CHAPTER I. INTRODUCTORY.

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[Illustration: _Frontispiece._ *Plate I. Electrical Conventions.*]

WIRELESS TELEGRAPH

CONSTRUCTION FOR

AMATEURS

BY

ALFRED POWELL MORGAN

EDITOR MECHANICAL AND ELECTRICAL DEPARTMENT OF THE "BOYS’ MAGAZINE"

AUTHOR OF "WIRELESS TELEGRAPHY AND TELEPHONY"

_WITH 167 ILLUSTRATIONS_

*Third Edition, Revised and Enlarged*

WITH A COMPLETE DESCRIPTION OF THE

NEW WIRELESS LAW

NEW YORK:

D. VAN NOSTRAND COMPANY

25 PARK PLACE

1914

Copyright, 1910, 1913, by

D. VAN NOSTRAND COMPANY

_Stanhope Press_

F. H. GILSON COMPANY

BOSTON. U.S.A.

PREFACE.

In this work, the author has endeavored to present a book embracing

practical information for those who may wish to build for private or

experimental use a set of wireless instruments which are more than toys

but yet not so expensive as the commercial apparatus.

Many books have been published on the subject of wireless telegraphy,

but in them the interests of the novice have been rather neglected and

in order to build an outfit he has been forced to rely upon a series of

disconnected articles published in the amateur periodicals.

It is the object of this book to show the construction of simple,

efficient instruments by means of clear drawings, and to give enough

elementary theory and practical hints to enable the experimenter to

build a size and type in keeping with his needs and resources.

The tiresome "how to make" style has been avoided as far as possible.

History and all unimportant details are omitted to give in their place a

concise explanation of the parts played by the different instruments and

the influence of developing their various factors.

A small lathe and a set of taps and dies are necessary to produce

apparatus having a good appearance, but a little ingenuity displayed in

adapting screws and parts of old electrical instruments oftentimes at

hand will make these tools unnecessary.

Ordinary precaution and plenty of time should be used in the work. It is

obvious that if a large coil is to be made, it is well to insure its

successful completion by painstaking care and the use of proper

materials. Neither is it wise to strain an instrument through becoming

impatient and using it before it is properly completed and adjusted.

Wherever possible instructions have been given regarding the adjustment

of the apparatus, but it is only by actual practice that the operator

will acquaint himself with the most efficient manipulation.

Extracts from articles contributed by the Author to _Popular

Electricity_ have been used in the chapters on Spark Gaps, Oscillation

Detectors and Telephone Receivers, through the courtesy of the editor,

Mr. H. W. Young.

In conclusion, the writer wishes to express his thanks to the United

Wireless Telegraph Company for views of their apparatus and to the other

firms who have loaned electrotypes and supplied information. To those

who have assisted in the preparation of the book, more especially to Mr.

Safford Adams, for numerous suggestions and criticisms, the Author

desires to express his full acknowledgments.

ALFRED POWELL MORGAN.

UPPER MONTCLAIR, N.J.

_June_, 1910.

PREFACE TO THE THIRD EDITION.

The success of the previous editions of this book has made a new and

third one necessary.

There have not been any startling changes or new discoveries made in the

field of wireless telegraphy since the first edition was published, but

the art has undergone a number of small changes and improvements which

have increased the efficiency and selectivity of the apparatus.

Since then a federal law restricting and controlling wireless telegraphy

has been passed. Its effect has been to place wireless telegraphy upon a

more certain basis, and to give a recognized standing to the amateur

experimenter.

This new law has been included in this edition in the form of an

appendix. The amateur will do well to read it carefully. Compliance with

its regulations will prove beneficial rather than a hindrance.

A fully illustrated chapter explaining exactly how to comply with the

law and how to build the apparatus required has been added. Complete

descriptions of several new types of detectors are also included.

All old matter has been thoroughly revised and several illustrations

replaced by ones more up-to-date and of direct interest.

ALFRED P. MORGAN.

UPPER MONTCLAIR, N.J.

_May_, 1913.

PREFACE. ..........................................................

PREFACE TO THE THIRD EDITION. .....................................

CHAPTER I. INTRODUCTORY. ..........................................

