Vision by Radio

C. FRANCIS JENKINS

The earliest attempts to send pictures and to see electrically date back some fifty years, being practically coincident with efforts to transmit sound electrically.

At first a metallic circuit was employed to carry the impulses representing picture values, but when radio was available several workers immediately began the adaptation of their apparatus to radio circuits.

Some remarkably fine examples of pictures transmitted by both wire and radio have been produced in recent months; most of them showing the lines, but some of them without lines at all,i. e., true photographic results.

And as the transmission of images from living subjects in action differs from “still” pictures only in that they are more rapidly formed, it naturally followed that the solution of this problem should also be undertaken.

When radio service to the eye shall have a comparable development with radio service to the ear, a new era will indeed have been ushered in, when distance will no longer prevent our seeing our friend as easily as we hear him.

Our President may then look on the face of the King of England as he talks with him; or upon the countenance of the President of France when exchanging assurances of mutual esteem.

The general staff of our Navy and Army may see at headquarters all that a lens looks upon as it is carried aloft in a scouting airplane over battle front or fleet maneuvers.

And from our easy chairs by the fireside, we stay-at-homes can watch the earth below as a great ship, like the Shenandoah, carries our flag and a broadcasting lens, over the mountains and plains, the cities and farms, the lakes and forests, of our wonderful country.

In due course, then, folks in California and in Maine, and all the way between, will be able to see the inaugural ceremonies of their President, in Washington; the Army and Navy football games at Franklin Field, Philadelphia; and the struggle for supremacy in our national sport, baseball.

The new machine will come to the fireside as a fascinating teacher and entertainer, without language, literacy, or age limitation; a visitor to the old homestead with photoplays, the opera, and a direct vision of world activities, without the hindrance of muddy roads or snow blockades, making farm life still more attractive to the clever country-bred boys and girls.

Already audible radio is rapidly changing our social order; those who may now listen to a great man or woman are numbered in the millions. Our President recently talked to practically the whole citizenship of the United States at the same time.

When to this audible radio we add visible radio, we may both hear and see great events; inaugural ceremonies, a football, polo, or baseball game; a regatta, mardi gras, flower festival, or baby parade; and an entire opera in both action and music.

Educationally, the extension worker in our great universities may then illustrate his lecture, for the distant student can see as well as hear him by radio.

It is not a visionary, or even a very difficult thing to do; speech and music are carried by radio, and sight can just as easily be so carried.

To get music by radio, a microphone converts sound into electrical modulation, which, carried by radio to distant places, is then changed back into sound and we hear the music.

To get pictures by radio, a sensitive cell converts light into electrical current, and at radio distances changes these currents back into light values, and one may see the distant scene; for light is the thing of which pictures are made, as music is made of sound.

To further show the close relation, it might be added that in receiving sets these same electrical values can be put back either into sound with headphones or into light with a radio camera; although it may be admitted that such radio signals do not make much sense when with headphones one listens to the pictures.

Already radio vision is a laboratory demonstration, and while it is not yet finished and ready for general public introduction, it soon will be, for it should be borne in mind that animated pictures differ from still pictures only in the speed of presentation, and the sending of “still” pictures by radio is now an accomplished fact, radio photographs of no mean quality, examples of which appear as illustrations in this volume.

Just as is done in radio photographs the picture surface is traversed by a small spot of light moving over the picture surface in successive parallel adjacent lines, with the value of the lines changed by the incoming radio signals to conform to a given order, the order being controlled by the light values of the scene at the distant sending station.

In sending pictures electrically, there have been but two methods employed, perhaps the only methods possible; namely (a) a cylinder mechanism; and (b) a flat surface.

Without exception, every scheme which had attained any degree of success, before the author adopted flat surfaces, has depended upon synchronous rotation of two cylinders, one at the sending station with the picture thereon to be sent; and the other at the receiving station where the picture is to be put.

Perhaps the very obviousness of the cylinder scheme, and that there are no patents to prevent, explains why it has been employed by so many. And there have been many workers in this line of endeavor; for example, in England, Lord Northcliff, Sir Thompson, Mr. Evans and Mr. Baker; in France, MM. Armengaud, Ruhmer, Rignoux, Fournier, and Belin; in Germany, Paul Nipkow, Dr. Anchutz, and Dr. Korn.

In America, Mr. Ballard, Mr. Brown, and Mr. Amstutz, the latter deserving particular mention, for, from a distant picture, a swelled gelatine print, he engraved a printing plate which could be put directly on a printing press for reproduction.

