Wireless telegraphy and Hertzian waves

Front matter

Google' books

Wireless telegraphy and Hertzian waves

LIBRARY

Preface

In the following pages will be found an attempt to set forth, in simple language, the principles on which those phenomena known as “ electric waves,” <l signalling through space without wires,” etc., are dependent. No mathematical formulae will be introduced ; those who desire to investigate the mathematics of the subject are referred to Dr. Heinrich Hertz’s classical work, Electric Waves. The subject will be divided into four portions, viz. a brief outline of what electricity is ; a short historical sketch ; a description of the phenomena themselves, with an account of the appliances needed for their production ; and lastly, constructional details of the various pieces of apparatus required

As electricians generally, with regard to their views of the nature of the medium through which the electrical disturbance takes place, are pretty equally divided into two camps, we shall not favour any particular hypothesis* but simply refer to the space between the body setting up the wave and that receiving it, as the medium.

S. R. Bottone.

WattingUm, Surrey.

216043

Preface To Second Edition

Great as was the interest awakened by the first announcement of the practicability of Wireless Telegraphy, it has increased rather than decreased with the lapse of time. With a view to satisfy this interest, an account of PopofF s recent experiments in the direction of Wireless Telephony has been embodied in this new edition.

S. R Bottone.

Wallington.

Preface To Third Edition

The marked advances made in Wireless Telegraphy, which have fitted it for transatlantic communication ; its peculiar adaptability to the transmission of messages, not only between ships in motion but also between ships and land, or vice versd, have greatly increased its value as a means of knitting mankind together, and enhanced the interest awakened by the original discovery. In the following pages we have endeavoured not only to give a clear idea of the fundamental principles, but also to describe and illustrate the more recent developments of “Wave” Telegraphy and Telephony.

S. R Bottone.

Wallington,

Preface To Fourth Edition

A new chapter on recent improvements in Wireless Telegraphy apparatus has been added, with particulars of a few modern types of coherers, and the application of tuning coils and capacities, with a formula for ascertaining wave lengths.

Contents

CHAP. PAOB

i. Preliminary Notions . . . 1

ii. Historical Considerations . . . 13

Index To Sections

sse.

PAO»

1. Knowledge essential

•

2. Wliat Electricity is

3. General Laws of Electricity

.

. 5

4. Two Conditions ever present .

5. Transference

. 6

6. Induction

7. Electrical Strain

. 11

8. Magnetism an aspect of Electricity

9: Action of Charged Conductor

10. Lateral Discharge

. •

11. Iron-filings affected by Discharge .

•

. 19

12. Syntony, or ‘Tuning* .

• .

13. Guarini’s Discovery

.

. 24

14. The Hertzian Waves

15. Waves compared

. .

. 25

16. Leyden Jar Discharges .

.

17. Coherence ....

. 34

18. Practical Applications .

.

19. Effect of Light on Discharge

.

. 44

20. Identity of Electricity and Light

.

21. Chunder Bose’s Experiments

.

. 51

22. Hertzian Waves and Photography

X

INDEX TO SECTIONS

23. Hertzian Waves and Nikola Tesla . . .53

23a. Application to Telephone .... 59

23b. Abbb Michel’s System . . . .119

~^23c. Collins’s System ..... 123

24. Desire of making Apparatus . . . .65

25. The Induction Coil ..... 65

26. The Wheatstone Commutator . . . .80

27. The Tapping-Key ..... 85

28. The Wimshurst Machine . . . .86

29. The Transmitter ..... 94

30. The Receiver . . . . .96

32. The Bell and Decoherer ..... 109

33. The Batteries required . . . .110

34. Instruction for putting together . . .111

35. To use the Apparatus • . . .116

36. Condenser, where applied . . . • . 118

37. Recent Improvements . . . . . 119

Aerials, 120. Liquid Aerial, 123. Blake Detector, 123. Magnetic Detector, 124. Electrolytic Coherer, 125. Marconi Coherer, 126. Italian Navy Coherer, 126. Lodge-Muirhead Coherer, 127.

