'WIRELESS.

Evening Star, Issue 14963, 24 August 1912, Page 10

'WIRELESS.

MR RAWSON E. STARK'S LECTURE. OTAGO INSTITUTE. The lecture given by Mr R. E. Stark before the Technological Branch of the Otago Institute on Tuesday night is of such general interest and so clear that we present it practically in full. Mi Stark chose as the title of his lecture ' Radio Telegraphy and the. Problem of Interference':— The practical application of electric waves to the purposes of wireless telegraph transmission over long distances has been greatly extended during the last few years, and many of the difficulties which, say ten years ago, seemed almost insurmountable have been overcome one by one. This improved condition has been brought about by our greater experience with,'* and practical knowledge of, the principle underlying radio telegraphic transmission. Before diving right into our subject, ' The Problem 'of Interference in Wireless," let us consider what some of the underlying principles are, and some of the different systems and their characteristics. There are a few points we must make clear, however. When wc speak of a sound, we mean a vibration of the air surrounding us. A musical sound :'s one in which the rate of vibration exceeds 30 vibrations a second. The range of vibrations which the ear can appreciate as a musical sound is between 2? taps per second and 4,000 taps per second. Above the latter speed the ear hears nothing; while below the former speed the ear hears the sound as onlv beats, or as a rattle without pitch. " The speed of propagation of sound waves is about 1.125 ft a second. For sound we must have air as the medium of conveyance. Those waves which we are- to deal with most particularly here are not vibrations of the air, but" vibrations of the ether. When we speak of vibrations of tho ether _ we mean the vibrations of that medium which enters so closely into the pores of all matter and which fills all space. In ether we have a wide variation in the speed of oscillation or of vibration—from, say, one a second to billions a second. * The slower electro-magnetic vibrations we know as Hertzian waves, and the higher speeds are known to us as heat, light, X-rays. etc. The speed of propagation of all these vibrations is about 1 So".000 millions a second. When we speak of waves, we mean the waves in the ether; and when we speak of "wave-length" we mean the distance between the successive ' crests." At this point a slide was thrown >;i thi' screen to show the relative tloii of these various rays as comd with the positions occupied by , un's rays in a rainbow.] .; there "are several scores of dift systems at present in operation, will' deal with only three, which nplify the chief types. i h;> Marconi system has two distinc- . e combinations. We will start with ihe low-power device. In this combination we have a set of apparatus which I will enumerate. You will understand that a pressure of, say, 60 volts is stopped and started at a rate of 140 times a second entering our induction coil or transformer, and this pressure is increased many hundreds of times and comes out of the other .side at several thousand volts. This pressure is necessary so as to charge our condenser or Leyden jar, which is merely sheets of glass with pieces of tinfoil inserted between. The condenser or Leyden jar stores up the electricity until it reaches a very high pressure, when it discharges itself instantaneously across the spark-gap. This Bpark-gap is composed of two large knobs or hemispherical pieces of iron about a quarter of an inch apart. The discharge is not a simple one, but is followed by a reaction discharging backwards and forwards many times until the energy is dissipated as heat. These oscillations of electricity traverse the inductance coil—or, in other words, the tuning coil—which is composed of many turns of copper wire, and which may" be connected either directly or indirectly with the aerial wires which are suspended between the towers or masts. Let us return one moment to these oscillations caused by the discharge of the condenser 140 times a second. Each complete discharge of the condenser takes place in a very small fraction of each 1-140 second, and the pauses when no electricity is flowing occupy a longer time than the actual useful discharges occupy. I have explained that these impulses have a maximum, and die down during the successive discharges and reactions. IS'.e same effect is, however, not noticed at the receiving end, where the effect they notice is not the maximum of the :' ; c !:r<iL;e, but the average between its ! !:.•;!'■■•.! and lowest points; and to get I -ruce average effect with this com- . .'.ion it is necessary to have a large Jn-rr, and in turn a large amount ::i)v,.-r at short intervals to charge ;r G. Marconi has, however, at his ..'.'r;n and at his Glace Bay stations ..-■•• accumulator plants, consisting of iii.iii- batteries capable of giving 1,200 it:, foutinuous current, and he uses tin.- Ip connection with a small rotating Fi'ii'.rk-gap. somewhat similar to this one tin Jk table. This rotating spark-gap starts and stops the flow of the highpressure electricity about 500 or 600 times a second. Each time the contacts come opposite one another there is a discharge of the condenser, and the result is that a train of but "feebly damped" oscillations are produced. 'These waves have a length of about 1,900 metres. By "damping" we mean checking of the successive electrical impulses. [At this point a slide was thrown upon the screen showing the graphical representation of the various waves produced bv different systems.] Before f describe how the actual transmission of messages takes place I wish to deal with the other characteristic systems. Let us consider tho Poulsen system, the generator of which is a producer of continuous oscillations but feebly damped; that is to say, there are no intervals between the successive impulses. The oscillations in this system are produced by an ingenious device which includes an arc light, an electromagnet, and a condenser. One advantage of this combination is that it may be used for wireless telephony over short distances. For wireless telephony a telephone transmitter is used which is capable of withstanding the strong electrical currents necessary. When one speaks into the transmitter, the air waves produced hy the voice cause thin plates to vibrate in sympathy. These plates are part of the oscillating electrical circuit, and when they vibrate they partially break the circuit, causing a variation in the amount of current passing through the aerial wires, thereby altering the amounts of current in sympathy with the pitch of the voice. In this way the oscillations get broken up and go forth into the ether as electromrg itic impulses, occurring at a very nii.cn smaller frequency. In fact, the frequency has been reduced so much that when these impulses aro again tcken up by the aerial wires at the receiving station they finally act upon a telephone receiver, producing air waves which the ear can hear as a sound. The Telefunken system is similar to the first-described Marconi combination, with the exception that there is a charge imparted to the condensers each 1-1000 second, and also that instead of a single spark-gap there are a number of these in series, thereby allowing the oscillations to pass more freely. Another difference is that the transformer has a special design without any iron, with the effect that no oscillations are wasted by reaction coming back into the generator.