All these many workers have adopted the cylinder method of sending and receiving, and all have arrived at approximately the final stage of development permitted by concurrent science.

It may be well to explain that, in these older schemes, the picture to be sent is wrapped around the cylinder, usually a cylinder of glass where light sensitive cells are employed, mounted on a rotating shaft, which also has longitudinal displacement.

The light values which make up the picture are converted into electric current of correspondingvalues and put upon a wire or other channel which delivers them to the distant receiving station.

At the receiving station a suitable film-like sheet (paper, for example) is wrapped around a cylinder similar to that at the sending station. As this cylinder is rotated and longitudinally advanced under a stationary point in contact with the paper on the cylinder, a spiral is traced thereon. As the incoming electrical current represents picture values, and as the two cylinders are turning in exact synchronism, a picture duplicate of that at the sending station appears thereon. After the picture is completed the paper sheet can then be taken off the cylinder and flattened out for such use as may be desired.

It is quite obvious that vision by radio and radio movies can never be attained by a cylinder method, for as the picture must appear to the eye complete, by reason of persistence of vision, it naturally follows that the eye must make up the whole picture from a single focal plane.

The attainment of “television” or Radio Vision, as it is now coming more commonly to be called, requires that the sending shall be from a flat plane, and reception on a flat plane, and a modulation which will give not only the high-lights and shadows but the halftones as well.

These “flat planes” may, of course, be the focal planes of the lenses employed at the receiving station, and from the focal depth of the lens at the sending station where the picture may perhaps be taken from living actors in the studio or from an outdoor scene.

At the receiving station the “flat surface” may be a photographic plate, a white wall, or a miniature of the usual “silver sheet” of the motion picture theatre.

It may aid in a clearer and quicker understandingof the text if the words telephone and television be limited to metallic circuit service, while radio phone and radio vision is applied to radio carried signals, and this designation will be employed in the following pages.

[Projector]

[Photographs]

This and succeeding pages are examples of photographs received by radio from a distance, by the Jenkins system, some of them from Washington to Philadelphia, and represent the best work done in 1922, 1923, and 1924.

[Photographs]

INITIAL ACTIVITIES:

The author’s work began with the publication in theMotion Picture News, of October 4, 1913, of an article entitled “Motion Pictures by Wireless.” This contemplated the employment of a flat receiving surface, but in the light of subsequent experience the scheme proposed therein is believed to be impractical. It did, however, provoke discussion of the subject and initiated the work which was thereafter rather continuously prosecuted, except for interruption to aid in the great World War.

After failure to find a practical, workable mechanism made up of devices already in use in applied science, diligent effort was made to discover the necessary, missing part.

At length a device described as a prismatic ring was developed, a new contribution to optical science. In use it is comparable to a solid glass prism which changes the angle between its sides, giving to a beam of light passing therethrough a hinged or oscillating action on one side of the prism while maintaining a fixed axis of the beam on the other side of the prism.

As a convenience in fabrication this prismatic ring is ground into the face of a glass disc of suitable size, of selected mirror plate, which gives the ring its own support on the rotating shaft upon which it is mounted.

Success in sending pictures by radio from flat photographs and receiving them on flat photo negativeplates (and subsequently of radio vision), really began with the perfection of automatic machines for the making of these prismatic rings, for by means of these prisms and a light sensitive cell at the sending station the light values which make up the picture are converted into electrical values, and broadcast.

So to put this picture on a radio carrier wave we simply slice up the picture (figuratively) into slices one-hundredth of an inch in width, in the best pictures, by sweeping the picture across the light sensitive cell by means of these rotating prismatic rings. With each downward sweep the picture is moved one-hundredth of an inch to the right until the whole picture has crossed the cell, the cell converting the light strengths of the different parts of each such slice into corresponding electrical values.

The process very much resembles a bacon slicer in the market, each slice showing fat and lean. Similarly these imaginary slices of our picture show light and dark parts, and these lights and shadows moving across the sensitive cell produce corresponding strength of electric current, modulating the radio carrier wave of the broadcasting set accordingly.

Further, of course, it is immaterial whether the current modulation is taken directly from a flat photograph, from a solid object, or from an outdoor scene at which the transmitter is pointed.

To put these light values back together again at the distant receiving station to make up a negative of the picture being broadcast from the sending station, it is only necessary to reverse the process; first, with a point of light to draw lines across a photographic plate, which the rotating prismaticrings do; and, second, to vary the density of the different parts of the successive lines corresponding to lights and shadows of the picture at the sending station, and this the varying strength of the incoming radio signal does by varying the intensity of the light.

