Wireless Telegraphy and Telephony

Front matter

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The Secret of Wireless Telegraphy

Wireless Telegraphy

Chapter I

The Secret of Wireless Telegraphy

c11. Nature’s Wonderful Medium

The term wireless telegraphy is commonly used in contradistinction to the electric telegraph in which it is necessary to employ a wire as an artificial conducting medium between stations.

Nature provides a medium, called the ether through which intelligence may be communicated over sea or land; and by means of which otherwise impossible results are accomplished. The ether exists between the planets and the stars, and all the other heavenly bodies, and has no conceivable end; hence when we speak of the ether, we tacitly refer to the universe.

This infinite sea of ether is continually in a state of intense turmoil, performing its mission of transferring energy, radiated from the sun, to our earth and the other planets, as well as other influences due to myriads of heavenly bodies outside of our own solar system.

Transmission of energy through the ether takes place in the form of wave motion, and these waves are known as electromagnetic waves. Since light is one form of wave motion, we may make use of it in illustrating the presence of ether, by describing the following well-known experiment.

An ordinary electric bell and small battery are placed in what is known as a receiver, the cover being a glass dome, and an air-pump is then started which will eventually pump out all the air from the receiver in which the electric bell is ringing. Although the cover, or glass dome, is over the bell, it can be plainly heard, and the clapper can be seen vibrating rapidly. As the air is pumped out, however, the sound of the bell becomes fainter and fainter, until, finally, it is no longer heard, although the bell can still be seen as plainly as before, and the clapper vibrating as rapidly as ever. Therefore it is very evident that something remains in the receiver after the air has been removed; for, while we cannot hear the bell, we can see it, and we could not see it if there were not some medium to convey the reflected light waves from the bell to the eye.

While the earth’s atmosphere extends but a comparatively small distance, the extent of the ether is infinite.

The universe is a sea of absolute darkness, and if it were possible for us to place ourselves, say, midway between the moon and the earth (in which position we would be over one hundred thousand miles from any air), we would see all the heavenly bodies, and even our own forms, as the light waves would be intercepted and reflected by them; but, otherwise, all would appear as an intense starlit night, with the unusual features of having the sun visible in greatly increased brilliancy, as well as the moon appearing four times its usual size, and the novelty of seeing our own earth rivalling the moon in size and splendor, even under these conditions, more than ten-fold.

Every manifestation of power on the earth at some time came through the ether in the form of waves, and even when we enjoy the cheerful sunshine we are, in reality, experiencing the result of the absorption of the ether waves from the sun by our own persons.

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2. Vibrations in the Ether

Light, heat, and wireless waves are electromagnetic waves (and, therefore, wave motions of the ether), the only difference being in their relative rates of vibration; their velocity, in free ether, being the same, viz., 186,000 miles per second. They do not, however, traverse all substances with like velocities.

The sun is constantly disturbing the sea of ether, not by a single train of waves, but by wave trains of vary- ing frequencies, and the higher rates of vibration are so intense that the mind can scarcely conceive them. When we think of 767 trillions of vibrations per second, it has little definite meaning to the lay mind; yet science has devised means to accurately determine so high a rate, which represents the extreme violet of the spectrum, and is the highest rate of vibration which our eyes are capable of detecting. The lowest rate of vibration to which our eyes are sensitive is 392 trillions per second, which represents the extreme red of the spectrum.

Our eyes are sensitive to all the other rates of vibration between the extreme violet and the extreme red and all these complex waves, acting together, produce what we call daylight. The waves which produce a sensation of light when they fall upon the eye are, naturally, called light waves, although the photographic plate is affected by light waves which are outside the rates of vibration to which the eye is susceptible.

When the ether waves impinge upon a body of ordinary opaque mass, it may be warmed, the energy being transformed into heat. While the light waves may produce heat, there are also ether waves called dark heat waves, which come at a rate of vibration much too slow to produce a sensation of light.