The Transmission and Reception of Electric Waves. ...............

CHAPTER II. THE APPARATUS. ........................................

CHAPTER III. AERIALS AND EARTH CONNECTIONS. .......................

CHAPTER IV. INDUCTION COILS. ......................................

CHAPTER V. INTERRUPTERS. ..........................................

CHAPTER VI. TRANSFORMERS. .........................................

Construction of a 2-K.W. Transformer. ...........................

CHAPTER VII. OSCILLATION CONDENSERS AND LEYDEN JARS. ..............

CHAPTER VIII. SPARK GAPS OR OSCILLATORS. ..........................

CHAPTER IX. TRANSMITTING HELIXES. .................................

CHAPTER X. KEYS. ..................................................

CHAPTER XI. AERIAL SWITCHES AND ANCHOR GAPS. ......................

CHAPTER XII. HOT WIRE AMMETER. ....................................

CHAPTER XIII. OSCILLATION DETECTORS. ..............................

"UNIVERSAL" DETECTOR. ...........................................

ELECTROLYTIC DETECTORS. .........................................

TANTALUM DETECTOR. ..............................................

CRYSTAL DETECTORS. ..............................................

LEAD PEROXIDE DETECTOR. .........................................

THE MARCONI MAGNETIC DETECTOR. ..................................

CHAPTER XIV. TUNING COILS AND TRANSFORMERS. .......................

CHAPTER XV. RECEIVING CONDENSERS. .................................

CHAPTER XVI. TELEPHONE RECEIVERS AND HEADBANDS. ...................

CHAPTER XVII. OPERATION. ..........................................

ELECTRICAL TERMS AND DEFINITIONS. ...............................

CHAPTER XVIII. THE AMATEUR AND THE WIRELESS LAW. WHAT IT IS; HOW TO

COMPLY; HOW TO SECURE A LICENSE. ..................................

APPARATUS REQUIRED IN MANY INSTANCES IN ORDER TO COMPLY WITH THE

WIRELESS LAW. ...................................................

Receiving Apparatus. ............................................

_Frontispiece._ *Plate I. Electrical Conventions.* ................

Fig 1. Hertz Oscillator and Resonator. ............................

Fig. 2. Hydraulic Oscillator. .....................................

Fig. 3. "Hydraulic" Transmitter and Receptor. .....................

Fig. 4. Simple Wireless Telegraph Transmitter and Receptor. .......

Fig. 5. Electric Waves and Lines of Strain. .......................

Fig. 6. Resonance Tube. ...........................................

Fig. 7. Lag and Lead. .............................................

Fig. 8. Tuned Hydraulic Transmitter and Receptor. .................

Fig. 9. Tuned Wireless Telegraph Transmitter and Receptor .........

Fig. 10. Long-distance Receiving Set. .............................

Fig. 11. Murdock Receiving Set. ...................................

Fig. 12. Clapp-Eastham Receiving Set. .............................

Fig. 13. Prague Receiving Set. ....................................

Fig. 14. Receiving Set. ...........................................

Fig. 15. Receiving Set. ...........................................

Fig. 16. Murdock Transmitting and Receiving Set. ..................

Fig. 17. United Wireless Portable Outfit. .........................

*Plate II. Aerial Systems.* .......................................

Fig. 18. Electrose Insulators. ....................................

Fig. 19. Guy Insulator. ...........................................

Fig. 20. Insulating Tube. .........................................

Fig. 21. High-tension Cable and Insulator. ........................

Fig. 22. Flat-top T Aerial. .......................................

Fig. 23. Ground Clamp. ............................................

Fig. 24. Switch for Lightning Protection. .........................

Fig. 25. Diagram of an Induction Coil. ............................

Fig. 26. Induction Coil Core. .....................................

Fig. 27. Theoretical and practical form of secondary. .............

Fig. 28. Layer Winding for Small Coils. ...........................

Fig. 29. Section Winder. ..........................................

Fig. 30. Impregnator for Silk Covered Wire. .......................

Fig. 31. Methods of Connecting the Secondary Sections. ............

Fig. 32. Coil Case. ...............................................

Fig. 33. Simple Interrupter. ......................................