LIST OF ILLUSTRATIONS no. PACK

1. Lateral Discharge . . . . . .16

2. Induction Discharge ..... 17

3. Preece’s System . . . . . .19

4. Kinging Bell by a Discharge ... 22

5. Preece’s System : another form . . . .30

6. Syntonic Jars ...... 32

7. Hertz’s Oscillator and Resonator . . . ,35

8. The Branly Tube ..... 38

9. Marconi’s Original Arrangement . . . . 41

10. Miller’s Decoherer ..... 43

11. General Arrangement of Receiver . . . .44

12. Hertz’s Device to show effect of Light . . 45

13. Oliver Lodge’s Arrangement . . , .47

14. Ch under Bose’s Explosion Experiment . . 53

15. Tesla’s Hertzian Wave Boat: plan . , .54

16. Tesla’s Transmitter and Boat : elevation • . 55

17. Tesla’s Boat : section . . . . .57

17a. PopofPs Wireless Telephone .... 61

17b. PopofFs Radio Conductor . . . . .63

19. Section of Condenser . . . . .69

20. Finished Condenser ..... 70

21. Bobbin-Winder .... 73

22. Contact-Breaker ..... 78

LIST OF ILLUSTRATIONS xii no.

23. Completed Coil : section .

PAGE . 80

24. Commutator, Roller of .

.

25. Commutator, Wheatstone’s

. 84

26. Tapping-Key .....

27. Wimshurst Machine

28. Lodge’s Transmitter ....

29. Bracket for Armature

30. Completed Relay ....

31. Simple Coherer ....

•

. 105

32. Frame for Multilocnlar Coherer

•

33. Multilocular Polytonic Coherer .

. 108

34. Bell with Decoherer ....

35. Receiver, completed

. 116

36. Michel’s “Earth” go and return System

.

37. Armorl’s Electro-Capillary Relay

. 130

WIRELESS TELEGRAPHY AND HERTZIAN WAVES

Chapter I

§ 1. A certain amount of knowledge of the laws which govern electrical and magnetic actions is essential to the proper comprehension of the phenomena under consideration. Without pretending to write a treatise on electricity, we give here a brief outline of the principal facts bearing on the subject. Those readers who possess a fair acquaintance with electrical science can skip this chapter.1

§ 2. Electricity appears to be a vibratory motion in the ultimate molecules of bodies — of this we have no positive proof, but can judge only by analogy. The name is derived from Elektron , the Greek for amber, this being

1 Those desiring fuller information are referred to the author’s work, Electricity and Magnetism.

B

Digitized by AjOOQle

[chap.

the substance in which electrical manifestations were first noticed. The principal phenomena which are of interest to us in connection with the study of electric wave action are — excitation, attraction, repulsion, transference, induction, and magnetic effects. When by any means we are able to set up an electrical condition in a body, that body is said to be excited. The means are several: for instance, friction, cleavage, change of temperature, chemical action, motion in a magnetic field, etc. As examples of these, we may take the following —

(1) If we rub briskly a stick of sealing-wax 'with a dry flannel, we shall find that it acquires the property of attracting light bodies, and of giving feeble sparks if the knuckle be presented to it.

(2) If a roll of brimstone be broken across the middle, the broken surfaces will generally be found to attract strongly light bodies ; and many natural crystals, if riven along the line of cleavage, will exhibit similar properties.

(3) If some melted sulphur be poured into a dry eggcup, a stick being held centrally to serve as a handle, it will be found on withdrawing the sulphur, when it has set and is cold, that the under surface is strongly electrified.

(4) If a plate of zinc and one of copper be immersed in a vessel containing dilute sulphuric acid, or any other fluid capable of acting on the zinc, and the two plates be connected together outside by a wire, a current of electricity will flow along this wire, from the copper to the zinc, and in the fluid from the zinc to the copper ; and the existence of this electrical flow may be shown by appropriate means which will be described later on.

(5) If we coil a few hundred feet of cotton-covered copper wire round a bobbin, in the same manner as cotton is wound on a reel, and bring the free extremities of the wire very close to one another, and then move this bobbin sharply before the poles of a powerful horseshoe magnet across the imaginary lines of force, connecting the two poles, this motion will set up an electrical disturbance in the wire, which will be evidenced by the occurrence of bright sparks at the gap between the two ends of the wire, every time the bobbin is moved to or from the magnet.

