A similar method is employed for the cylinders. They are driven by electric motors, and the motor at the receiving end is adjusted so as to run very slightly faster than the motor at the sending end. The result is that the receiving cylinder completes one revolution a minute fraction of a second before the transmitting cylinder. It is then automatically held back until the sending cylinder completes its revolution, and then both commence the next revolution exactly together. The pause made by the receiving cylinder is of extremely short duration, but in order that there shall be no break in the spiral traced by light upon the film, the pause takes place at the point where the ends of the film come together. In actual practice certain other details of adjustment are required to ensure precision in working, but the main features of the process are as described.
Although the above photo-telegraphic process is very satisfactory in working, it has been superseded to some extent by another process of a quite different nature. By copying the original photograph through a glass screen covered with a multitude of very fine parallel lines, a half-tone reproduction is made. This is formed of an immense number of light and dark lines of varying breadth, and it is printed in non-conducting ink on lead-foil, so that while the dark lines are bare foil, the light ones are covered with the ink. This half-tone is placed round a metal cylinder having the same movement as the cylinders in the previous processes, and a metal point, or “stylus” as it is called, is made to rest lightly upon the foil picture, so that it travels all over it, from one end to the other. An electrical circuit is arranged so that when the stylus touches a piece of the bare foil a current is sent out along the line wire. This current is therefore intermittent, being interrupted each time the stylus passes over a part of the half-tone picture covered with the non-conducting ink, the succeeding periods of current and no current varying with the breadth of the conducting and the non-conducting lines. This intermittent current goes to a similar arrangement of stylus and cylinder at the receiving end, this cylinder having round it a sheet of paper coated with a chemical preparation. The coating is white all over to begin with, but it turns black wherever the current passes through it. The final result is that the intermittent current builds up a reproduction in black-and-white of the original photograph. In this process also the cylinders have to be “synchronized,” or adjusted to run at the same speed. Both this process and the foregoing one have been used successfully for the transmission of press photographs, notably by the Daily Mirror.
Professor Korn has carried out some interesting and fairly successful experiments in wireless transmission of photographs, but as yet the wireless results are considerably inferior to those obtained with a line conductor. For transmitting black-and-white pictures, line drawings, or autographs by wireless, a combination of the two methods just mentioned is employed; the second method being used for sending, and the first or selenium method for receiving. For true half-tone pictures the selenium method is used at each end.
CHAPTER XX
WIRELESS TELEGRAPHY AND TELEPHONY—PRINCIPLES AND APPARATUS
Wireless telegraphy is probably the most remarkable and at the same time the most interesting of all the varied applications of electricity. The exceptional popular interest in wireless communication, as compared with most of the other daily tasks which electricity is called upon to perform, is easy to understand. The average man does not realize that although we are able to make electricity come and go at our bidding, we have little certain knowledge of its nature. He is so accustomed to hearing of the electric current, and of the work it is made to do, that he sees little to marvel at so long as there is a connecting wire. Electricity is produced by batteries or by a dynamo, sent along a wire, and made to drive the necessary machinery; apparently it is all quite simple. But take away the connecting wire, and the case is different. In wireless telegraphy electricity is produced as usual, but instantly it passes out into the unknown, and, as far as our senses can tell, it is lost for ever. Yet at some distant point, hundreds or even thousands of miles away, the electrical influence reappears, emerging from the unknown with its burden of words and sentences. There is something uncanny about this, something suggesting telepathy and the occult, and herein lies the fascination of wireless telegraphy.
The idea of communicating without any connecting wires is an old one. About the year 1842, Morse, of telegraph fame, succeeded in transmitting telegraphic signals across rivers and canals without a connecting wire. His method was to stretch along each bank of the river a wire equal in length to three times the breadth of the river. One of these wires was connected with the transmitter and with a battery, and the other with a receiver, both wires terminating in copper plates sunk in the water. In this case the water took the place of a connecting wire, and acted as the conducting medium. A few years later another investigator, a Scotchman named Lindsay, succeeded in telegraphing across the river Tay, at a point where it is over a mile and a half wide, by similar methods. Lindsay appears to have been the first to suggest the possibility of telegraphing across the Atlantic, and although at that time, 1845, the idea must have seemed a wild one, he had the firmest faith in its ultimate accomplishment.