Dense areas in the negative are built up where the light is successively very bright at the same place in adjacent lines; halftones where the light is less intense; while where the light is very faint, little or no exposure occurs, and shadows will result.

It is thus the lights and shadows which make the picture are built up, line by line, for when this negative is developed, and paper prints made therefrom, the dense areas produce high-lights in the picture; the less dense areas the halftones; and the thin areas the shadows of the picture, person or scene broadcast at the sending station. It is simply that a photographic negative has been made of what the lens at the sending station is looking at.

So, then, to receive pictures by radio, it is only necessary (1) to cover a photographic plate in parallel adjacent lines, and (2) to vary the density of the lines, to build up the shadows, the halftones, and the high-lights of the picture.

If one puts a nickel under a piece of paper and drawsstraightlines across it with a dull pencil, a picture of the Indian appears. And that is exactly the way photographs by radio are received, except that a photographic plate is used instead of a piece of white paper, and a pencil of light instead of the pencil of lead, the light pencil changing the exposure in various parts of the successive adjacent parallel lines by reason of the variation of the incoming radio signals.

The scheme is just a long camera with miles insteadof inches between lens and plate. For example, the lens in Washington and its photographic plate in Boston; with this exception, that the one lens in Washington can put a negative on one, ten or one hundred photographic plates in as many different cities at the same time, and at distances limited only by the power of the broadcasting station, radio instead of light carrying the image from lens to plate.

The time for transmitting a picture depends upon the size of the picture and strength of light, say, from three to six minutes, using a filament lamp as a source.

The radio photograph receiving instruments are rather simple and inexpensive and, like a loudspeaker, can be attached to any standard amplifying audio-radio receiving set.

For the light source for radio photographs a filament lamp is employed, and in a single turn coil enclosed in a hydrogen atmosphere. This miniature filament coil is imaged on a photo negative plate, and the variation in the light is caused by putting the incoming radio signals through this lamp, perhaps after the filament has been brought to a red glow by a battery current. By adjusting the speed of the motor to the temperature change of this filament soft gradations of light and shade are obtained which probably can never be equaled by any other device, a photograph of true photographic value, entirely free of lines.

The author wishes to take this occasion to express his appreciation of the splendid assistance of the General Electric Company, under the personal supervision and hearty cooperation of Mr. L. C. Porter, who from the very first has shown his confidencein the ultimate successful conclusion of this development.

For the high speed radio photograms, where only blacks and whites are needed, a corona glow lamp of very high frequency has been developed. This lamp is lighted by the plate current of the last tube of the amplifier; and as the lamp can be lighted and extinguished a million times a second, it is obvious that the permissible speed is almost limitless, and a thousand words per minute is believed ultimately possible.

This lamp has been developed for the author by Professor D. McFarlan Moore, an expert in lamps incorporating this phenomena, and who some years ago, it may be remembered, produced a lamp of this type more than two hundred feet long. It is probably safe to predict that no other lamp will ever be able to compete in speed.

As photography is the quickest means of copying anything; and radio the swiftest in travel, it seemed logical that the two hitched together should constitute the most rapid means of communication possible.

Of course, the sending machine and the receiving machines must run in exact synchronism. This synchronous control of the sending and receiving motors is maintained by the vibration of a rather heavy fork at each station, and adjusted to beat together, with such slight automatic correction by radio as may be required to keep all receiving forks in step with the fork of the station which at the moment is sending. It is a very simple and dependablemechanism, by which any number of motors, of any size, separated by any distance, can be made to run in synchronism.

Another scheme of the rotary type, perhaps even better adapted to the distant control of large motors, is a small synchronous radio motor driven by power carried by radio from the broadcasting station to the receiving stations. It is, of course, rotated partly by radio power from the distant station, and partly by local current, just as a loudspeaker is operated. These small motors, rotating in synchronism with the motor at the sending station, control the rotation of a larger motor in each receiving camera, and so all stations keep in step.

Of course, it would be fatal if it were necessary to wait until the picture was developed before it could be discovered that the receiving camera was getting out of control. So a special “neon” lamp is located to shine on a revolving marker on the motor shaft of the receiving instrument, and flashed by the incoming radio signals, which latter bear a definite relation to the rotation of the sending station motor.

It should be noted that the same radio wave carries both the picture frequency which builds up the photograph and the synchronism frequency which controls the motors, and also that it lights the stroboscopic lamp.

