CHAPTER II
TRANSMITTING APPARATUS
Let us now consider the requirements necessary for transmitting photographs by means of the wireless apparatus in use at the present time.
The connections for an experimental syntonic wireless transmitting station are shown in the diagram Fig. 4. A is the aerial; T, the inductance; E, earth; L, hot-wire ammeter. The closed oscillatory circuit consists of an inductance F, spark-gap G, and a block condenser C. H is a spark-coil for supplying the energy, the secondary J being connected to the spark-gap. A
mercury break N and a battery B are placed in the primary circuit of the coil. The Morse key K is for completing the battery circuit for signalling purposes. When the key K is depressed, the battery circuit is completed, and a spark passes between the balls of the spark-gap G producing oscillations in the closed circuit, which are transposed to the aerial circuit by induction. For signalling purposes it is only necessary for the operator by means of the key K to send out a long or short train of waves in some pre-arranged order, to enable the operator at the receiving station to understand the message that is being transmitted.
If a photograph could be prepared in such a manner that it would serve the purpose of the key K, and could so arrange matters that a minute portion of the photograph could be transmitted separately but in succession, and that each portion of the photograph having the same density could be given the same signal, then it would only be necessary to have apparatus at the receiving station capable of arranging the signals in proper sequence (each signal recorded being the same size and having the same density as the transmitted portion of the photograph) in order to receive a facsimile of the picture transmitted.
The following method of preparing the photograph[[3]] is one that has been adopted in several
systems of photo-telegraphy, and is the only one at all suitable for wireless transmission. The photograph or picture which is to be transmitted is fastened out perfectly flat upon a copying-board. A strong light is placed on either side of this copying board, and is concentrated upon the picture by means of reflectors. The camera which is used for copying has a single line screen interposed between the lens and sensitised plate, and the effect of this screen is to break the picture up into parallel lines. Thus a white portion of the photograph would consist of very narrow lines wide apart, while the dark portion would be made up of wide lines close together; a black part would appear solid and show no lines at all. From this line negative it will be necessary to take off a print upon a specially prepared sheet of metal. This consists of a sheet of thick lead- or tinfoil, coated upon one side with a thin film of glue to which bichromate of potash has been added; the bichromate possessing the property of rendering the glue waterproof when acted upon by light. The print can be taken off by artificial light (arc lamps being generally used), but the exact time to allow for printing can only be found by experiment, as it varies considerably according to the thickness of the film. The printing finished, the metal print is washed under running water, when all those parts not acted upon by light, i.e. the parts between the lines, are
washed away, leaving the bare metal. We have now an image composed of numerous bands of insulating material (each band varying in width according to the density of the photograph at any point from which it is prepared) attached to a metal base, so that each band of insulating material is separated by a band of conducting material. It is, of course, obvious that the lines on the print cannot be wider apart, centre to centre, than the lines of the screen used in preparing it. A good screen to use is one having 50 lines to the inch, but one is perhaps more suitable for experimental work a little coarser, say 35 lines to the inch. To use a screen having 50 or more lines to the inch, the transmitting apparatus, as will be evident later on, will require to be very nearly perfect.
Before proceeding further it will perhaps be as well to make an experiment. If we take one of the metal prints or, more simple, draw a sketch in insulating ink upon a sheet of metal A, Fig. 5, and connect a battery B and the galvanometer D as shown, we shall find on drawing the free end of the wire across the metal plate that all the time the wire is in contact with the lines of insulating material the needle of the galvanometer will remain
at zero, but where it is in contact with the metal plate the needle is deflected.
From this experiment it will be seen that we have in our metal line print, which consists of alternate lines of insulating and conducting material, a method by which an electric circuit can be very easily made and broken. It is, of course, necessary to have some arrangement whereby the whole of the surface of the metal print is utilised for this purpose to the best advantage. One type of transmitting machine used for this purpose is represented by the diagram, Fig. 6. The cylinder A is fastened to the steel shaft B, which runs in the two bearings D and D', the bearing D' having an internal thread corresponding to that on the shaft. The stylus in this class of machine is a fixture, the cylinder being given a lateral as well as a revolving movement. As it is impossible to use a rigid drive, a flexible coupling F is employed between the shaft B and the motor.