We say that glass is transparent because it does not appreciably obstruct the ether waves. We may sit near a closed window and enjoy the warmth of the sun because the glass, being transparent, does not arrest the ether waves, and, therefore, the glass is not warmed; but our own bodies, being opaque, do absorb them, thus producing heat, which our sense of feeling makes manifest to us.

The rate of vibration in the ether, as artificially produced in wireless telegraphy, is extremely slow as compared with that of the light waves, ranging from approximately 1,500,000 to 100,000 per second. The wave length, in any such case, is found by dividing the velocity by the rate of vibration; hence for the extreme violet and for the extreme red, 1/3000 inch, while those employed in wireless telegraphy vary from about 650 feet to nearly two miles.

As is well known, the ether in proximity to the earth is in continual disturbance, which creates the magnetic field surrounding the earth. This manifests its presence in many ways; the most common is its influence on the magnetized needle of a compass, which causes it to point north and south. Wireless waves are propagated through this magnetic field, and follow the curvature of the earth.

When we contemplate the wonders of nature, and gradually fathom her mysteries, that which before seemed impossible now becomes an established fact, and even so wonderful an art as wireless telegraphy loses its magic when we study into the laws which govern it, and see it take its place, with marvels of the past, in the service of man.

Principle and Theory of Wireless Telegraphy

Chapter II

Principle and Theory of Wireless Telegraphy

c33. Principle

The art of wireless telegraphy is based upon wave motion, and an analogy is found in the wave motion of water, as the following explanation should make clear. Picture a small pond of still water, with a chip or twig floating upon its surface, mm full view of the observer. Now if a stone be thrown into the water, the sudden impact of the stone would cause ripples, or small waves, to radiate from the point of impact of the stone with the surface of the water, the waves becoming weaker as the circles become larger, i.e., as the distance from the point of impact becomes greater. As the waves arrive at the point where the chip is floating, they will impart motion to the chip; hence the observer will be aware that there has been some disturbance in the water. (See ) After the waves have ceased, the chip will again lie motionless upon the surface of the water. It is obvious that the distance over which the signals may be sent by this means will depend (a) upon the force employed to start the waves, and (b) upon the lightness of the chip, or its sensitiveness to the motion of the waves. Moreover, if there were grass or other obstructions in the pond between the point where the waves are started and the pot where the chip is located, some of the energy would be absorbed in swaying the grasses: hence the effect upon the chip would not be so great, and the signalling distance would be lessened. Or if there were an

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Fig 1.—Wave Motion in Water.

obstruction in the path of the waves, as, for instance, a protruding rock, the waves would be distorted by this obstruction; hence less energy would reach the chip.

It is also obvious that any number of chips might be placed at any number of points within the affected radius of wave motion, and all would be moved by the waves.

When it is considered that these water waves cover an ever-increasing area as the circles expand, and that the actual energy which disturbs the chip is an extremely small part of the total energy in the entire circular wave, it should be clear why so great an amount of energy is required at a wireless sending station in order to operate a very sensitive receiver many miles away.

If we consider that the light chip resting upon the surface of the water has practically no inertia, it will respond to almost any wave length, and, therefore, if the water were disturbed from some other source than the stone referred to, and while signals were being sent by means of the stone, confusion would result, as the chip would respond to the waves from both sources, and, for this reason, no accurate signals could be made out by watching the motions of the chip. However, this difficulty may be overcome by employing a transmitting device which will send out waves of a certain length, and a receiving apparatus which shall respond only to the wave length of the transmitter.

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4. Synchronous Wave Motion and Tuning

If a weight suspended by a spiral spring, or a rubber band, be given a blow so as to cause it to move up and down, the weight will oscillate uniformly; that is, a definite number of times per minute, the frequency depending upon the elasticity of the spring, or rubber band, and the weight of the suspended mass. Now assume this device placed over the pond of still water, as depicted at the left in Fig. 2, and set in motion as described above. On each downward movement of the weight it will touch and disturb the water, and, since

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Fig 2.—Principle of Synchronous Wave Motion

it oscillates uniformly, it will create, or generate, a definite number of waves per minute, all being of uniform length and size.