A further advance step was made when an audible message was added to the same radio wave whichcarried the picture. This is done by modulating the carrier wave to give audibility, while interrupting the same carrier wave at a frequency far above the audible range, say, two hundred thousand cycles, to make our picture.

By means of this duplex employment of the same radio wave, it is possible to get, for example, both the gesture and the voice of an inaugural address; the play and the cheers of a national sport; or the acting and song of grand opera.

Perhaps it might be explained that synchronism in visual-audible radio reception is accomplished by the simple expedient of keeping the radio picture “framed,” exactly as this is done in the motion picture theatre.

But continuing the description of the still picture processes a little further, before taking up Radio Vision and Radio Movies, it might be added that while photographs by radio is the more interesting and impressive process, there is little doubt but that radio photo letters will be of much greater immediate service in business.

Commerce, like an army, can go forward no faster than its means of communication. The history of industrial advance in all ages shows that with every addition to communication facilities the volume of business has increased. Obviously a third electrical means of communication will enlarge business, and speed up commerce and industry.

As an aid in national defense the chief of staff of the Signal Corps of the Army, in a recently published report to the Secretary of War, said (Washington Star, November 22, 1924):

“Looking into the future of signal communication for a moment, it appears that the basic method of breaking messages up into words, words into letters,letters into dots-and-dashes, and then passing these through the wrist of an operator, as has been the practice since Morse’s fundamental invention of the electric telegraph, seems to be nearing the end of a cycle. Mechanical transmitters with higher speed qualities are becoming stabilized and American invention seems to be making further and rapid progress in associating photography with radio, which bids fair to revolutionize fundamental methods of transmission.

“The message of the future, whether it be written, printed, of mixed with diagrams and photographs, including the signature of the sender, will, it seems certain, soon be transmitted photographically by radio frequency at a rate tens of times faster than was ever possible by the dot-and-dash methods of hand transmission.

“Military messages of the future, particularly in active operations, may contain diagrams and sketches, or even entire sheets of maps, all transmitted as part of the same message and by means of which detection or listening-in will be reduced to a very low minimum.”

The author suggests that it might be added that the newcomer, the radio photogram, has merits distinctly its own, e. g.:

(1) It is autographically authentic; (2) it is photographically accurate; (3) it is potentially very rapid; (4) it is little effected by static; (5) it is not effected by storms; and (6) it is automatic and tireless.

It can also be used to enlarge the individual newspaper’s influence and prestige by the establishment of photostat branch printing plants at strategic points, like summer camps, and winter resorts, and at ridiculously little cost.

Such copies of the news, financial and market report pages of the paper could be distributed in these distant places before they could possibly appear on the streets of the home city of the paper.

Of course, produce market reports, stock market news, and similar matter could be so distributed very much quicker than could be done by any other system, certainly so to the farmer and gardener.

Radio Photographs and Radio Vision, when both are done by the flat-plate method, are identical in principle, the difference being only in the speed of the apparatus, with such modification in the apparatus as will permit of the required speed.

Just as in the Radio Photograph the picture surface of the Radio Vision is covered with a small spot of light moving over the picture surface in successive parallel lines, with the light value of the lines changed by the incoming radio signals to conform to a given order, the order being controlled by the distant scene at the sending station.

And as the whole picture surface is covered in one-twelfth to one-sixteenth of a second, persistence of vision of the human eye is sufficient to get the picture from the white receiving screen—a photographic plate is not necessary.

When the machine of Radio Vision is turned over slowly, the little spot of light on the screen which makes up the picture looks for all the world like a tiny, twinkling star as it travels across the white surface of the screen in adjacent parallel lines, changing in light value to correspond in position and intensity to the light values of the scene before the lens at the broadcasting station.

But when the machine is speeded up until the succession of lines recur with a frequency which deceives the eye into the belief that it seesallthese linesallthe time, then a picture suddenly flashes out on the white screen in all the glory of its pantomime mystery.

To accomplish this, the apparatus must be speeded up until a whole picture can be assembled on the screen, say, in one-sixteenth of a second, to be seen by the eye directly.

It was necessary to modify the Radio Photo apparatus to permit this increase in speed. So a lens disc is substituted for the fast pair of prismatic plates. Each lens draws a line while the relatively slow rotation of the prismatic plates distributes the lines over the whole picture surface, just exactly as the plates do in the Radio Photo Camera.

The Radio Vision receiving set and the Radio Movies set are identical, and one may, therefore, see in one’s home what is happening in a distant place, an inaugural parade, football, baseball, or polo game (and we call it Radio Vision); or one may see the motion picture taken from the screen of a distant theatre (and we call it Radio Movies).