Fig. 34. Independent Interrupter. .................................

Fig. 35. Details of Magnets. ......................................

Fig. 36. Details of Moving Parts. .................................

Fig. 37. Details of Standard and Screws. ..........................

Fig. 38. Diagram of Connections for an Independent Interrupter. ...

Fig. 39. Construction of a Paper Condenser. .......................

Fig. 40. Wenhelt and Simon Electrolytic Interrupters. .............

Fig. 41. Construction of Electrolytic Interrupters. ...............

Fig. 42. Details of Electrolytic Interrupters. ....................

Fig. 43. Electrolytic Interrupter. ................................

Fig. 44. Assembly and Dimensions of Core. .........................

Fig. 45. Fiber Head and Separator. ................................

Fig. 46. Section Form. ............................................

Fig. 47. Methods of Connecting Sections. ..........................

Fig. 48. Assembly of Leg. .........................................

Fig. 49. Transformer with One Secondary removed. ..................

Fig. 50. Wiring Diagram. ..........................................

Fig. 51. Clapp-Eastham 1/4-K.W. Transformer. ......................

Fig. 52. United Wireless Motor-Generator set for supplying

Alternating Current to the Transformer. ...........................

Fig. 53. Simple Condenser. ........................................

Fig. 54. Leyden Jar. ..............................................

Fig. 55. "Aerial Switch." .........................................

Fig. 56. Amco Oscillation Condenser. ..............................

Fig. 57. Clapp-Eastham Oscillation Condenser. .....................

Fig. 58. Methods of Varying Capacity. .............................

Fig. 59. Spark Gaps. ..............................................

Fig. 60. Spark Gap. ...............................................

Fig. 61. Closely Coupled Helix. ...................................

*Plate III. Transmitting Circuits.* ...............................

Fig. 62. Prague Transmitting Helix. ...............................

Fig. 63. Closely Coupled Tuning Circuit. ..........................

Fig. 64. Loosely Coupled Transmitting Helix and Contact Clip. .....

Fig. 65. Loosely Coupled Transmitting Circuit. ....................

Fig. 66. United Wireless Helix, Spark Gap and Condenser. ..........

Fig. 67. Morse Key fitted with Extension Lever. ...................

Fig. 68. Wireless Key. ............................................

Fig. 69. "United Wireless Type Key." ..............................

Fig. 70. Connections for Aerial Switch. ...........................

Fig. 71. Aerial Switches. .........................................

Fig. 72. Detail of Contacts. ......................................

Fig. 73. Details of Switch Parts. .................................

Fig. 74. Method of Fastening Knife. ...............................

Fig. 75. "T" Aerial Switch. .......................................

Fig. 76. "United" Wireless Lightning Switch. ......................

Fig. 77. Shoemaker Tuning Coil and Aerial Switch. .................

Fig. 78. "United" Wireless Anchor Gaps. ...........................

Fig. 79. Anchor Gap. ..............................................

Fig. 80. Simple Hot Wire Meter. ...................................

Fig. 81. Meter with Case Removed. .................................

Fig. 82. Glass Compensating Strip. ................................

Fig. 83. Details of "Hot Wire" Supports. ..........................

Fig. 84. Details of Movement. .....................................

Fig. 85. Complete Movement. .......................................

Fig. 86. Side View of Hot Wire and Movement. ......................

Fig. 87. United Wireless Hot Wire Ammeter. ........................

Fig. 88. Universal Detector. ......................................

Fig. 89. Details of Universal Detector. ...........................

Fig. 90. Parts of Universal Detector. .............................

Fig. 91. Bare Point Electrolytic Detector. ........................

Fig. 92. Effect of Exposing too much Wire. ........................

Fig. 93. Electrolytic Detector Circuits. ..........................

Fig. 94. Electrolytic Detector. ...................................

Fig. 95. Forming "Glass" Point. ...................................

Fig. 96. Shoemaker Detector. ......................................

Fig. 97. Shoemaker Detector Circuits. .............................

Fig. 98. Lamp Detector. ...........................................

Fig. 99. Simple Electrolytic Detector. ............................

Fig. 100. Electrolytic Detector. ..................................

Fig. 101. Details of Electrolytic Detector. .......................