It will be well to bear in mind the differences in»the character of the impulses generated by the different systems. In tho Marconi system first described here the impulses occur at a rate of, say, 140 trains of oscillations a second, with comparatively long intervals between. In the\ high-power Marconi system the trains of waves succeed each other much more rapidly—in the neighborhood of 1,000 traius per second. With the Telefunken system the impulses are of the same character and speed as the high-power Marconi system. With the Poulsen system the chief characteristic is that the oscillations are continuous; that is, there are no intervals between successive impulses. [The above was ably shown by use of an interesting diagram.] One of the disadvantages of the lowpower Marconi system is that electrical discharges in the atmosphere sound so much like the note heard at the receiving station. The high-power combination has not that disadvantage, but has a more, serious drawback, and that is the natural inherent drawbacks of a high-power storage battery plant. With the Poulsen system the chief drawback is the limitation of the power that can be sent out into the ether. At the receiving station of the Poulsen system there is a small piece of apparatus called a "ticker," which breaks up the continuous impulses so that the ear can hear them. With the Telefunken system we have not the disadvantage of an unwieldy storage battery plant, with the advantage that, unlike tho high-power Marconi system, we can alter our power to suit the necessity at hand. It may make it clear at this point to state that when power is turned on at a transmitting station trains of waves are produced that go out through space as monotonous, unintelligible impulses, and it is not until these impulses are broken up by the telephone transmitter or by the Morse key of tho telegraph operator that we get impulses of different values which can be translated by the receiving station into an intelligible message. The function of tho aerials is to radiate into and to receive from the open ether our electrical impulses. We have already spoken of how the oscillations are produced and applied to our aerials. When the condenser discharges and the current jumps tho air gap, it proceeds around the circuit and also charges the aerial at a high electrical potential, at the samo time thrusting the opposite kind of electricity into tiie earth. The earth and the aerial stand charged with the opposite kinds of potentials, and the ether between them is subjected to a considerable strain, and the minute electrons with which the ether is charged are forced to assume definite directions—being polarised, so to speak, in a manner comparable to the polarisation of a compass needle by the. earth's magnetism. Then comes the reaction ? when the aerial and tho earth respectively exchange their electrical qualities. Tho electrons are called upon to instantly reverse their directions, thereby causing a sharp disturbance or shock in the ether. Thus it is that the waves in the ether arc caused, not unlike the waves caused on the surface of a pool of water when a stone is thrown in. * The waves travel in all directions radially from the transmitting aerial until the energy of the impulso is entirely dissipated. Anywhere within the radius where the oscillations are strong enough to be detected they may again bo picked up by a receiving aerial—in fact, every tree, every lightning conductor, or" factory chimney intercepts a portion of these oscillations. The distances covered by various transmitting stations under the same local conditions depend in general upon the power utilised. Very great distances have been covered. The record, I believe, is between Italy and Argentina, practically 8,000 miles. It is interesting- to note that with the same apparatus much greater disi tances may be covered by night than by day. Also, it is noticed that greater distances are covered over the water than over the land. Let us return to these impulses flying through space. A receiving station happens to be within the effective radius, and the impulses, passing the aerial wires at the receiving station, set tho ether in the wires in vibration. These oscillations pass down the aerial wires and through the tuning coil to earth. The function of this tuning coil is to transfer by induction to your detecting device the waves of that length to which you are tuned. The tuning is done by increasing or decreasing the number of turns of wiro in the various coils, as we shall see later, and this has the same effect electrically as slackening or straining the strings of a musical instrument. When the transmitting and the receiving station are tuned to the same pitch, if I may use the expression, or as we say to the same wave-length, the receiving apparatus resonates to the tune of the transmitting station, as ono tuning fork does to the vibrations caused by another of the same pitch. The impulse selected by the tuning coil and transferred inductively to a second coil of wire placed near the first pass through a detector. There are various types of detectors, and their actions are somewhat similar. In the one I have here the electrical oscillations cause a local heating where the circuit is completed between points of silicon and steel. The reason for this local heating is assigned to the fact that there is considerable resistance at these points, which has the effect of checking the oscillations. When wo have two dissimilar metals in contact and the junction is heated, a direct current is started—that is to say, a current without oscillations—and this current '.a turn affects our telephone receivers, causing the metal plates to vibrate in pitch with the trains of waves. All systems must be considered as consisting of two parts—(l) the aerial and (2) the source of oscillations. With a close coupling system—that is, one where the aerial circuit is connected intimately with tho source of oscillations—there are two wave tones to which the system as a whole will respond, the reason being that when the maximum effect is taking place in the spark-gap the aerial circuit is still dormant, and by the time the energy reaches the aerial circuit the condenser has ceased to discharge across the spark-gap until the energy is again built up. While tho oscillation circuit is in this state it is out of resonance with the aerial, and will produce a different wave length, setting up this shorter wave length, thereby dividing the power between the two wave lengths and weakening the original. In order to prevent this double wave length being produced, recourse to an inductive coupling is often made, which means that one coil is made to influence another without contact, ono coil being connected with the spark-gap and the other with the aerial cirouit. It is useful to have these coils relatively nearer or further apart. When they are remote it is called " loose coupling " ; when they are adjacent it is called "close." In order to further prevent the aerial oscillations from reacting back on the generator it is_ advisable not to have exactly tho equivalent turns on the two coils; that is to say, they will not be exactly in tune. The mistuning must not, however, exceed several %. The majority of the receiving tuning coils are constructed on these same principles. The same aerial is used for receiving as for transmitting, though unless special devices are installed both operations are not carried on sirmiltaneously. The various tuning coils on the market, though differing in details, all depend for their tuning on two or three principles, which are easily demonstrated by the " loose coupler " tuning coil, as we shall call it. The tuning coil which I have here is made pasteboard cylinders, which are wound with turns of wire. The primary coil