If now we substitute a similar spring and weight for the chip as a receiving device (shown at the right in ), and place this within the radius of the transmitted waves, these waves in passing will set it in motion, as it oscillates at exactly the same frequency as the transmitting weight. If the receiving device did not oscillate at the same rate as the transmitter, and, therefore, was not in harmony with the transmitted waves, these would tend to counteract any motion imparted to the receiving spring-suspended weight, as the following example should make clear. Assume the receiving weight to be of such dimensions that it will oscillate once per second. Now if the sending weight be generating waves at the rate of two per second, the first wave will give the receiving weight an upward motion at its own frequency; but just as it starts on its downward stroke, the second wave will strike it, thus preventing any further motion of the weight.

It is, therefore, evident that the oscillations of the receiving device would be destroyed if the frequency did not harmonize with that of the sending device. Tuning is absolutely necessary for the successful operation of wireless telegraphy, and it should be thoroughly understood before continuing.

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5. Theory

Wireless signals are a wave motion in, or disturbance of, the magnetic forces of the earth, and are propagated through this magnetic field, following the curvature of the earth, just as a tidal wave would follow the surface of the ocean. Practice indicates that the nodel points of the waves are at, or near, the earth’s surface.

As explained in , ether waves do not traverse all substances with like velocities; this explains why wireless signals are propagated many times farther over water than over land, as the waves traverse air and water at practically the same velocities. In land the waves travel at a much slower rate.

Now to produce, electrically, the results described by the analogy of water, we must employ means for creating waves in the earth’s magnetic field, and use an electrical spring and weight, so to speak. The electrical spring effect is obtained by the electrical phenomena of capacity. Any surface of metal possesses capacity, which is the power to retain a charge of electricity. When this is disturbed it has the same elastic principle as the spring.

The inertia of the weight is represented, electrically, by the term inductance, which effect is produced when a constantly changing current is passed through a coil of wire. This causes the continually changing current to react upon itself, and, consequently, produces a retarding effect.

Referring to , let C represent a capacity connected to the ground MN through the adjustable inductance I. If means be employed to cause the residual charge of this capacity to oscillate, it will, in turn, cause a wave-like motion of the electromagnetic forces of the earth similar to the wave motion in water.

If either the capacity or inductance is increased, the vibrations will be slower, and the wave length will be

f3Fig. 3.—Theory of Wireless Telegraphy.

greater. The waves thus generated are propagated through the earth’s forces in ever-increasing circles, exactly as in the case of the water waves,

C in represents the receiving capacity connected to the ground through the inductance I, in the same manner as at the sending station. This capacity, of course, also contains a residual charge which is dormant under normal conditions, but as the wave front glides by the station, the rising and falling of the waves will impart a slight oscillatory motion to the residual charge.

Means for manifesting these oscillations permit us to correctly read all signals sent out from the transmitting station.

To clearly receive all signals transmitted from the sending station, it can be readily understood that the capacity and inductance must be adjusted to give exactly the same frequency: otherwise the natural frequency of the circuit would counteract the forced oscillations set up by the received waves, thus causing an interference or prevention of oscillations in this particular receiving station. Of course this would have no detrimental effect on other receiving stations which might be adjusted to harmonize with the waves.

The Apparatus Used

Chapter III

The Apparatus Used

c66. General Outline

The production of electromagnetic waves requires a source of current, means for interrupting a unidirectional current (or an alternating current may be used), means for changing the interrupted or alternating current into low-frequency high-pressure currents, means for transforming these into high-frequency high-pressure oscillations, and means for utilizing these oscillations to form the electromagnetic waves.

At the receiving station means must be provided for intercepting the waves, and retransforming them into electrical oscillations; means for detecting the enfeebled oscillations, and for manifesting and translating them into readable signals.