Fig. 102. Increasing the Sensitiveness of an Electrolytic Detector.

Fig. 103. Tantalum Detector. ......................................

Fig. 104. United Wireless Carborundum Detector (horizontal type). .

Fig. 105. United Wireless Carborundum Detector (vertical type). ...

Fig. 106. Clapp-Eastham Ferron Detector. ..........................

Fig. 107. Silicon Crystal in Cup. .................................

Fig. 108. Silicon Detector Circuits. ..............................

Fig. 109. Perikon Detector Elements. ..............................

Fig. 110. Perikon Detector. .......................................

Fig. 111. Peroxide of Lead Detector. ..............................

Fig. 112. Marconi Magnetic Detector. ..............................

Fig. 113. Details of Transformer. .................................

Fig. 114. Method of Joining Ends of Band. .........................

Fig. 115. Pulley. .................................................

Fig. 116. Pulley Bearings. ........................................

Fig. 117. Circuit of Magnetic Detector. ...........................

Fig. 118. Fleming Oscillation Valve. ..............................

Fig. 119. Flame Audion. ...........................................

Fig. 120. Circuit of Flame Audion. ................................

Fig. 121. Double-slide Tuning Coil. ...............................

Fig. 122. Sliders. ................................................

Fig. 123. Double-slide Tuning Coil Circuits. ......................

*Plate IV. Receiving Circuits. (Straightaway Aerial.)* ............

Fig. 124. Murdock Double-slide Tuning Coil. .......................

Fig. 125. United Wireless Receiving Set. ..........................

Fig. 126. United Wireless Portable Receiving Set. .................

Fig. 127. Oscillation Transformer. ................................

Fig. 128. United Wireless Receiving Transformer. ..................

Fig. 129. Details of Receiving Transformer. .......................

Fig. 130. Slider for Loose Coupler. ...............................

Fig. 131. Loosely Coupled Tuning Circuits. ........................

Fig. 132. Combination Loosely and Closely Coupled Tuner. ..........

Fig. 133. Clapp-Eastham Loose Coupler. ............................

Fig. 134. A Highly Efficient Form of Loose Coupler. ...............

Fig. 135. Potentiometer. ..........................................

Fig. 136. Amco Potentiometer. .....................................

*Plate V. Receiving Circuits.* ....................................

Fig. 137. Tuning Circuit with and without an Adjustable Condenser. .

Fig. 138. Tubular Condenser. ......................................

Fig. 139. Variable Condenser. .....................................

Fig. 140. Details of Variable Condenser. ..........................

Fig. 141. Sliding Plate Variable Condenser. .......................

Fig. 142. Types of Permanent Magnets. .............................

Fig. 143. Grinding Tool. ..........................................

Fig. 144. Parts of a Holtzer Cabot Receiver. ......................

Fig. 145. Holtzer Cabot Head Set. .................................

Fig. 146. Adjustable Head Band. ...................................

Fig. 147. Marconi Station at Siasconset, Mass. ....................

*Plate VI. DeForest and Marconi Systems.* .........................

Fig. 148. Experimental Amateur Station of W. Haddon, Brooklyn, N. Y.

Fig. 149. Complete Receiving Outfit Consisting of Receiving

Transformer, Detector, Fixed Condenser, Loading Coil, Two Variable

Condensers, Potentiometer, Battery, Switches, etc. ................

Fig. 150. Receiving Outfit Consisting of Receiving Transformer,

Fixed Condenser and Detector. .....................................

Fig. 151. Amco Oscillation Helix. .................................

Fig. 152. Details of Oscillation Helix Construction. ..............

Fig. 153. Quenched Gap. ...........................................

Fig. 154. Quenched Gap. ...........................................

Fig. 155. Details of Disk and Ring. ...............................

Fig. 156. Explanatory Drawing of Quenched Gap. ....................

Fig. 157. Amco Rotary Gap. ........................................

Fig. 158. Details of Revolving Parts of Rotary Gap. ...............

Fig. 159. Details of Rotary Gap. ..................................

Fig. 160. Methods of Preventing "Kick Back." ......................

Fig. 161. Variometer. .............................................

