyden jar, 42, 202

Light, 23, 60, 64, 133

Light absorbed, 148

Light, artificial, 140, 142

Light, reflected, 148

Lightning, 42, 48

Lines in the spectrum, 152, 154, 160, 162

Lorentz's declaration, 206

Magnetic attraction, 78, 205

Magnetic field, 68, 76, 118

Magnetic repulsion, 205

Magnetism, 73, 76, 203, 204

Magnetism and electricity, 73

Magnets, electro-, 76, 81, 83, 205

Magnets, permanent, 83

Mariner's compass, 77

Matter, 52, 54

Metal electrified, 37, 38

Molecules of matter, 181, 183

Moon's birth, 52, 54

Morse telegraph, 88

Motion in line of sight, 162

Motor, electric, 116, 122

Negative electricity, 23, 32, 39

Oersted's discovery, 204

Oxygen atoms, 182

Permanent magnets, 82

Positive electricity, 23, 32, 39, 52, 180

Prism of glass, 147, 152

Radiant heat, 131, 133, 142

Radiant matter, 63, 205

Radium, 188

Rainbow, 147

Rays from radium, 189, 190

Reflection of light, 148

Repulsion, electrical, 202

Repulsion, magnetic, 205

Roentgen rays, 167

Roentgen's discovery, 168, 206

Sea, cause of saltness, 56

Shock, electric, 47

Silk, electrified, 38

Sodium atoms, 56, 182

Spark, electric, 44

Spectroscope, 152, 154, 207

Spectrum, 144, 147, 152, 154

Speed of electrons in conductor, 70

Stars approaching the earth, 162

Stars, constituents of the, 52, 55, 146

Stoney, Dr. Johnstone, 206

Sun, constituents of the, 154

Sun's heat, 128, 131

Telegraph signals, 90

Telegraphy, wireless, 95

Telephone, 109

Telephony, wireless, 110

Temperature, 180

Tramway, electric, 117, 118, 122

Transparent substances, 149

Vacuum tubes, 60, 61, 132, 205

Velocity of electrons, 70

Volta's discovery, 203

Waves in the aether, 96, 133, 137, 146, 148, 163

Wireless messages from the stars, 162

Wireless telegraphy, 95

Wireless telephony, 110

X-rays, 166, 206

X-rays from radium, 189

X-ray photography, 173

Zeeman proves existence of electrons, 161, 206

Printed by BALLANTYNE, HANSON & CO. Edinburgh & London

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Transcriber's Note

The following changes have been made to the original text:

Page xi: "always necessary, How" changed to "always necessary. How"
Page 205: "vacuum tubes, When" changed to "vacuum tubes. When"
Page 214: "Negative electricity, 23, 32, 9" changed to "Negative
electricity, 23, 32, 39"

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