Not all bodies allow electricity to pass freely along them. Some offer very little resistance to its passage, and hence are called conductors . Others, on the contrary, present great resistance to its passage, and are therefore called non-conductors or insulators. As in all probability electricity, like light and heat, is simply a vibratory motion in the ultimate particles of matter, we can well liken the electrical conductivity of certain bodies to the facility which many elastic bodies have in transmitting sound, . For instance, nearly all the hard and elastic bodies are sonorous and transmit sound easily — a stretched steel wire, a hard dry road, etc., are examples of this ; while bodies which do not enter into vibration freely, as moist clay, sawdust, cotton-wool, etc., as they cannot take up the sound vibrations, are practically non-conductors of sound. The best conductors of electricity are the metals; among the worst are dry air, resinous bodies, such as shellac, gutta-percha, indiarubber, and its congeners, also sulphur, etc. Of the metals, silver is the best conductor ; while paraffin wax is one of the solid bodies which pre-

[chap.

sents the highest resistance. If we express the resistance offered by silver to the passage of electricity as 1, the resistance of paraffin wax and of shellac is certainly not less than 1,500,000,000,000. It must- be borne in mind, however, that there is no perfect conductor and no absolute insulator ; in other words, the best conductor we have presents some resistance to the passage of electricity, while, on the other hand, the best insulator will permit the passage of some electricity; in the same way, as if we stuff our ears with cotton-wool, or insert sawdust between our walls, in the hopes of deadening a sound, some vibrations still reach our auditory nerves and some sound is still heard. A short table of the principal conductors and non-conductors, beginning with the former and ending with the latter, the indifferent ones taking the middle place, is here inserted: —

All metals, as —

'Silver

Copper

Gold

Aluminium Zinc - Brass Platinum Iron Nickel .Lead Antimony Bismuth Graphite Acids

Metallic sulphides Water

Metallic salts Linen Alcohol Ether Dry wood Dry ice

Metallic oxides

Ice at 25° cent.

Fats and oils

Indiarubber

Gutta-percha

Dry air

Wool

Ebonite

Diamond

Silk

Glass

Wax Amber

Sulphur Shellac

Resin Paraffin wax

§ 3. It will be evident from this, that if we attempted to excite a metal rod held in one hand, by rubbing it with a piece of flannel or silk, we should obtain no manifestations of electricity, because, both the metal and the hand being good conductors, what electricity was; evoked by friction would flow away through the rod and the body to the earth. To obtain any evidence of the electrical disturbance set up in the metal rod, it will be necessary to “ insulate ” 1 it and cut it off from connection with any other conductor, by fitting to it a handle of some nonconductor such as a rod of glass or ebonite. Then on rubbing the metal rod the usual manifestations of electricity can be obtained. With insulators or non-conductors this precaution is not necessary, since the electricity elicited on friction cannot escape to the hand. If we excite a stick of sealing-wax by rubbing with a piece of dry flannel, we shall find on presenting it to any light bodies, such as small bits of paper, pith, or straw, that it will first attract these strongly, and after having been in contact with them for a short time will as strongly repel them. In like manner a rod of glass rubbed with silk, will first attract and then repel light bodies. If, however, in performing these experiments we place the pieces of paper on a sheet of ebonite or a pane of glass, which, being insulators , will prevent the pieces of paper, etc., frotn losing any charge they may have taken up, we shall 1 From %ntvla> an island, “ cut off from all other land by the sea.19

[chap.

notice a very peculiar fact, namely, that a piece of paper that has been repelled by the excited sealing- wax, will be attracted by the excited glass, while, on the other hand, that piece of paper which has been repelled by the excited glass, will be attracted by the excited sealing-wax. From this we gather that although both the sealing-wax and the glass are electrified, yet there must be some difference in the electrification; and if we agree to call the electricity of the glass positive , and that of the sealing-wax negative , we can lay down the following • law: — Bodies charged with electricity of like name repel one another, while those charged with electricity of different name attract one another; or we may put the matter more briefly, though perhaps not so exactly, by saying "like electricities repel, unlike attract”

§ 4. It must be noticed here that when by any means we set up a charge of electricity of either name in any given body, we always call forth a similar charge, but of contrary name in the body which produces excitation. Thus on rubbing the sealing-wax rod with flannel we call forth negative electricity on the sealing-wax and positive on the flannel, and in the case of glass rubbed with silk, the former becomes positively and the latter negatively electrified.