Another type of machine is shown in Fig. 7. The drum in this case is stationary, the table T moving laterally by reason of the screwed shaft
The steel point Z (ordinary gramophone needles may be used and will be found to answer the purpose admirably) is made to press lightly upon the metal print, and while the pressure should be sufficient to make good electrical contact, it should not be sufficient to cause the needle to scratch the surface of the foil. The pressure is regulated by means of the milled nut H. The electrical connections are given in Fig. 9. One wire from the battery M is taken to the terminal T, and the other wires from M and F lead to the relay R. The current flows from the battery M through the spring Y, through the drum and metal print, the stylus Z, spring A, down to the relay R, and from R back to the battery M. As the drum carrying the single line half-tone print is revolved, the stylus, by reason of the lateral movement given to the table or cylinder as the case may be, will trace a spiral path over the entire surface of the print. As the stylus traces over a conducting strip the circuit is completed, and the tongue of the relay R is attracted, making contact with the stop S.
On passing over a strip of insulation the circuit is broken and the tongue of the relay R returns to its normal position.
As already stated, the conducting and insulating bands on the print vary in width according to the density of the photograph from which it is prepared, so that the length of time that the tongue of the relay R is held against the stop S, is in proportion to the width of the conducting strip which is passing under the stylus at any instant. The function of the transmitter is therefore to send to the relay R an intermittent current of varying duration.
The two photographs Figs. 10 and 10a are of a machine designed and used by the writer in his experiments. In this machine the drum is 3.5 inches long and 1.5 inches in diameter. The lead screw has 30 threads to the inch, and the reduction between it and the drum is 3:1, so that the table has a movement of 1/90th inch per revolution of the drum.
From the brief description of the various types of machines that have been given it will be apparent that in the design of the machine proper there is nothing very complicated, although the addition of the driving and synchronising apparatus complicates matters rather considerably. The questions of driving and synchronising the machines at the two stations is fully dealt with in Chapter IV.
Although the design of the machines is rather simple great attention must be paid both to accuracy of construction and accuracy of working, and this applies, not only to the machines (whether for transmitting or receiving) but for all the various pieces of apparatus that are used. Too much care cannot be bestowed upon this point, as in the wireless transmission of photographs there is a large number of instruments all requiring careful adjustment, and which have to work together in perfect unison at a high speed.
The machine shown in Figs. 10 and 10a was designed and used by the writer solely for experimental work. It will be noticed in the description given in the appendix of the method of preparing the metal prints that a 5" × 4" camera is recommended, while the machine, Fig. 10, is designed to take a print procured from a quarter-plate negative. This size of drum was adopted for several reasons, and although it will be found quite large enough for general experimental work the writer has come to the conclusion that for practical commercial work a drum to take a print 5" × 4" will give better results.
In making a negative of a picture that is required for reproduction purposes, the line screen in the camera is replaced by a "cross screen," i.e. two single line screens placed with their lines at an angle of 90° to one another, and this breaks the
image up into small squares instead of lines. By looking at any ordinary newspaper or book illustration through a powerful magnifying glass the effects of a cross screen will readily be seen. With a cross screen a certain amount of detail is necessarily lost, but with a single line screen the amount lost is much greater. If there is any very small detail in the picture most of this would be lost in a coarse screen, hence the necessity of employing as fine a line screen as practicable in order to get as much detail in as possible. It is mainly on this account that a 5" × 4" print is recommended, as, if fairly bold subjects are used for copying, the small detail (this is, of course, a very vague and indefinable term) will not be too fine, and the time required for transmitting reasonable. For obvious reasons it is a great advantage to put the print under pressure to cause the glue image to sink into the soft metal base and leave a perfectly flat and smooth surface. It is essential that the bands on the print lie along the axis of the cylinder, so that the stylus traces its path across them, and not with them.
We have now an arrangement that is capable of taking the place of the key K, Fig. 4, and the diagram, Fig. 11, gives the connections for the complete transmitter. A is the aerial, E earth, T inductance, L ammeter. The closed oscillatory circuit consists of a spark-gap G, inductance F,
In transmitting over ordinary conductors where the initial voltage is fairly high and the self-induction of the circuit very great, the use of the condenser will be found to be absolutely essential. It has also been noted that the angle which the stylus presents to the drum has a marked effect upon the sparking, an angle of about 60° being found to give very good results.