(or the ono connected with the aerial) is the outer one, and consists of a single-layer of No. 20 magnet wire, arranged so that ono may make connection with any one of the turns, thus enabling one to send the electrical oscillations through the whole or any smaller number of the 208 turns. Ihe secondary coil, which is connected with the detecting device, consists ot six sections, each made up of a great number of turns of finer wire, and by means of a small switch we may include from one to six sections. When the secondary is pushed right in we. say that the coupling is "close," and when the secondary is entirely withdrawn we say it is " loose." The nearness together or the distance of the coils apart is of vital importance to tuning. The lecturer continued by explaining very carefully and particularly, by the aid of a few pictures of apparatus and diagrams, how to tune 'the various instruments, and then proceeded to show how this affected the problem of " Interference " : It is necessary for the designer of a station to consider which system is more advisable in view of the circumstances. If he has to transmit and receive over very long distances he must havo a close coupling, so as to make use of all the available energy, but by having a high and a low wave length he runs the risk of interference with neighboring stations by encroaching on their wave lengths; and his power being so great, they have difficulty in rejecting his oscillations, thereby compelling them to suspend their business. If ho uses a loose coupling he produces wave lengths not widely dissimilar in length, but his radius of action will bo curtailed. It may be added that more harm is done by usiug-power in excess to the requirements than we are likely to credit. How is it we hear so much about amateur interference? Is it that lie wilfully uses more power than is absolutely necessary for experimental purposes, and thereby creates a wave length which is of a value similar to what commercial stations have tc use for their work, and so "jams" their message? Or is it that the efficiency of the commercial receiving apparatus is low!' Or is it that the commercial operators have not studied the problem of tuning sufficiently to enable them to tune out the intruding wave length ? The lecturer concluded with the following remarks:— I have shown you there is a method, by the proper use of condensers and tuning coils, to enable an operator to eliminate disturbances. But the time will come when every operator, amateur and commercial, will be required to be entirely familiar with the latest scientific methods, and thus we shall hear less of "interference." In these modern days, when wireless is one. of the commonplaces of life, we aro apt to forget the debt we owe to Professor Hertz, who was the first one to demonstrate that wireless signalling was possible, and in whose honor the waves have been called " Hertzian waves." [A photo of the eminent professor was thrown upon the screen.]