§ 5. By transference is meant the actual passage of electricity from one charged body to another. It can only occur between bodies that are conductors. When transference occurs suddenly , it is generally spoken of as a “ discharge ” ; when it is effected slowly and with more continuity it is termed a “flow,” or a current. When it is

I]

desired to make use of a current , it is usual to employ wire to effect the connection between the positive and negative bodies, as the wire, being a good conductor, and presenting but little resistance, wastes but little electric energy. In the last section we pointed out, that whenever we set up an electrical condition (either positive or negative) in one body, we also produce an opposite condition in another body : so, for the sake of illustrating what relation the two phenomena of excitation and transference bear to one another, we may liken excitation to the pumping up of water from a pond, into a vessel standing at a higher level. The vessel will then be positive, and the pond negative. We have not produced any water, but simply altered its level If now we knock the bottom out of the vessel, so that the water, finding no appreciable resistance to its passage, suddenly returns to its original level, we have an example of “ discharge 99 or rapid transfer ; but if instead we open a small hole in the vessel, the contained water again finds its level, but slowly, as a fine stream; and this exemplifies an electrical current. In every case, then, transference, whether it take the form of a sudden discharge, or of the slower and steadier current, always indicates that a levelling up of a difference in electrical condition is taking place. There is one point which it is important to notice with regard to the more sudden forms of discharge, and that is, that just in the same way as when, if we raise a body of water from a pond to some height, and then let it suddenly fall back again, the water does not immediately come to a dead level, but rebounds, splashing up again to some extent, producing

[chap.

surging waves, which only gradually die away, so a sudden electrical discharge does not immediately result in a dead electrical level, but sets up electrical “ surgings,” of infinitesimally short duration, as compared to those set up in the case of water, nevertheless not only capable of measurement, but also endued with far-reaching powers.

§ 6. The phenomenon of induction is the one to which particular attention must be directed, with a view to obtaining a clear insight into the principles which underlie electric wave signalling.

The French name for induction is influence , and this really gives us a very fair idea of what the effect is. We may define induction as being “the action which an electrified insulated body has upon all other bodies/’ This action (like all others dependent on radiating forces) will diminish inversely as the square of the distance. The inducing body must be insulated from the body induced, otherwise transference or “ discharge ” will take place instead of induction. A medium of some kind must exist between the inducer and the induced; but provided it be a non-conductor sufficiently resistant to prevent direct transference, induction will take place. We are not able to produce a perfect vacuum ; but the highest vacuum we can produce, or of which we have any cognizance, permits inductive effects to take place through it. From our knowledge of other phenomena, it is, however, highly probable, that were it possible to produce a perfect vacuum , id est, a space with nothing in it, induction would not take place across that space. Hence, as induction is found to take place across every known space, the ij

modem theory is that a subtle, weightless, and highly elastic medium, to which the name of ether has been given, pervades all space. Now for facts. If we take a large metal ball, and suspend it by a silken thread, we can charge it with positive electricity, either from a rubbed glass rod, or the positive conductor of any electrical machine. If now we bring near it (not touching) another insulated brass ball, we shall find that this second brass ball acquires and retains, so long as it is near the first one, certain electrical properties. In the first place, we shall notice that the surface of the second ball which is nearer the first or charged ball, is negatively electrified ; while the further side is in a positive condition. This can be shown even more strikingly by using two suspended bails instead of only one for the second ball. In this case the two balls are kept together by their silken suspensions while approaching the charged ball, and in a line with the direction of approach. If they now be separated before they are withdrawn from the influence of the charged ball, it will be found, on testing, that the ball that was nearer the charged ball will have acquired a negative charge, while the further bajl will be charged positively. No matter at what distance the charged or inducing ball shall be placed from the others, this effect will always take place provided no good conductors intervene ; but of course as the distance increases, so the effect rapidly diminishes. What is this action ? Does it really take place without any intermediary ? A little consideration will show that action at a distance without any medium to convey that action is a physical impossi-