Fig. 162. Silicon Detector. .......................................

Fig. 163. Pyron Detector. .........................................

Fig. 164. Galena Detector. ........................................

Fig. 165. Audion. .................................................

Fig. 166. Audion Circuit. .........................................

Fig. 167. Rotary Variable Condenser. ..............................

CHAPTER I. INTRODUCTORY.

Being desirous of keeping this book as far as possible within the limits

prescribed by the title, it is not possible to go deeply into the theory

of the propagation of electric waves, but at the same time it is not

deemed advisable to plunge suddenly into the construction of wireless

apparatus without giving some explanation of the underlying principles.

If the reader desires information upon this subject he is referred to

Fleming’s "Electric Wave Telegraphy" or the same author’s "Elementary

Manual of Radio-telegraphy and Radio-telephony."

The explanations given in this chapter do not involve any actual theory

of the transmission and reception of electric waves. They are merely

intended to show the train of actions which take place and may be

observed in a physical sense. With this purpose in view, several

references have been made to simple hydraulic apparatus and an analogy

drawn to render the explanation clearer.

The Transmission and Reception of Electric Waves.

Wireless telegraphy by means of electromagnetic waves may be divided

into four distinct operations, namely:

1. The generation of electrical oscillations.

2. The transformation of electrical oscillations into electrical

waves.

3. The transformation of electrical waves into electrical

oscillations.

4. The detection of the electrical oscillations.

The first two operations comprise those taking place at the transmitter,

while the last two, which are the converse of the first, are in evidence

only when receiving.

[Illustration: Fig 1. Hertz Oscillator and Resonator.]

Fig. 1 illustrates the original Hertz oscillator and resonator, which is

the simplest form a wireless installation may take. _T_ represents the

transmitting apparatus and _R_ the receptor. At the transmitting station

a telegraph key is placed in series with a battery and an induction

coil. Two large metal plates, _t_ and _t’_, are connected to the

opposite sides of the spark gap, which in turn is connected to the

secondary of the induction coil. When the key is pressed the electrical

circuit is completed and the voltage of the battery is raised

sufficiently by the induction coil to charge the metal plates _t_ and

_t’_.

The key serves to break the current into periods corresponding to the

dots and dashes of the telegraph code. When the high voltage of the

induction coil is impressed upon the plates they become charged, and

being of opposite polarity, when at a maximum the energy rushes across

the gap and produces a disruptive spark. Each discharge, although

appearing like a single spark passing in one direction, is in reality

made up of a large number of rapid oscillations or surgings. The first

passage of current serves to more than discharge the plates and they

become charged in the opposite direction. A reverse discharge then

occurs which also oversteps itself, and thus the oscillations go on, but

gradually become weaker and weaker until they die completely or are

damped out. The heated air of the spark gap becomes a conductor during

the passage of the spark, and the oscillations are enabled to surge back

and forth at the rate of 15,000 to 1,000,000 per second, although the

actual discharge may take only a fraction of a second.

[Illustration: Fig. 2. Hydraulic Oscillator.]

The generation of electrical oscillations may perhaps be made more clear

by reference to the hydraulic apparatus illustrated in Fig. 2. _T_ and

_T’_ are communicating tubes divided by an elastic membrane M. The tubes

may be likened to the metal plates _t_ and _t’_ or the arms of the

oscillator. The membrane may be likened to the layer of air between the

knobs which separates the opposite arms of the oscillator. _P_ is a pump

connected to the two tubes _T_ and _T’_, and the broken lines in the

apparatus represent water. The pump corresponds to the induction coil in

Fig. 1, and the water to the secondary currents of the induction coil.

When the pump is set in operation, the water is drawn from the tube _T_

and injected into _T’_. The pump valves prevent it from flowing back.

When the level becomes very high in _T’_, the great pressure distends

the membrane in the direction shown by the dotted line until finally it

bursts and the water is allowed to flow with a rush into the tube T. But

the inertia of the water causes it to rise higher in the tube than its

final position of equilibrium, while in returning and endeavoring to

seek its level its inertia carries it below this position. Thus the

water oscillates back and forth until finally it comes to rest.