The Radio Vision receiving set, as now designed, is very simple; namely, a mahogany box, or small lidded cabinet, containing, beside the radio receiving set and a loudspeaker, only a small motor rotating a pair of glass discs, and a miniature, high frequency lamp for outlining the pantomime picture on a small motion picture screen in the raised lid of the cabinet, synchronism being maintained by the simple expedient of “framing” the picture on the screen exactly as this is done in a moving picture theatre.

The author wishes to acknowledge his indebtedness to his friend, Professor D. McFarlan Moore, for a word name for this new device, i.e., “telorama” for the radio vision instrument, and “teloramaphone” for the instrument when it includes simultaneous reproduction of the music or sound accompanying the living scene.

COL. PAUL HENDERSON, OCTOBER 1, 1924. ASSISTANT POSTMASTER GENERAL, WASHINGTON, D.C. MY DEAR COL. HENDERSON:—THIS IS AN EXAMPLE OF OUR NEW RADIO-PHOTO LETTER, A METHOD OF TRANSMITTING MESSAGES BY RADIO INSTEAD OF BY STEAMSHIP. WASHINGTON TO PANAMA IN FIVE MINUTES. IT HAS THE AUTHENTIC CHARACTER OF AN AUTOGRAPHED LETTER, AND THE SPEED OF RADIO. IT IS THE BEGINNING OF A RADIO SERVICE TO THE EYE, WHERE HERETOFORE RADIO HAS BEEN AN ADDRESS TO THE EAR ONLY. WILL THE TIME SOON COME WHEN THE POST OFFICE DEPARTMENT WILL DELIVER BY RADIO PHOTOGRAPHIC COPIES OF OUR BUSINESS LETTERS AT THE SPEED OF LIGHT RATHER THAN THE LAGGARD DELIVERY OF THE ORIGINALS BY MAIL-PLANE. SUCH AN EXCHANGE OF INTELLIGENCE WOULD WONDERFULLY SPEED UP INDUSTRY BECAUSE, LIKE AN ARMY, INDUSTRY CAN GO NO FASTER THAN ITS MEANS OF COMMUNICATION. Jenkins

Maj. Mauborgne, Washington When a radio message is received as a photo copy of an autographed order, it is known to be authentic and can be obeyed at once. In war this is vital. Combined with simplicity, ruggedness and speed the radio photo deserves attention. Jenkins Oct. 20, 1924.

[Photographs]

RADIO SERVICE TO THE EYE:

Since the initiation of broadcasting, a veritable army of engineers have been devoting themselves to the development of radio as a service to the ear.

The author for several years has been, rather lonesomely, devoting his efforts to the development of radio as a service to the eye.

Incidentally it is suggested that there are undreamed of possibilities in radio in the unlimited frequencies above audibility, in which speed transmission is greatly accelerated by the tolerance of eyesight, not possible in an appeal to the ear. Witness, the motion picture theatre screen upon which a picture is taken off and put back again forty-eight times per second without discovery by the eye; while the slightest error in a note in the orchestra is detected at once and grates harshly on the ear.

Just as the motion picture depends for success on the fact that the eye is easily deceived, so in Radio Vision the eye is fooled into the belief that it sees the radio picture as a whole, though in fact the eye sees at any one moment only the tiny spot of light by which, with almost lightning like speed, the picture is made up.

Audio radio engineers have been working in the very limited audio-frequency band below, say, ten thousand cycles, whereas the workable range where light instead of sound is employed goes away up to millions of cycles. It is confidently predicted that the next great development in radio is in this area.

When the “teloramaphone” is made generally available, then pictures at the fireside sent from distant world points will be the daily source of news; the daily instructional class; and the evening’s entertainment; and equally the long day of the sick and shut-ins will be more endurable, and life in thefar places less lonely, for the flight of radio is not hindered by rain, or storm, or snow blockades.

The successful study of the problem of the transmission of light effects electrically (vision, pictures, light signals, etc.) might well begin with the division of the subject into its elements and sub-elements.

The major division is, naturally, into (a) the sending station apparatus; and (b) the receiving station apparatus. A great variety of devices have been invented for analysis of the picture at the sending station, and the translation of the light values (which make up the picture) into electrical modulation; and likewise a variety of methods for receiving these electrical signals at a distant place (or places) and there changing the electrical modulations back into light values with which the picture is built up.

Before the refinement of light sensitive cells, actual electrical contact was oftenest employed in sending the impulses which represented light gradations in the picture.

