Historic Inventions

by Rupert Sargent Holland

Let us now see what it was that Marconi had actually done.

Wireless signals are in reality wave motions in the magnetic forces of the earth, or, in other words, disturbances of those forces. They are sent out through this magnetic field, and follow the earth’s curvature, in the same way that tidal waves follow the ocean’s surface. Everywhere about us there is a sea of what science calls the ether, and the ether is constantly in a state of turmoil, because it is the medium through which energy, radiating from the sun, is carried to the earth and other planets. This energy is transmitted through the free ether in waves, which are known as electromagnetic waves. It was this fact that Professor Hertz discovered, and the waves are sometimes called the Hertzian waves. Light is one variety of wave motion, and heat another. The ether must be distinguished from the air, for science means by it a medium which exists everywhere and is to be regarded as permeating all space and all matter. The ether exists in a vacuum, for, although all the air may have been withdrawn, an object placed in a vacuum can still be seen from outside, and hence the wave motions of light are traveling through a space devoid of air.

Professor Hertz proved in 1888 that a spark, or disruptive discharge of electricity, caused electro-magnetic waves to radiate away in all directions through the ether. The waves acted exactly like ripples that radiate from a stone when it strikes the water. These Hertzian waves were found to travel with the same velocity as light, and would circle the world eight times in a second. As soon as the existence of these waves was known many scientists began to consider whether they could not be used for telegraphy. But the problem was a very difficult one. The questions were how to transmit the energy to a distance, and how to make a receiver that should be sensitive enough to be affected by it.

Let us picture a body of still water with a twig floating upon its surface. If a stone is thrown into the water ripples radiate in all directions, these waves becoming weaker as the circles they form become larger, or in other words as they grow more distant from the point where the stone struck the water. When the waves reach the floating twig they will move it, and when they cease the twig will be motionless again. Should there be grasses or rocks protruding up from the water the motion given to the twig by the waves would be lessened, or distorted, or changed in many ways, depending on the intervening object. Whether the waves will actually impart motion to the twig will depend on the force by which these waves were started and upon the lightness of the twig, or its sensitiveness to the ripples as they radiate. If the water were disturbed by some other force than the stone the twig would be moved by that other force, and the observer could not tell from what direction the motion had come, or how it had been caused. Applying this to wireless telegraphy one may say that a device must be used that will send out waves of a certain length, and that the receiver must be constructed so that it will respond only to waves of the length sent by that transmitter.

There must therefore be accurate tuning of the two instruments. Let a weight be fastened at the end of a spiral spring and then be struck. The weight will oscillate at a uniform rate, or so many times a minute. If this be held so that it strikes the water the movement of the spring will create a certain number of waves a minute. If now a second weight, attached to a second spring, be hung down into the water, the waves caused by the first will reach the second, and if the springs be alike the movements or oscillations will correspond. But if the springs were not alike, or if, in other words, the two instruments were not in tune, the wave motions would not be received and copied accurately. Therefore in wireless telegraphy the instrument that is to impart the motion to the electro-magnetic waves that fill the ether must be tuned in accord with the instrument that is to receive the motion of those waves.

The sending of the wireless message requires a source of production of the electro-magnetic waves. This is obtained by what is known as capacity, or in other words, the power that is possessed by any metal surface to retain a charge of electricity, and by inductance, procured when a constantly changing current is sent through a coil of wire. This capacity and inductance must be adjusted to give exactly the same frequency of motion to the waves, or the same oscillations, if the receiver that is tuned to vibrate to those waves is to receive that message accurately. The receiving station must have the means to intercept the waves, and then transform them again into electrical oscillations that shall correspond to those sent out from the transmitting station.

As early as 1844 Samuel F. B. Morse had succeeded in telegraphing without wires under the Susquehanna River, and in 1854 James Bowman Lindsay, a Scotchman, had sent a message a distance of two miles through water without wires. Sir William Henry Preece, by using an induced current, had telegraphed several miles without a connecting wire. But the discoveries made in regard to the Hertzian waves placed the subject on a different footing, and the possibility of an actual usable wireless telegraph was now looked at from a new view-point.

Professor Hertz had used a simple form of apparatus to obtain his free ether waves. A loop of wire, with the ends almost touching each other, had been his receiver, or detector. When he set his generator, or instrument to create the oscillations, in operation, and held the detector near it, he could see very minute electric sparks passing between the ends of the loop of wire. This proved the existence of the electro-magnetic waves.

In 1890 Professor Eduard Branly found that loose metallic filings became good conductors of electricity when there were electric oscillations at hand. He demonstrated this by placing the filings between metal plugs in a glass tube, and connecting this in circuit with a battery and electric indicator. Professor Oliver Lodge named this device of Branly’s a “coherer,” and when he found that it was more sensitive than the Hertz detector he combined it with the Hertz oscillator. This was in 1894, and the combination of oscillator and coherer actually formed the first real wireless set.

Wireless stations on shore are marked by very tall masts, which support a single wire, or a set of wires, which are known as the antenna. The antenna has electrical capacity, and when it is connected with the other apparatus needful to produce the oscillations it disturbs the earth’s magnetic field. For temporary service, as in the case of military operations, the antenna is frequently attached to captive balloons or kites, and so suspended high in air. On ships the antenna is fastened to the masts. The step that led to this addition was taken by Count Popoff in 1895, when he attached a vertical wire to one side of the coherer of the receiver of Professor Lodge, and connected the other side with the ground. He used this to learn the approach of thunder-storms.