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7. The Induction Coil

The function of the induction coil is to change, say, a battery current of low pressure and comparatively large flow, to a current of great pressure and small flow; or, in other words, it transforms, or changes the character of, electrical energy.

An is shown in , at the left. This is also known as a Ruhmkorff coil in honor of . By its use electrical energy at pressures which might scarcely be felt even when placed across the tongue may be transformed into pressures so great as to render a person unconscious, or to even cause death.

Another form of induction coil is called a . The Ruhmkorff coil is operated by means of an interrupted unidirectional current, while the transformer is operated by an alternating current, i.e., a current which flows rapidly and alternately in opposite directions. Both of these devices are operated by, and consequently deliver, currents of very low frequency, as compared with the frequency required to generate the wireless waves.

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8. Leyden-Jar Battery

A frequency of at least 100,000 vibrations per second is required to form the wireless waves, and since it is impossible to practically obtain this frequency by mechanical means, the is employed for this purpose. This device consists of two pieces of tin-foil separated and insulated from each other by glass, or other suitable material.

A group of these jars, when connected together, con- stitutes a battery of Leyden jars, which has the same effect as a much larger single jar. Instead of being in the form of a round jar, this device is sometimes made in a flat form; that is, the glass and, consequently, the sheets of foil are flat.

When the terminals of a Leyden jar are connected to a source of electrical energy, it will receive and retain a

f4Fig. 4.—Types of Leyden Jars.

of the Leyden jar; the smaller the capacity, the greater the frequency.

The oscillatory discharge of the Leyden jar was first noticed by in 1842. in 1847 said: “We assume that the discharge of a Leyden jar is not a simple motion of the electricity in one direction, but a back-and-forward motion between the coat-

f5Fig. 5.—Battery of Leyden Jars.

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9. The Spark-gap

The device through which the oscillatory discharge of the Leyden jars takes place is known as the spark-gap. This consists of two metal rods insulated from one another, and with their ends about one inch apart, although this

f6Fig. 6.—Spark-gap with Muffler, Surrounded by Inductance.

As the high-frequency discharge across the spark-gap emits a loud, crashing sound, it is usually surrounded by a “muffler” to deaden the noise. The muffler is shown in , and is provided with peep holes, in which glass or mica is set, in order that the operator may at all times be able to watch the condition of the “spark.”

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10. Production of Oscillatory Discharge

In is shown the induction coil, Leyden jars, and spark-gap properly connected to produce the oscillatory

f7Fig. 7.—Connections of Apparatus to Produce the Oscillatory Discharge.

coil or transformer, a current of low pressure is transformed into a current of high pressure, but of low frequency, and this high-pressure current is utilized to charge the Leyden jars.

When the Leyden jars are fully charged (which action takes place almost instantaneously), the resistance of the spark-gap is “broken down,” and the oscillatory discharge takes place between the points of the spark-gap.

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11. The Inductance

In order to successfully utilize the high-frequency oscillations due to the discharge of the Leyden jars across the spark-gap, a controlling device is necessary which shall vary the electrical inertia of the circuit into which these oscillations are to be delivered.

Adjustment is obtained by varying the number of turns of wire in the oscillating circuit. In the inductance is represented by the spirals in the connecting wire between the Leyden jars and the spark-gap. In practice the inductance usually consists of a dozen or so turns of copper wire, about ⅛ inch in diameter, wound spirally around a wooden frame. In the inductance is shown placed around the spark-gap; this, however, is simply a matter of design. Referring again to , one side of the inductance is permanently connected to the ground. There are two other wires flexibly connected to the inductance, one of which is connected to the spark-gap; the other is the antenna. These wires are so arranged that they

f8Fig. 8.—High-power Outfit with Rotating Spark-gap Mounted on Leyden-jar Battery Frame, and Hot-wire Current Meter Mounted on Inductance Frame.——Navy Yard, Washington D. C.