Similarly the difference of potential of the oscillator arms is not

immediately equalized upon the breaking down of the air gap, and the

apparatus becomes the seat of extremely rapid electrical oscillations,

as explained above.

All space is supposed to be filled with a highly attenuated, invisible

and weightless medium called ether. When the electrical oscillations

surge back and forth through the arms of the oscillator, portions of the

energy are thrown off from the apparatus and travel in enlarging circles

like the ripples on a pond. These consist of lines of dielectric stress

or electrostatic flux which pass through the ether and constitute

electromagnetic waves.

The receptor or resonator _R_, Fig. 1, consists of a circle of wire

having in it a small spark gap capable of minute adjustment. Two metal

plates _r_ and _r’_ are sometimes attached to the opposite sides of the

spark gap. When the key is pressed at the transmitting station and waves

are sent out through the ether, they strike the resonator and set up

therein electrical oscillations which pass across the gap in the shape

of sparks.

[Illustration: Fig. 3. "Hydraulic" Transmitter and Receptor.]

To make the explanation clearer, let us consider Fig. 3 in which two

floats or blocks of wood are represented as resting on the surface of a

tank or pool of water. One float, _A_, is connected by a rope and pulley

so that by jerking the rope the float may be made to _oscillate_ and

cause little ripples or waves to pass outwards in a gradually enlarging

circle. When the waves reach the float, _B_, they cause it to rise and

fall with each wave or to oscillate and reproduce the movements of the

float, _A_. Likewise the oscillations set up by a wireless transmitter

are sent out into space to be caught and duplicated at the receiving

station. Of course this analogy to the propagation and reception of

electric waves is not the same as the true electrical actions, but is

merely a graphical, representation.

[Illustration: Fig. 4. Simple Wireless Telegraph Transmitter and

Receptor.]

The wireless telegraph outfit illustrated in Fig. 1 would not serve for

more than short distances of a few feet, and so a somewhat similar but

more efficient apparatus is employed in practice. Fig. 4 shows such a

system in its simplest form. In this case the secondary or high

potential leads of the induction coil are connected, one to an earth and

the other to an aerial or antenna composed of a number of bare copper

wires insulated and suspended from a mast.

All electrically charged bodies are surrounded by an electrostatic field

of force, the nature of which in theory is a state of strain.

The action of an induction coil connected as in Fig. 4 is to charge the

upper part of the aerial above the spark gap, say with negative

electricity and establish a field of force in its vicinity varying in

area from a few feet to several miles. When the charge reaches a certain

potential it is sufficient to puncture the layer of air in the gap and a

spark takes place, setting up electrical oscillations.

[Illustration: Fig. 5. Electric Waves and Lines of Strain.]

Previous to the rupture of the spark gap, _lines of electric strain or

force_ stretch from the aerial to the earth on all sides as in the

center of Fig. 5. A line of force may be defined as a curve drawn in the

electric field so that the direction of the curve is the same as that of

the electric intensity at that point.

The aerial and the earth act like the two metal plates in Fig. 1 or like

the opposite plates of a condenser. As soon as the air gap is punctured

it becomes conductive and the aerial charge rushes down into the earth.

With the discharge, the strain in the electrostatic field is released

and the aerial charge rushes down into the earth, but in so relaxing

produces a new current and builds up a strain around the antenna

opposite in direction to the first. This process repeats itself very

rapidly and electrical oscillations are thus set up in the antenna.

Every oscillation changes the direction of the magnetic flux or

dielectric strain and causes the imaginary lines which originally

stretched from the aerial to the earth to be displaced and the ends

terminating at the aerial to run down it and form semi-loops or inverted

"U’s" standing with their ends on the earth in a circular ripple around

the aerial and moving away from it with the speed of light. In Fig. 5

three oscillations are supposed to have taken place. The shortest

distance between two adjacent points at which the electric strain is at

a maximum in the same direction and period of time is the _wave length_

emitted by the aerial. The separate standing groups of dielectric strain

moving away from the antenna are electromagnetic waves. In the figure,

the adjacent groups are separated by half a wave length. These waves are

emitted at right angles to the transmitting aerial, whence they pass

through the ether to the other station. When they reach the receiving

aerial they set up electrical oscillations therein which are _too weak

to be perceptible in the shape of sparks_ as in the original Hertz

oscillator and resonator because of the great distance separating the

stations, so they are made to flow through a _detector_, which in Fig. 4

is represented as being a crystal of a mineral called silicon. When the

high frequency currents strike the silicon, they set up a weak pulsating

direct current. This action is due to a peculiar rectifying property of

the mineral. The direct current flows through the telephone receiver and

produces an audible sound. If the aerial and ground were connected

directly to the terminals of the telephone receiver, without the

silicon, the oscillations would not pass because of the impeding or

choking action of the electro-magnets in the telephone receivers.