If the size of the single line print used is 5 inches by 4 inches, and a screen having 50 lines
to the inch is used for preparing it, then the stylus will have to make 250 contacts during one revolution of the drum. Assuming the drum to make one revolution in three seconds, then the time taken to transmit the complete photograph can be found from the equation T = w × t × s, where w is the width of the print, t the travel of the stylus during one revolution of the drum, and s the time required for one revolution of the drum. In the present instance this will be T = 4 × 90 × 3 = 1080 seconds = 18 minutes. The number of contacts made by the stylus per minute is 5000, and in working at this speed the first difficulty is encountered in the use of the two relays. The relay R is lightly built, and capable of working at a fairly high speed, but R' is a heavier pattern, and consequently works at a slightly lower rate. This relay must necessarily be heavier, as more substantial contacts are needed in order to pass the heavy current taken by the spark-coil.
Relays sensitive and accurate enough to work at this speed will in all probability be beyond the reach of the majority of workers, but there are several types of relays on the market very reasonable in price that will answer very well for experimental work, although the speed of working will no doubt be slower.
For the best results the duration of the wave-trains sent out should be of the same duration as
the contact made by R, and therefore equal to the time taken by the stylus to trace over a conducting strip; but if the duration of the contact made by R is t, then that made by R' and consequently the duration of the groups of wave-trains would be t - v where v equals the extra time required by R' to complete its local circuit. The difference in time made by the two relays, although very slight, will be found to affect very considerably the quality of the received pictures. Renewing the platinum contacts is also a great expense, as they are soon burnt out where a heavy current is passed. If the distance experimented over is short so that the power required to operate the spark-coil is not very heavy, one relay will be sufficient providing the contacts are massive enough to carry the current safely. It is useless to expect any of the ordinary relays in general use to work satisfactorily at such a high speed, and in order to compensate for this we must either increase the time of transmitting, or, as already suggested, make use of a coarser line screen in preparing the photographs.
For reasons already explained, all points of make and break should be shunted by a condenser. The effective working speed of an ordinary type of relay may be anything from 1000 to 2500 dots a minute, depending upon accuracy of design and construction.
In the wireless transmission of photographs it
is absolutely essential to use some form of rotary spark-gap, as where sparks are passed in rapid succession the ordinary type of gap is worse than useless. When a spark passes between the electrodes of an ordinary spark-gap, Fig. 12, we find that for a fraction of a second after the first spark has passed, the normally high resistance of the gap has been lowered to less than one ohm. If the column of hot gas which constitutes the spark is not instantly dispersed, but remains between the electrodes, it will provide an easy path for any further discharges, and if sparks are passed at all rapidly, what was at first a disruptive and oscillatory discharge will degenerate into a hot, non-oscillatory arc.[[4]]
Two forms of rotating spark-gaps are shown in Figs. 13 and 14, and are known as "synchronous" and "non-synchronous" gaps respectively. In the synchronous gap the cog-wheel is mounted on the shaft of the alternator, and a cog comes opposite the fixed electrode when the maximum of potential is reached in the condenser, thus ensuring a discharge at every alternation of current. With this type of gap a spark of pure tone is obtained which
Owing to the large number of sparks that are required per minute in order to transmit a photograph at even an ordinary speed, it is necessary that the contact breaker be capable of working at a very high speed indeed. The best break to use is what is known as a "mercury jet" interrupter, the frequency of the interruptions being in some cases as high as 70,000 per second. No description of these breaks will be given, as the working of them is generally well understood.
In some cases an alternator is used in place of the battery B, Fig. 4, and when this is done the break M can be dispensed with. In larger stations the coil H is replaced with a special transformer.
The writer has designed an improved relay which will respond to currents lasting only 1/100th part of a second, and capable of dealing with rather large currents in the local circuit.[[5]] This relay has not yet been tried, but if it is successful the two relays R and R' can be dispensed with, and the result will be more accurate and effective transmission.
The connections for a complete experimental station, transmitting and receiving apparatus combined, are given in Fig. 15. The terminals W, W are for connecting to the photo-telegraphic receiving apparatus Q, being a double pole two-way switch for throwing either the transmitting or receiving apparatus in circuit. There is another system of transmitting devised by Professor Korn, which employs an entirely different method from the foregoing. By using the apparatus just described, the waves generated are what are known as "damped waves," and by using these damped waves, tuning, which is so essential to good commercial working, can be made to reach a fairly high degree of efficiency.
The question of damped versus undamped waves is a somewhat burning one, and no attempt will be made here to deal with the merits or demerits of the claims made for the respective systems. A series of articles describing the production of undamped waves and their efficiency in working compared with damped waves will be found in the Wireless World, Nos. 3 and 4, 1913, and are well worth reading by any one interested in the subject.