Usually, therefore, a zinc etching of a pen and ink picture was made, and curved into a cylinder. This picture cylinder was then slipped onto a rotating mandril, which was also moved axially by a screw thread on the mandril shaft; or the cylinder rotated and a contact arm moved along by the screw, like the old wax cylinder phonograph.

A delicately suspended arm, moved along by the screw as just described and carrying, instead of the phonograph sound box, a very small and smooth point which was lifted by the high parts of the zinc etched picture. When so lifted the arm makeselectrical contact with an adjustable point and current is put into the inter-station wire circuit.

By this means the values of the picture are converted into corresponding duration values of an electric current, and put onto a wire connecting the sending machine with a distant receiving machine (described in detail later in the text).

It will thus be seen that the electric impulses sent out over a wire attached to the contact point represent the value of the light and dark portions of the picture.

The electric impulses are similar to letter-code dots-and-dashes, for the picture actually opens and closes the circuit, like a telegraph key, the dark portions of the picture sending dashes and the light portions of the picture sending dots. With the point set at one end of the cylinder, and the contact arm advancing longitudinally by reason of the thread on the shaft, the point traverses a spiral around the cylinder until the whole picture is covered.

In another process a swelled gelatine picture print was used to raise and lower the contact-making arm, and a carbon contact button was employed, but otherwise the sending machine was much the same.

Somewhat later halftones of photos were available, and these were similarly bent into cylinders. The interstices between the metal points of the halftones were filled with an insulating wax, and the whole smoothed off until the bright metal points (of different size and representing the different values of the picture) were exposed.

When this picture cylinder was rotated under acontact point, the cylinder and the point being parts of an electric circuit, current flowed in the circuit whenever the point touched the metal parts of the picture, but no current flowed when the insulation passed under the point.

Because the point does not jump up and down, but has a smooth surface to ride on, greater speed and accuracy is possible with the filled-in etching.

As is quite generally known there are certain “semi-metals” which have the property of changing their resistence to an electric current when light falls thereon.

Of this group selenium is typical, although there are several others, thallium, strontium, barium, etc.

More recently it was discovered that some of the rarer alkali metals had the property, under certain conditions, of actually converting light into electric current. In this group are potassium, sodium, caesium, rubidium, etc.

These light sensitive cells vary the electric current quite accurately in proportion to the intensity of the light falling thereon, and when available were quickly seized upon by the workers in “pictures-by-electricity.”

When these light sensitive cells were employed, a modification of the previous picture-translating methods and mechanisms was made, for now a modulation instead of an interruption of the electric current was possible, the modulation representing the values of the halftones of a picture transparency as well as its blacks and whites.

The rotating cylinder now employed was of glass, around which the picture, on transparent film, was wrapped. Inside the cylinder a light was put toshine through the passing picture film as a minute point of light falling on the light sensitive cell located in a dark box.

Just as in the other cylinder schemes the picture is made to traverse this point of light until the whole picture is converted into electric current of corresponding values, which, as before, can be put on a wire, or can be made to modulate a radio wave.

One of the oddities of picture analytical translation consists of running a perforated paper strip between a source of light and a light sensitive cell, the paper ribbon perforated with a series of groups of holes.

It is intended that the number of holes in successive groups along the ribbon shall represent successive values of light in the different parts of the picture to be transmitted.

While it is possible to perforate such a ribbon it is quite likely that the experienced engineer would adopt some of the simpler forms of picture translation, for there are enough of them which may be used without hesitation, such basic patents as have ever existed having long since expired.

An unusual scheme consists in writing the message in ink made of saltpeter, and then setting fire to the ink line. The ink line of the message burns itself out leaving the paper intact. Thereupon the paper is carefully laid on the metal cylinder of the sending machine, or on “silver paper” which is put on the sending cylinder. The contact point drops through the burnt lines making contact, and the out-going signals, received on a like cylinder at a distant station, make a duplicate of the original message.

A more satisfactory scheme is to put a thin coating of hard wax on a thin sheet of metal, or metal coatedpapers. These sheets as wanted are laid on an electric hot-plate and the message, picture, or sketch, is written through the warm wax coating with a lead pencil or stylus. Then the paper with its message etched therein, is wrapped around the sending cylinder and rotated under the contact-making finger, which sends out the electrical impulses.

One may also take the sketch, line drawing, or pen picture, to the zinc etcher (halftone engraving plant), and have him make a print on very thin metal, and develop and harden it, but not etch it. This will give a photographically accurate copy. This copy on the metal sheet can then be bent around the cylinder of your sending machine, and sent out by wire or radio, to be received at all stations tuned in. If etched the etching may be filled in with hard wax and this put on the cylinder, and run under the contact finger.