*Tuning.*—It is sometimes desirable that messages should be made

selective or secretive. It is obvious that if there were several large

stations in the same neighborhood they could not all operate at the same

time unless some means of preventing the stations from receiving more

than one message at a time were possible. This is the object in view of

the so-called "tuning" of wireless telegraphy. It also accomplishes a

second purpose which is perhaps considered more important than the

first. The length of the aerial may be too great or too short for the

amount of energy and the length of the waves which it emits or receives.

When this is the case, the oscillations are quickly damped out and do

not generate very powerful waves or produce strong signals at the

receiving station and thus by properly adjusting the circuit all

undesirable messages may be cut out as well as the signaling range

greatly increased. Every electrical circuit has a definite period or

electrical length, determined by its inductance and capacity. A circuit

emits waves of only one length for given values of inductance and

capacity, and must also be of a certain length before it will respond to

waves sent out by another transmitter. The careful adjustment of a

circuit to emit or receive a given wave constitutes _tuning_.

[Illustration: Fig. 6. Resonance Tube.]

This may be made more clear by the comparison of an electrical circuit

with a column of air. Fig. 6 represents a cross section of a glass tube,

_T_, lying in a horizontal position and containing a cork, _C_, which

can be slid to various positions. By adjusting the cork we are able to

obtain various depths of air in the tube from its open end, _M_, to the

cork, _C_.

When a vibrating tuning fork, _F_, is held opposite the open mouth and

the cork slid back and forth it is found that the sound of the tuning

fork is greatly increased in volume at a certain position of the cork.

If the cork is then removed from this position the sound decreases in

intensity. When the cork is in such a position that the sound of the

fork is reenforced, we have secured resonance. When in this condition

and the prong of the vibrating fork is moving toward the open mouth of

the tube a "condensed" pulse of air travels down the tube and back

again, having been reflected at the cork and reaching _M_ just as the

prong of the fork begins its excursion away from the open mouth of the

tube. When the prong of the fork is moving away from _M_ a "rarefied"

pulse of air moves from _M_ to _C_ and back again by the time the prong

is ready to begin its next vibration. When the tube is not in resonance,

the successive condensations and rarefactions passing up and down the

air column interfere with one another and decrease instead of increase

the sound of the tuning fork.

If we substitute the sound waves emitted by the tuning fork for high

frequency oscillations and the air column for the electrical circuit we

may readily see that by adjusting its length, resonance can be produced.

If the length of the air column is measured it will be found that the

reenforcing of the sound of the fork reaches a maximum when the depth of

the air column is _one-fourth_ of the sound wave length given by the

fork. Likewise resonance is produced in wireless telegraphy when the

length of the circuit is _approximately one-fourth_ the length of the

waves. Vice versa, the wave emitted from an ordinary closed circuit

transmitter is _approximately four times_ the length of the aerial wire.

For example, an aerial 25 meters long will emit waves having a length in

the neighborhood of 100 meters.

As stated above, tuning is accomplished and resonance or syntony

established by varying the inductance and capacity of the circuit. The

capacity of a circuit may be defined as its relative ability to retain

an electrical charge, while inductance is the property of an electric

circuit by virtue of which lines of force are developed around it.

Capacity and inductance are opposite or reactive in their effects upon a

circuit. If the value of one is decreased the influence of the other in

increased. Fig. 7 and the following explanation will serve to illustrate

this.

[Illustration: Fig. 7. Lag and Lead.]