Amongst those who followed Lindsay’s experiments with keen interest was the late Sir William, then Mr. Preece, but it was not until 1882, twenty years after Lindsay’s death, that he commenced experiments on his own account. In March of that year the cable across the Solent failed, and Preece took the opportunity of trying to signal across without a connecting wire. He used two overhead wires, each terminating in large copper plates sunk in the sea, one stretching from Southampton to Southsea Pier, and the other from Ryde Pier to Sconce Point. The experiment was successful, audible Morse signals being received on each side. In this experiment, as in those of Morse and Lindsay, the water acted as the conducting medium; but a year or two later, Preece turned his attention to a different method of wireless communication, by means of induction. This method was based upon the fact that at the instant of starting and stopping a current in one wire, another current is induced in a second wire placed parallel to it, even when the two wires are a considerable distance apart. Many successful experiments in this induction telegraphy were made, one of the most striking being that between the Island of Mull and the mainland, in 1895. The cable between the island and the mainland had broken, and by means of induction perfect telegraphic communication was maintained during the time that the cable was being repaired. Although this system of wireless telegraphy is quite successful for short distances, it becomes impracticable when the distance is increased, because the length of each of the two parallel wires must be roughly equal to the distance between them. These experiments of Preece are of great interest, but we must leave them because they have little connexion with present-day wireless telegraphy, in which utterly different methods are used.
All the commercial wireless systems of to-day depend upon the production and transmission of electric waves. About the year 1837 it was discovered that the discharge of a Leyden jar did not consist of only one sudden rush of electricity, but of a series of electric oscillations, which surged backwards and forwards until electric equilibrium was restored. This discovery was verified by later experimenters, and it forms the foundation of our knowledge of electric waves. At this point many readers probably will ask, “What are electric waves?” It is impossible to answer this question fully, for we still have a great deal to learn about these waves, and we only can state the conclusions at which our greatest scientists have arrived after much thought and many experiments. It is believed that all space is filled with a medium to which the name “ether” has been given, and that this ether extends throughout the matter. We do not know what the ether is, but the important fact is that it can receive and transmit vibrations in the form of ether waves. There are different kinds of ether waves, and they produce entirely different effects. Some of them produce the effect which we call light, and these are called “light waves.” Others produce the effect known as heat, and they are called “heat waves”; and still others produce electricity, and these we call “electric waves.” These waves travel through the ether at the enormous speed of 186,000 miles per second, so that they would cross the Atlantic Ocean in about 1/80 second. The fact that light also travels at this speed suggested that there might be some connexion between the two sets of waves, and after much experiment it has been demonstrated that the waves of light and electricity are identical except in their length.
Later on in this chapter we shall have occasion to refer frequently to wave-length, and we may take this opportunity of explaining what is understood by this term. Wave-length is the distance measured from the crest of one wave to the crest of the next, across the intervening trough or hollow. From this it will be seen that the greater the wave-length, the farther apart are the waves; and also that if we have two sets of waves of different wave-lengths but travelling at the same speed, then the number of waves arriving at any point in one second will be greater in the case of the shorter waves, because these are closer together.
A tuning-fork in vibration disturbs the surrounding air, and sets up air waves which produce the effect called sound when they strike against the drums of our ears. In a similar way the discharge of a Leyden jar disturbs the surrounding ether, and sets up electric ether waves; but these waves produce no effect upon us in the shape of sight, sound, or feeling. There is however a very simple piece of apparatus which acts as a sort of electric eye or ear, and detects the waves for us. This consists of a glass tube loosely filled with metal filings, and having a cork at each end. A wire is passed through each cork so as to project well into the tube, but so that the two ends do not touch one another, and the outer ends of these wires are connected to a battery of one or two cells, and to some kind of electrically worked apparatus, such as an electric bell. So long as the filings lie quite loosely in the tube they offer a very high resistance, and no current passes. If now electric waves are set up by the discharge of a Leyden jar, these waves fall upon the tube and cause the resistance of the filings to decrease greatly. The filings now form a conducting path through which the current passes, and so the bell rings. If no further discharge takes place the electric waves cease, but the filings do not return to their original highly resistant condition, but retain their conductivity, and the current continues to pass, and the bell goes on ringing. To stop the bell it is only necessary to tap the tube gently, when the filings immediately fall back into their first state, so that the current cannot pass through them.