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TELEVISION

From the collection of the

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Prelinger

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JJibrary

San Francisco, California
2006

PRESENT METHODS or PICTURE TftAMswsstoM

H. HORTON SHELDON, PH.D.

Chairman of the Department of Physics, Washington Square

College, New York University; Science Editor,

New York Herald Tribune

AND

EDGAR NORMAN GRISEWOOD, M.A.

Instructor of Physics, New York University

1929

D. VAN NOSTRAND COMPANY, INC.

NEW YORK

Printed in U. S. A.

PRESS OF

BRAUNWORTH & CO., INC.

BOOK MANUFACTURERS

BROOKLYN, N. Y.

PREFACE

IN preparing a book on Television one is confronted
with the possibility of a revision before the book is off the
press. The kaleidoscopic manner in which advances are
being made is, of course, responsible for this.

Nevertheless, since the authors feel that for the good
of the subject a review of the field is at the moment impera-
tive, they have undertaken this difficult task. At the outset,
and during the preparation of the manuscript, they have
seen clearly the reason why no one else in this country has
ventured such a book to date. They have rushed in where
angels fear to tread in spite of the condemnation which is
likely to fall upon their heads. The mass of material which
could be included in a book of this kind is enormous and
we have been forced to use our best judgment as to what
to leave out. There will be many to tell us that we have
omitted material which ought to have been included and
included other material which might well have been excluded.
Our argument is that it depends upon the point of view.
This, of course, necessitates an explanation of our own view-
point. We have attempted to give a true picture of the
state of television to-day and to give the reader the neces-
sary background to enable him to begin a special study of
any one feature. Our style, we hope, is also of a sufficiently
popular nature to enable the layman, or the possible investor,
to gain a proper perspective of the subject. The book is in
no sense a compendium. We have tried to give credit where
credit is due and to subdue the blatant claims of pretenders
who fill the daily press with extravagant statements. If in

iii

iv PREFACE

any case we have done an injustice, it has not been in-
tentional.

We have finished the book without knowing what the
future of television is to be. Technical difficulties make us
somewhat skeptical yet past accomplishments make us, per-
haps, over optimistic. We believe, we have fairly curbed
our desires to prophesy considering the excellent opportunity
afforded by the nature of the subject.

We hope that the book will be found useful and that
we may later be able to improve it and keep it up-to-date.
Suggestions from readers will be greatly appreciated.

We have had much helpful cooperation in its prepara-
tion from individuals and corporations. Without them its
writing would have been impossible. We are particularly
indebted to the following to whom we extend our sincere
thanks: J. L. Baird, of London; Professor Arthur Korn, of
Berlin; Jenkins Laboratories, Washington, D. C. ; and also
to the following corporations: Bell Telephone Laboratories,
Case Research Laboratory, Inc., General Electric Co., Radio
Corporation of America, Raytheon Manufacturing Co.,
Westinghouse Electric and Manufacturing Co.; and to the
Institute of Radio Engineers.

With these few words on behalf of what is to follow;
we leave the book to your tender mercies.

H. H. SHELDON
E. N. GRISEWOOD

CONTENTS

CHAPTER PAGE

INTRODUCTION . . . . . . . vii

I. ESSENTIAL ELEMENTS OF TELEVISION AND PICTURE

TRANSMISSION . . . . . . . .1

II. HISTORICAL BACKGROUND OF THE DEVELOPMENT OF

TELEVISION . . •. .' . . . . . . 5

III. OFFICIAL SYSTEMS AND THE EYE . . . . 19

IV. ELECTROMAGNETIC WAVES . . . . . .31

V. THE SELENIUM CELL . . . . . . . . 39

VI. THE PHOTOELECTRIC CELL . . . . r. . 46

VII. GLOW LAMPS . . . ... . . . -59

VIII. OSCILLOGRAPHS . . . . . . . . 71

IX. SCANNING . . . . * . . . . 81

X. SYNCHRONIZATION . . . ' .. . . . . 91

XI. TELEPHOTOGRAPHY .» . »". *. 101

XII. THE BAIRD TELEVISOR . . . . . .-' .118

XIII. THE BELL SYSTEM . . . ... . . . 132

XIV. THE JENKINS SYSTEM . . . ... . . 159

XV. ALEXANDERSON SYSTEM . 166

XVI. RELAYS . 175

XVII. AMATEUR EQUIPMENT 179

XVIII. THE FUTURE OF TELEVISION 185

INDEX 191

INTRODUCTION

Who wants television? Will television serve any useful
purpose, become popular as a source of entertainment; or
will it, having been perfected, serve a very narrow field?