Digitized by v^ooQle bility. The air, we know, is a non-conductor, so there can be no transference ; and even if it permitted the slow passage of the charge from the inducer to the induced, this would not account for the opposite condition of the nearer surface of the induced body. Clearly then the air or other surrounding medium must play a part, and a very important part, in this action. It will be remembered that we have noticed that "like electricities repel, and unlike attract.,, Now if we have a positively charged insulated body, it acts upon the molecules of the air or other surrounding medium, polarizing them, and causing them to present their negative faces to the charged body (by the attractive effect of the positive for the negative), while the positive faces are turned the opposite way under the repulsive influence of the positive upon positive. No discharge can take place, since, as we have seen, air is a non-conductor, but each molecule of air in like manner puts a strain on its neighbour, until the surface of the induced body is reached ; forming as it were a chain (or a number of chains) of polarized air molecules, reaching from the inducer to the induced body. This strain in its turn puts a stress upon the conductor, which constitutes the induced body, and its molecules can part with their strain, being conductors ; the final effect is that the face of the induced body becomes negatively charged, the positive electricity flowing to the opposite extremity, and this condition of things obtains as long as the charged inducing body is in its vicinity. As soon as the charged body is removed, or in any way discharged, the strain on the medium, being relieved, things return (after a few

I]

surges, so rapid as to be almost imperceptible except by the most delicate appliances) to their original condition. Of course if the induced body consists of a number of parts originally in contact but afterwards separated from one another, before the removal of the inducing body, then the nearer parts will be in an opposite electrical condition to that of the inducer, while the further portions will be found charged with electricity of like name. And this latter statement holds good whether the inducing body be charged negatively or positively.

§ 7. It is not within our province here to give an account of the many pieces of apparatus or divers applications to which this property of electrical induction can be put ; we wish, however, to emphasize in the reader’s mind, that the presence of a charged body immediately reacts on all surrounding bodies, by putting a strain on the molecules of the circumambient medium, which strain in its turn reacts on the molecules of the bodies subjected to its influence.

§ 8. Another property of electricity which must be noted in this connection is that every current, nay, every manifestation of electricity, is accompanied by magnetic effects. In point of fact we may lay down broadly that magnetism is but one aspect of electricity; and that whenever a current of electricity is set up, magnetic conditions exist at right angles to the direction of flow. It is easy to verify this statement. A piece of copper wire in its ordinary condition has no attractive influence on iron filings or magnetic bodies ; the same wire when traversed by a current will be found to attract iron

[chap.

filings and to behave magnetically. If we coil some insulated wire round a small ruler or pencil so as to produce a long helix (from which the pencil, etc., may be afterwards removed), a current being sent through this helix will impart to it strong magnetic properties, precisely similar to those of a bar magnet. This magnetism lasts as long as the flow of electricity is stronger in proportion as the current is stronger, and ceases with its cessation. Lastly, if we place some iron filings in a tube around which we have loosely coiled some insulated copper wire, on sending a current through the wire the iron filings will be seen to arrange themselves in lines at right angles to the flow of the current, while the entire tube under the influence of the current will exhibit all the properties of a magnet.

The main facts we have to bear in mind are, that electricity appears to be a species of wave motion in the molecules of bodies ; that we have many means of setting up this wave motion. That not all bodies convey this wave motion equally well ; those that transmit it freely are called conductors ; those that present great resistance are termed insulators, or non-conductors ; that induction is the result of strain or stress put upon the molecules of the medium by the presence of a charged (rapidly vibrating) body, which strain or stress sets up a similar vibrating motion (electrified condition) in bodies in contact with the medium ; and that this strain or stress is always accompanied by magnetic effects, and often affects largely the conductivity of the bodies themselves.