It is possible to write on paper with copper sulphate (blue vitral) solution, for the acidulated line carries the current through the paper to the metal cylinder beneath, and completes the circuit. The acid may even be strong enough to eat through the paper exposing the metal cylinder underneath.

Salt water with a little glycerine to keep it from drying up too fast will also perform.

Another method which has been proposed is to print or write on paper with sticky material, like Japan drier, and sprinkle thereon a fine powdered wax, battery sealing wax, for example. This will stick to the tacky lines and can be melted over a hot plate or in an oven. The melted wax leaves standing lines which will raise a contact-closing pen passing over it. If the lines are sprinkled with metallic powder a double contact pen can be usedand the mechanism is still simpler, less delicate, and more dependable.

One of the newer methods of photogram transmission is to use a rotating table, like a talking machine table, with a rectangular piece of paper thereon (tucked under at the corners), from which to send a communication; market bulletins, for example, broadcast by a progressive newspaper to the farmers and truck gardener patrons in their vicinity.

The contact point is advanced from the outer edge to the centre by a spiral cut on the under side of the table; or by a threaded edge of a detachable tabletop and a reducing gear to move the contact arm across the message, or other scheme.

(Of course, the receiving machine should be a duplicate of the sending machine, with suitable receiving surface.)

The bulletin sheet can not be advantageously used to the very centre, any more than a music record can, but this space can be employed by the broadcaster for printed announcements (as music disc records are so used), the receiving paper being furnished by the broadcaster.

But of all the schemes it is very doubtful if any will ever equal the writing of the message or sketch in lead pencil on paper, and rotate it under a two-contact collector. The graphite of the lines makes contact across the twin-blade terminals, effecting the transmitter as would a telegraph key in the circuit.

Coming now to the design of a suitable receiving machine, it will be found that an even greater variety of schemes have been tried.

INK PEN RECEIVERS:

Upon a rotating and longitudinally moving cylinder, similar to that of the sending machine first described, a paper is put, and upon this paper, as the cylinder rotates, an ink pen, mounted on a pivoted arm, touches intermittently, being drawn down to ink the paper with every incoming electrical impulse, and lifted off the paper by a gentle spring.

In another ink and pen scheme the electric current is passed through a capillary ink tube to make it flow and black the paper; no lifting of the pen arm is necessary.

As the order of these dots-and-dashes is controlled by the impulses put into the line by the picture at the sending station, a picture is built up on the paper on the receiving machine cylinder, a copy of the picture on the cylinder of the sending machine.

In another and similar scheme a chemically treated paper is put on the cylinder, and upon this, as it rotates, a metallic point is gently pressed.

When the incoming electric current from the sending station passes through the paper under the contact point an electrolysis occurs which appears as a discoloration of the paper.

And as these discolored dots-and-dashes appear, as before, in an order controlled by the distant station, a picture is again built up on the paper, a copy of the picture at the sending station.

One of the best solutions for the purpose is made up of Iodide of Potassium one-half pound; Bromide of Potassium two pounds; Dextrine or Starch one ounce; and Distilled Water one gallon. (Use aniron contact needle). There are other solutions made of Ferricyanide, but are not so satisfactory.

Still another scheme, the simplest of mechanisms, consists of a metal disc upon which electrolytic paper is clipped. This plate is then put on the rotating table of a talking machine. A rubbing electrical contact is made with the disc, and the other wire attached to the tone arm to complete the circuit through the steel needle of the sound box.

As no groove is available to carry the arm toward the centre of the disc, a spurred-wheel is attached thereto, so as to engage the paper on the disc. The wheel can be adjusted diagonally of the tone arm to give any separation required in the convolutions of the spiral line.

One of the schemes employed, and with considerable success for its time, consisted of an engraving tool, moved up and down radially of a coated cylinder, cutting a groove of varying width in the soft coating of the cylinder.

When this coating was stripped off the cylinder, laid out flat and hardened, it was mounted on a printing block, inked, and impressions taken therefrom on a suitable printing press.

But doubtless photographic paper, wrapped around the cylinder, has been used oftener than any other medium. With photo paper or film a point of light is usually employed to expose the film.