Hardly has the talking movie made its appearance in al-
most every theatre, than we are told that it is a passing
fancy, that it will soon complete its run. This is not a novel
statement; we were told the same thing about the auto-
mobile, the airplane and the radio; but they are still here.

Television is not like collecting postage stamps, an avoca-
tion which may appeal to a few or be in vogue for a few
years and then subside. It is not like police dogs or short
skirts which are for the moment fashionable. It is the real-
ization of a desire that has existed in man from his earliest
beginnings. The automobile, giving man the speed he has
always desired, has stayed. The airplane, giving him the
wings he has long sought, has stayed. Television, giving
him the distant sight he has longed for ever since, in his
ape-like form, he climbed to the topmost, swaying branches
of the tallest tree to look afar, is here to stay.

Seeing and hearing things at a distance is as natural a
desire as eating. The Indian put his ear to the ground to
hear the approach of his enemies so as to be prepared.
Lookouts from high hills were common. A great advance
in vision was made by the invention of the telescope. This
has been greatly improved up to the present time; but still
we are not satisfied. We want to see more and further.

The modern newspaper is, after all, nothing more than
an instrument to enable us to see and hear more and further.

vii

viii INTRODUCTION

We can visualize from its printed pages what has happened:
but would we read the account as written in the newspaper,
had we the time and facilities to be present at the event?
Television brings us somewhat nearer to a perfect realiza-
tion of an event; wherefore its stay will be permanent.

A stadium may seat from seventy to one-hundred thou-
sand people, an auditorium is large if it seats four-thousand,
a theatre is built to accommodate considerably less than the
last named figure. Why? A theatre is built so that all
may hear the spoken word, the auditorium is used by a
symphony orchestra which may be heard at greater distances
than the human voice and so may be housed in a larger
building. One comes to a stadium, however, to see; hence
it may be made very much larger. The attendance at
stadiums is a good guarantee of the success of television.
The advantage lies all with the latter when it is coupled
with radio so that both eye and ear may be satisfied.

There are three distinct phases of any message delivered
orally. There is the spoken word which may be written or
printed on paper. The statement that such words may have
several meanings requires no elaboration. Lawyers fre-
quently take advantage of this to convey one idea to a jury;
whereas a totally different conception might be gleaned from
the statement appearing on the records. However, if we
know the tone of voice, the accent, the pauses in such a state-
ment, would we know the correct story? Thus authors of
fiction write, "'Villian!' he hissed," for the purpose of
conveying the idea of venom, even though you cannot hiss
such a word if you tried. Or turning to the heroine, we
might find, " 'John,' she sobbed." Though it is difficult
to sob such a word, we get the idea. This helps, but for
fullest realization we must see the expression on the
heroine's face as well as hear how she sobbed the word.
At this stage we double or triple the amount of information
the single word "John" can convey. It is obvious, then,

INTRODUCTION ix

that for full comprehension television is essential. Up to
this point we can only fill in the details from past experience;
a fact which accounts for our greater enjoyment of a radio
play when it is one that we have seen as compared to one
that we have not seen.

