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.

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.

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.

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.

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.

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.

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.

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.

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.

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.