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12. The Antenna

What is probably the most striking characteristic of a shore station is the very tall mast which towers above the operating building. This mast supports a wire, or group of wires, known as the antenna.

The antenna possesses electrical capacity (also inductance), and, therefore, when connected with other apparatus, as in , it disturbs the earth’s magnetic field, as was fully described in .

The antenna is connected to the inductance through one of the flexible connections, as shown in . The length of the antenna varies according to conditions, the supporting mast in some cases being nearly 200 feet high, and in at least one case the height is 418 feet. The antenna is sometimes attached to captive baloons or to kites, and suspended in this manner for temporary service, as in military operations. On boats the antenna is attached to the masts.

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13. Tuning the Transmitting Apparatus

As explained in , it is necessary to have the oscillations of the Leyden jars in synchronism with the antenna circuit. The adjustment is made on the inductance coil as shown in . It can be readily seen that the Leyden jars and antenna circuits can be adjusted independently of one another, but always having more or less

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Fig. 9.—Mast for Supporting Antenna, Fessenden Trans-Atlantic Station, Brant Rock, Mass.

f10Fig. 10.—Method of Suspending Antenna between Masts of Vessels.

There are several means by which it may be determined when the two circuits are in harmony with one another. One method is to insert a hot-wire current meter between the antenna and the inductance, which will indicate the strength of the oscillatory current set up in the same. By manipulating the flexible connections, a maximum reading will be obtained, which will indicate that the two circuits are in synchronism.

In the other method a device is used which accurately indicates the wave length. With this instrument the frequency of one circuit can be measured, and then the other circuit adjusted to give a corresponding wave length.

Since the wave length is dependent on the frequency of oscillations, which in turn is dependent upon the capacity and inductance of the oscillatory circuits, it should be clear that the larger the antenna, the longer will be the wave length, and, necessarily, the greater the capacity of the Leyden jars. The power required is always in proportion to the wave length; that is, for the most efficient results.

In practice it is customary to use a short wave length for low-power short-distance equipments, and a long wave length for high-power long-distance systems. This may be readily understood when we consider that more energy is required to make long, deep waves in water, than is required for the short and shallow waves.

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14. The Receiving Apparatus

While some wireless systems employ separate antennæ for sending and receiving the messages, the same antenna is used for both purposes in most cases, and we may, therefore, describe the receiving apparatus in the inverse order of the transmitting system.

explains how the oscillations are set up in the receiving antenna and, also, how they must be adjusted

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Fig. 11.—Microphone with Telephone Receivers Connected.

practice the condenser is also adjustable so as to increase the range of wave length.

When the two circuits are adjusted to harmonize with the received waves, an electrical pressure is created in the condenser, which pressure can be detected and made manifest by suitable apparatus. This part of the system is called a detector.

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15. Detectors

The function of the detector is to respond to, and make manifest in some manner, the electric oscillations set up

f12Fig. 12.—Microphone Connections

with a telephone receiver, the telephone being sensitive to very weak currents of electricity, which are made manifest by a “noise’” in the telephone receiver. This

f13Fig. 13.—Detector Mounted on Wave Meter.

One type of detector is known as the Microphone. This, in one of its simplest forms, comprises two blocks of carbon with sharp edges, across which rests a steel needle. This type is illustrated in , and its simplest connections are shown in . The steel needle resting across the carbon blocks forms an imperfect contact. When the high-frequency oscillations pass through the carbons and the needle, the contact is greatly improved,

f14Fig. 14.—Connections of Electrolytic Detector.

Another type used in connection with the telephone receiver is called the Electrolytic Detector. This consists of a small cup containing nitric or other acid, into which the end of a very fine platinum wire is slightly immersed. shows the principle. It will be observed that the connections are very similar, although the principle is quite different from that of the microphone. In the electrolytic detector, a film of gas forms between the end of the platinum wire and the acid, which acid is a conductor of electricity. This film of gas insulates the plati-

f15Fig. 15.—Magnetic Detector.

the telephone receiver produces sound, as already described.

