Wireless Telegraphy and Telephony/Chapter 3

by Walter W. Massie and Charles R. Underhill

 

Chapter III

The Apparatus Used

6. 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.

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 induction coil is shown in Fig. 7, at the left. This is also known as a Ruhmkorff coil in honor of its inventor. 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 transformer. 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.

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 Leyden jar 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, constitutes 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

Fig. 4.—Types of Leyden Jars.

charge equal in electrical pressure to that of the source of energy. If, after receiving a charge, its terminals be brought near one another, a sudden discharge takes place in the form of an electric spark which, while appearing to be single and momentary, has been found by experiment to consist of a series of alternating flashes in rapid succession, each flash lasting less than one hundred thousandth part of a second. The frequency of these oscillations is regulated by the capacity, or size,

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

The oscillatory discharge of the Leyden jar was first noticed by Prof. Joseph Henry in 1842. Von Helmholtz 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-

Fig. 5.—Battery of Leyden Jars.

ings in oscillation, which becomes continually smaller until the entire vis viva is destroyed by the sum of the resistances.” In 1853 Lord Kelvin proved the oscillatory discharge mathematically, and in 1859 Feddersen demonstrated it experimentally, by employing a rapidly revolving mirror.

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

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

distance may be varied at will by means of an adjusting device.

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 Fig. 6, 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.”

10. Production of Oscillatory Discharge

In Fig. 7 is shown the induction coil, Leyden jars, and spark-gap properly connected to produce the oscillatory

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

discharge. This takes place in the following manner. In Art. 7 we explained how, by means of the induction

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.

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 Fig. 7 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 Fig. 6 the inductance is shown placed around the spark-gap; this, however, is simply a matter of design. Referring again to Fig. 7, 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

Fig. 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.

may be connected to any point on the spirally wound wire of the inductance.

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 Fig. 7, it disturbs the earth’s magnetic field, as was fully described in Art. 5.

The antenna is connected to the inductance through one of the flexible connections, as shown in Fig. 7. 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.

13. Tuning the Transmitting Apparatus

As explained in Art. 5, 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 Fig. 7. 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

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

turns of the inductance coil common to both circuits. When the two circuits are adjusted to the same frequency, the discharge of the Leyden jars, through the few turns of wire in the inductance, will induce oscillations in the

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

antenna, which in turn cause the disturbance in the magnetic field of the earth.

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.

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.

Art. 5 explains how the oscillations are set up in the receiving antenna and, also, how they must be adjusted

Fig. 11.—Microphone with Telephone Receivers Connected.

to the same frequency as that of the passing waves from the transmitting station. Referring to Fig. 3, C' represents the antenna connected through the adjustable inductance I'. Adjustably connected with this inductance is also a small capacity, called a condenser, which with the inductance forms a closed oscillating circuit. The vibratory motion in the antenna is adjusted by moving the connection y. The frequency of the closed circuit is adjusted by changing the position of point x. In

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.

15. Detectors

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

Fig. 12.—Microphone Connections

in the receiving circuits. There are many devices that will serve this purpose. They are used in connection

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

Fig. 13.—Detector Mounted on Wave Meter.

noise, or buzzing sound, corresponds to the sound of the spark at the sending station; thus an operator may often recognize a distant station by the sound of the “‘spark”’ in his telephone receiver.

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 Fig. 11, and its simplest connections are shown in Fig. 12. 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,

Fig. 14.—Connections of Electrolytic Detector.

with the result that the local battery current will be strengthened in nearly direct ratio to the improved contact. This change of current causes a sound in the telephone receiver.

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. Fig. 14 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-

Fig. 15.—Magnetic Detector.

num wire from the acid. Hence there will be practically no current flowing through the telephone receiver. In the presence of the high-frequency oscillations, however, the resistance of the gas film is reduced, which allows an increased battery current to flow through the telephone receiver. The sudden rush of the battery current through

the telephone receiver produces sound, as already described.

The platinum wire employed in the electrolytic detector

Fig. 16.—Silicon Detector.

is so fine, and the method of its manufacture so unique, that we here describe it.

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-

Fig. 17.—Connections of Silicon Detector

tics in iron undergo a change under the influence of the high-frequency oscillations. This detector is illustrated in Fig. 15. It requires no local battery, but must be rotated by means of clockwork or a small electric motor. The regular telephone receiver is also used with this detector.

What is probably the simplest type of all is known as the Silicon Detector.^[1] 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 Fig. 16, and Fig. 17 shows its connections.

1. Patented by Prof. G. W. Pickard.