Alternating currents do not always keep step with the voltage impulses

of a circuit. If there is inductance in the circuit, the current will

lag behind the voltage, and if there is capacity, the impulses of the

current will lead. Fig. 7 _A_ illustrates the lag produced by inductance

and _B_ the lead produced by capacity. In _A_ the impulses of the

current, represented by the full line, occur a little _later_ than those

of the volts as represented by the dotted line. In _B_ the effect is

just the opposite and the current leads. These reactive effects of

inductance and capacity are very pronounced with the high frequency

currents of wireless telegraphy, and, as stated before, are the factors

which determine the period of the circuit.

[Illustration: Fig. 8. Tuned Hydraulic Transmitter and Receptor.]

Tuning is represented graphically in Fig. 8. The two floats _A_ and _B_

are not only resting on the surface of a pool of water as in Fig. 3 but

are also suspended from the springs _S_ and _S’_. The springs will have,

like a pendulum, a definite time of rising and falling, or period of

oscillation, depending upon their length. If we strike the float _A_ the

spring will cause the float to rise and fall at a definite rate and send

out a little wave or ripple with every oscillation. If the springs _S_

and _S’_ are of the same length, the float _B_ will be caused to

oscillate with every wave sent out by _A_, for, the periods of the

springs being equal, _B_ will be permitted to rise with a wave and fall

again just in time to be raised by the next oncoming ripple. On the

other hand, if the springs are of different lengths, _B_ may only rise

slightly and in falling meet an oncoming wave which will cause it to

rise before it has reached its lowest point and so dampen or weaken its

oscillations that they either do not become very strong or are entirely

obliterated. Thus several floats having different periods of oscillation

might be sending out ripples in the same pool, and the float _B_ could

be made to respond to any of them by adjusting the length of the spring.

We may also see in this illustration the part that tuning plays in

causing the apparatus to emit or receive more powerful impulses. When

the rope in the untuned apparatus illustrated in Fig. 3 is jerked, the

block _A_ oscillates only once or twice before a new jerk is required to

keep it in motion. In Fig. 8 it is quite the contrary, for when an

impulse has been given to the float _A_ it will oscillate much longer

than the untuned float before it requires to be set in motion again.

Likewise the float _B_ in Fig. 8 will oscillate longer and more

powerfully than the float _B_ in Fig. 3, when once it has been set in

motion.

[Illustration: Fig. 9. Tuned Wireless Telegraph Transmitter and

Receptor]

Fig. 9 shows a diagram of a simple wireless telegraph system employing

an inductance and capacity for tuning the circuits. When the induction

coil is in operation it charges a condenser. The condenser discharges

through the sending helix and across the spark gap. The sending helix is

merely a spiral coil of wire of large diameter, and constitutes the

greater part of the inductance in the circuit. Two movable contacts, _A_

and _B_, make connections with the helix. The spark gap, condenser and

lower portion of the helix up to the movable contact _A_ are known as

the closed circuit. By shifting _A_, more or less inductance may be

included in the closed circuit until resonance is secured. The aerial,

the inductance from the contact _B_ down, the condenser and the ground

compose the open circuit. By varying the contact _B_ more or less

inductance may be included in the open circuit and its period altered

until the oscillatory currents of both circuits flow in the same period

of time. The closed and open portions of the transmitting helix form an

auto transformer, and the voltages of the open circuit are raised above

those of the closed circuit.

The tuned receptor shown in Fig. 9 is the simplest form possible and is

known as the single slide system. The tuning coil or helix is much

longer in proportion to its diameter than the sending helix, and is made

of finer wire, since it does not carry such heavy currents. When the

contact is slid up or down on the tuning coil, the inductance of the

circuit is varied. Since the oscillating currents in the receiving

aerial have the same frequency as those in the radiating aerial, the

receptor must have the same relative values of inductance and capacity.

This condition is obtained by varying the slider until the signals in

the telephone receivers are the loudest.

In practice more than one sliding contact is used, and these together

with adjustable condensers make the circuit more complicated. These

devices are necessary because oscillations may be forced on a receptor

by a near-by transmitter unless other precautions than the "single

slider" are taken. Such circuits are illustrated in Plates IV and V.

With them it is possible to obtain a considerable degree of selectivity

and "tune out" an undesirable message.