A diagrammatic representation of the apparatus as arranged by Professor Korn is given in Fig. 16. The undamped or "continuous" waves are generated by means of a high-frequency alternator or Poulsen arc. In Fig. 16, X is the generator, F inductance, C condenser; the aerial inductance T is connected by the aerial A and earth E. By this means the waves are tuned to a certain period.
A metal print, similar to what has already been described, is wrapped round the drum D of the machine, and when the stylus Z traces over an insulating strip the waves generated are in tune with the receiving station, but when it traces over a conducting strip, a portion of the inductance T is short-circuited, the period of the oscillations is altered, and the two stations are thrown out of tune.
The receiving station is provided with an aperiodic circuit, which consists of an inductance F', condenser C', and a thermodetector N. A string galvanometer H (described in Chapter III.), and the self-induction coils B, B' are connected as shown, the coils B, B' preventing the high-frequency currents, which change their direction, from flowing through the galvanometer. The manner in which the string galvanometer is arranged to reproduce a transmitted picture is shown in Fig. 24.
The connections adopted by the Poulsen Company for photographically recording wireless messages are given in Fig. 17, a string galvanometer of the Einthoven type being used. The two self-induction coils S and S' are in circuit with the detector D and the galvanometer G. The condenser C' prevents the continuous current produced by the detector from flowing through the high frequency circuit; P is the primary of the aerial
inductance and F the secondary. The method of transmitting adopted by Professor Korn appears to be a simple and reliable arrangement, provided that an equally reliable method of producing the undamped waves can be found. Owing to the absence of mechanical inertia it should be capable of working at a good speed, while the absence of a number of pieces of delicate apparatus all requiring careful adjustment add greatly to its reliability.
In any spark system with a properly designed aerial a coil taking ten amperes is capable of transmitting signals over a distance of thirty to fifty miles, but where the number of interruptions of the break required per second is very high, as in radio-photography, it must be remembered that a much higher voltage is needed to drive the requisite amount of current through the primary winding of the coil than would be the case if the interruptions were slower. It is possible to use platinum
contacts for the relays, for currents up to ten amperes, but for heavier currents than this some arrangement where contact is made with mercury will be found to be more economical and reliable.
In the transmitter already described and given in Fig. 11, the best results would be obtained by finding the speed at which the relay R' works best, and regulating the number of contacts made by the stylus accordingly.
The method employed by De' Bernochi (see Chapter I.) of varying the intensity of a beam of light by passing it through a photographic film, which in turn alters the resistance of a selenium cell, has been very successfully employed in at least one system of photo-telegraphy. Its application has also been suggested for wireless transmission, and although with any system using continuous waves this would not be very difficult, it could hardly be adapted to work with the ordinary spark system. The apparatus for receiving from this type of transmitter would, on the other hand, necessarily be more elaborate than the methods that are described in the next chapter, and as far as the writer's experience goes, experiments along these lines would not prove very profitable, as simplicity is the keynote of success in any radio-photographic system.
It has been suggested that in order to decrease the time of transmission a cylinder capable of
taking a print 7 inches by 5 inches be employed, the print being prepared from rather a coarse line screen—say 35 to the inch—and a traverse of about 1/50 inch given to the stylus, thus reducing the time of transmission to about twelve minutes. It is questionable, however, whether the increase in speed would compensate for the loss of detail, as only very bold subjects could be transmitted. As already pointed out, wireless transmission would only be employed for fairly long distances, and the extra time and expense required to receive a fairly good detailed picture is negligible when compared with the enormous time it would take to receive the original photograph by any ordinary means of transit.
The public much prefer to have passable pictorial illustrations of current events than wait several days for a more perfect picture—the original, and the advantage of any newspaper being able to publish photographs several days before its rivals is obvious. There can also be no doubt but that a system of radio-photography, if fairly reliable and capable of working over a distance of say thirty miles, would be of great military use for transmitting maps and written matter with a great saving of time and even life. Written matter could be transmitted with even greater safety than messages which are sent in the ordinary way in Morse Code, as the signals received in the receiver
of an hostile installation would be but a meaningless jumble of sounds, and even were they possessed of radio-photographic apparatus the received message would be unintelligible, unless they knew the exact speed at which the machines were running and could synchronise accurately.