The platinum wire employed in the electrolytic detector

f16Fig. 16.—Silicon Detector.

A heavy platinum wire approximately one one-hundredth of an inch in diameter is coated with a suitable thickness of silver. The combined silver and platinum wire is then drawn down to the desired diameter. The close-fitting silver jacket prevents the rupturing of the platinum wire within during the drawing process. The silver is then removed by immersing in nitric acid, which leaves the platinum only. Platinum wires have thus been drawn down to .00006 of an inch.

Another type, known as the Magnetic Detector, is based upon the phenomena that certain magnetic characteris-

f17Fig. 17.—Connections of Silicon Detector

What is probably the simplest type of all is known as the Silicon Detector. This consists of a piece of silicon with a brass point resting against it. In the presence of the high-frequency oscillations, the particles at the point of contact are heated, which causes a sound in the telephone receiver. The silicon detector is illustrated in , and shows its connections.

The Apparatus Used

Chapter IV

Method of Operation

c1616. Switching Device

While, as before stated, in some cases separate antennæ are used for sending and receiving, it is common practice to use the same antenna for both. This is accomplished by means of a switching device so arranged that when the transmitter is connected in, the receiver is cut out. This is absolutely necessary, as otherwise the delicate receiving apparatus would be destroyed should the high-power currents of the transmitter pass through it.

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17. Sending the Message

In are shown several types of keys used in wireless telegraphy. The operator sends the message by pressing the key lever downward, or allowing it to remain up for certain periods of time. Pressing the key lever downward for a brief interval represents a dot, and a longer period of depression, a dash. The proper arrangement of dots and dashes forms letters, and combinations of letters of course form words, etc. the letters being spaced by holding the key lever up for a given period of time, and the words being spaced by still longer periods.

The proper arrangement of the dots, dashes, and spaces constitutes a code. The Morse code is extensively used in America, while a modification of it, called the Continental code, is employed in England and on the

f17Fig. 17.—Types of Keys.

When an operator desires to communicate with another station, he first “listens in” by connecting his receiving system with the antenna and the ground, and placing the telephone receiver to his cars. He then adjusts his receiving circuits for various wave lengths and, if he hears no signals, he assumes that no one else is sending within his radius. He therefore “throws in” the transmitting apparatus, which action automatically disconnects the receiving side. He then sends the letters which constitute the “call” of the station desired, signing the letters designating his own station, after sending the call several times. He then listens in again, and if the operator at the desired station has heard his call, the latter answers, and regular telegraphic communication ensues.

Some wireless systems are so arranged that no switching is necessary. The operators can “break” one another, by keeping the telephone receivers over their ears all the time. If the receiving operator should wish to correct the sender, he does so by pressing his key; this is heard by the sending operator when his key is in normal position.

Tuning not only increases the radius of operation, but maintains secrecy as well. By means of careful tuning, two distant stations may be sending simultaneously, and if they employ different wave lengths, the operator at the receiving station may, by adjusting his apparatus to the wave length of the station with which he desires to communicate, “tune out” the other message, receiving only the one desired.

However, if the stations above referred to be quite f19

Letters | Morse | Continental

A | 1 3 | 1 3

B | 3 1 1 1 | 3 1 1 1

c | 1 1 1 | 3 1 3 1

D | 3 1 1 | 3 1 1

E | 1 | 1

F | 1 3 1 | 1 1 3 1

G | 3 3 1 | 3 3 1

H | 1 1 1 1 | 1 1 1 1

I | 1 1 | 1 1

J | 3 1 3 1 | 1 3 3 3

K | 3 1 3 | 3 1 3

L | 33 | 1 3 1 1

M | 3 3 | 3 3

N | 3 1 | 3 1

O | 1 1 | 3 3 3

P | 1 1 1 1 | 1 3 3 1

Q | 1 1 3 1 | 3 3 1 3