Chapter II

§ 9. The action of a charged conductor on other bodies in its vicinity was known in the early part of the eighteenth century ; the Leyden jar was described by Kleist, Cuneus, and Muschenbroeck about 1727, and from the notices given by several experimenters of that period there is no doubt that the phenomenon of the lateral discharge, which is in itself an evidence of the influence which a disturbance in the electrical condition of a given body produces on that of the surrounding bodies, was also well known. We have an illustration of the apparatus required to produce this effect in a work entitled An Essay on Electricity , by Geo. Adams, published in 1780, at Fig. 66, plate 4, where a series of conductors not touching one another, and at a certain distance from a charged Leyden jar, are shown to allow a spark to pass between them, at the instant that the jar is being discharged by a separate discharging rod. How far this disturbing effect influenced the surrounding medium does not appear to have occupied the attention of these early experimenters, and although the possibility of producing inductive effects by means of the sudden discharge of so-called “static” charges has been shown experimentally by Matteucci, yet the bearing of this important fact on the problem of transmitting a signal through space not connected by any other medium but the intervening air does not appear to have struck any one of them. In 1853 J. B. Lindsay proved the possibility of transmitting a message across water at points 500 yards apart, without continuous wires ; and he patented this invention in 1854. But this can hardly be classed with the phenomena which we now understand by “ wireless telegraphy ” ; and even the results obtained by Mr. Preece between 1884 and 1894, and that culminated in his induction system, which consists virtually in the inductive effect of a wire carrying a momentary current of electricity on a similar wire at a distance from the first, but parallel to it, has but little affinity with the methods at present adopted. Riess and Henry, in 1842, had pointed out that the discharge of a Leyden jar was not a sudden and complete levelling up of the difference of potential between the coatings of the jar, but rather partook of the nature of a principal discharge in one direction, and then a series of surgings backwards and forwards, until equilibrium was obtained. Feddersen, Paalzow, and others, notably Oliver Lodge, proved experimentally the existence of these surging waves during the discharge ; and showed that as the resistance of the circuit increases, the number of these alternating discharges decreases, but at the same time their duration is greater. It was also known to Henry the great distance to which the inductive effect of these surging discharges could extend, for he calls attention to the fact that a single spark of about an inch in length from the prime conductor

IL1

of a machine passing to the end of a circuit of wire placed in an upper room, produced an induction sufficiently powerful to magnetize needles in a parallel circuit of iron placed in the cellar beneath, at a perpendicular distance of 30 feet, with two floors and ceilings, each 14 inches thick, intervening. This discovery of Henry’s, important as it was, lay dead and without practical application, until the attention of Dr. Heinrich Hertz, of Carlsruhe and Bonn, was directed to this subject, in consequence of a prize which had been -offered by the Berlin Academy of Science for the elucidation of the relation which exists between dielectric polarization and electro-magnetic induction. By means of a series of masterly experiments, following upon careful reasoning, Dr. Hertz, between the years 1886 and 1891, placed our knowledge of these electric waves and their effects on surrounding bodies on so firm a basis as to have enabled us to put them to practical use. The author has ever been of opinion that no knowledge is so easily acquired or can be so thoroughly retained, as that following actual experiment ; and for that reason he reproduces here illustrations of the principal experiments leading up to our present knowledge of electric waves, and strongly recommends those of his readers who have not hitherto performed these experiments, to make practical acquaintance with them now, as by this means they will obtain a grasp of the subject that no amount of mere book knowledge will ever give them.

§ 10. The phenomenon of the lateral discharge , as illustrated in George Adams’ work, is easily demonstrated by apparatus arranged as shown in our Fig. 1. A charged

Digitized by AjOOQle

[chap.

Leyden jar is placed on an insulating slab (a sheet of glass or ebonite will do), and in close proximity to it and each other, but not actually touching, are two elongated conductors, the last of which may be connected to earth. On discharging the jar by means of the discharging rod, and more especially if a chain intervene between the outer coating of the jar and the discharging-rod, a bright spark will be seen to pass along the gaps between the two con-