The point sources of light used and methods of modulation have been almost as varied as the temperaments of the several workers. One of the first was the employment of a steady light source which, reflected in the tiny mirror of an oscillograph,is caused to vibrate at a high frequency across a minute aperture which in turn is imaged on the film on the cylinder. As the amplitude of vibration of the mirror determines the amount of light passing through the aperture and falling on the film, it will readily be understood that the strength of the incoming electric signals, representing light values of the picture at the distant station, reproduce duplicate values on the exposed film. When this film is developed a copy of the picture on the cylinder of the sending machine is obtained.

Another scheme for modulating the light falling on the film on the cylinder, consists in placing a light wedge against the face of a lens which images the vibrating mirror (or light source) on the film.

As the light is constantly imaged on the film by the lens its slight displacement toward the dark end of the light wedge by the vibration of the mirror decreases the strength of the light falling on the film, while displacement toward the thin edge of the light wedge gives greater exposure on the film.

It is obvious, therefore, that the vibration of the mirror determines the exposure at successive positions on the film; and as these displacements follow the varying strength of the incoming electric current, and the latter in turn is determined by the light values of the picture at the sending station, it naturally follows that when the film is developed a duplicate of the distant picture results.

Another method of varying the light falling on the photo film on the cylinder consists in mounting two very minute overlapping shutters one on eachof the two wires of an electric circuit suspended in a strong magnetic field. On these overlapping shutters a light source is focused, so that greater or lesser displacement of the shutters, by reason of varying strengths of current in the adjacent runs of the wire, allows more or less light to pass there between.

Another lens images these tiny shutters onto the film covered cylinder, so that when the shutters are opened by the incoming currents in the two wires, the light is concentrated on the film.

As the exposure depends on (a) the shutter openings, and the shutter opening on (b) the incoming current strength, and the incoming current strength on (c) the light values at the sending station, development of the film again gives a duplicate of the picture at the sending station.

An interesting scheme of picture reception is known as the pneumatic light valve, the vibration of which causes a shadow band to oscillate across a lens opening into the camera.

In a circular center opening in a magnetized iron diaphragm is suspended an iron disc somewhat smaller than the opening. This small disc is magnetically held by its edge to the inside edge of the opening in the diaphragm, with its plane in the plane of the magnetic field of the diaphragm.

Upon the disc is mounted a tiny mirror, and as the suspension of the disc is in the magnetic field held there by the strength of the field itself, it is extremely easily disturbed, so that a small beam of light reflected from the mirror can be vibrated with a very little current through great amplitude.

As the beam of light has a transverse shadow band therein of a width to normally close the lensopening into the camera, the varying amplification of the vibration of the mirror, and therefore, of the shadow, admits a proportional amount of light.

One of the very simplest light sources for exposing the film on the receiving cylinder consists of a minute spark-gap located in contact with the moving film.

The strength of the incoming current charges a small condenser until the gap breaks down and the passing spark exposes the film (or perhaps perforates it). If the current is strong the sparks pass the gap at a high frequency, while if the current is weak the frequency is less. The range may be from 500 to 5,000 per second perhaps, depending on the current strength, and, of course, the film exposure correspondingly varies, and the different degrees of density of the picture results.

This scheme requires about as small current as is likely to be practical, perhaps, especially when the spark is in a suitable degree of vacuum, and, of course, the incoming radio signals require correspondingly small amplification.

The direct source of light which in the author’s laboratory has produced the most perfect photographic effects, i. e., photographs absolutely without lines, consists of a lamp about an inch in diameter and two inches long, fitted with a standard screw base. The tube contains a .6 mil filament with a small single turn coil in a hydrogen atmosphere.

The coil is offset until it almost touches the glass wall. Such location of the coiled filament permits the effective placing of a minute aperture in veryclose relation to the filament; whereas an aperture on the outside of a bulb with the light source in the centre of the bulb acts like a pin-hole camera, and sharpness of image is practically impossible (unless a lens is used).

The lamp described above will respond to fluctuations in current well above a thousand times per second, but requires voltages about four times normal. It was made for the author by courtesy of the General Electric Company, under the direction of Mr. L. C. Porter, of Harrison, N. J.

But the lamp that really elicits the author’s unqualified admiration is the corona glow lamp made for the author by Professor D. McFarlan Moore. It consists of a small glass bulb containing a neat-fitting metallic cylinder, one terminal of an attached circuit. Inside and concentric with the cylinder is a second cylindrical capsule made of a solid rod drilled to a predetermined depth.

The electron stream from the outside cylinder to the capsule naturally takes the long path, and as the longest path is down into the capsule, the result is that the small central opening of the capsule glows with great intensity while the other parts of the lamp remain dark. This lamp, Professor Moore advises, has a light and dark frequency of a million per second.

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