Thinking of television in its broadest sense, that is in-
cluding the transmission of pictures by wire, there is even
here an advantage over the usual telegraph message. Where
advertisements are to be duplicated in various cities, the
usual telegraph message is not sufficient; only a picturegram
can convey the exact form. Where one wishes to send a
message in, let us say, Sanskrit, whose alphabet is not
familiar to the telegraph operator, the advantage of being
able to send a picture of one's handwriting is apparent. Such
service has already become a part of the regular routine of
our large telegraph companies. That it is destined to sup-
plant the usual service with which we are familiar, may not
at the moment be obvious. When one realizes, however,
it is now possible to transmit a picture by wire in less than
one minute, in the laboratory; there can be little doubt that
putting such speed into commercial practice will greatly in-
crease the number of words it is possible to send over a
telegraph line in any given time. The tedious ticking off of
the individual letters for each word will eventually appear
as antiquated as an ox-cart alongside a powerful, electric
locomotive.

If one makes an inventory, one will soon discover that
there are a number of forms of entertainment from which
he is barred without television in conjunction with his radio.
Dancing, which forms such a large part of any musical
revue, is entirely absent. This was realized when one broad-
casting station tried to send over the air a series of Broad-
way musical shows, direct from the stage. We feel sure
that no one will deny this was a failure. Without the stage
setting, the display of color and dancing, it was lifeless.

x INTRODUCTION

We have long had movies without sound; today, they
are being combined. In radio we now have sound without
movies; the condition is rapidly being remedied. When
this page reaches you, the combination may have been
effected in your home; as it has been, for some time, in the
laboratory.

The radio expert or amateur who says he is not in-
terested in the development of television might as well say
he is not interested in the three electrode vacuum tube. Both
are part of his business.

In this book we have attempted to bring together, in a
manner that can be understood by all, a summary of the
achievements in the field of television together with a
description of accessory equipment. We believe, that at the
time of going to press, it presented fairly the accomplish-
ments to date. We realize that the next few years will
bring a multitude of changes that may at times leave us
somewhat behind. It is our hope, however, that public
acceptance of this book will make it possible, through fre-
quent revisions, to keep in step with the parade. We invite
our readers to assist us with suggestions.

TELEVISION

CHAPTER I

ESSENTIAL ELEMENTS OF TELEVISION AND PICTURE
TRANSMISSION

TELEVISION and picture transmission both require that
the light and shade of an object or of a picture be translated
into varying electrical impulses. These are transmitted by
wire or radio to their destination; where they are changed
from the electrical form back into the original light and
shade, either to be viewed directly or to be recreated into a
picture. To do this there are certain fundamental steps
which are unavoidable and which differ only in their method
of application. When we fully understand these, we have
mastered the entire problem.

All modern television systems require, first of all,
illumination of the object in order that the light and shade
used to produce the picture may be present. In the early
Baird experiments, in which the illumination covered the
entire subject at one time, the heat and glare were so great
that only dummies could be used. Increasingly sensitive
apparatus now makes it necessary to use illumination no
greater than strong sunlight. Most systems, however, do
not light up the entire object at once; but use a narrow beam
which rapidly traverses the object along successive strips.

Thus, if we were to place a series of parallel fine wires
before the object, the light would follow the length of each
of these successively. In effect this same thing happens

2 TELEVISION

when the entire object is illuminated; for, although the light
itself is unchanged, the successive stripes are viewed in the
same way by a moving lens system. We may say then,
without reservation, that present-day scanning, as this
process is called, is accomplished by viewing successive por-
tions of an object along a straight line and viewing the
consecutive lines in some convenient manner. This is not
at all unlike reading the words on this page along any line
and then reading the lines one after the other. In this
manner the entire page is covered. In an analogous way
every portion of the surface of this page would be covered
in order, if being scanned for television transmission.

The lights and shadows, or variations of light, which
are noticed as we pass over the picture in this or some
similar fashion, must next be changed into corresponding
variations of electrical current. This has presented the
greatest difficulty in the development of television apparatus.
In the first experiments, which promised success, the
selenium cell was used. Such cells, to be described later,
have the peculiarity of decreasing their resistance to the
flow of electrical current, in proportion to the intensity of
illumination. They are, however, temperamental devices
having a fatigue effect which causes them to fall off in
effectiveness after a short time of continuous use. To
recuperate, they must be left in the dark. Also the change
in resistance does not coincide with the illumination changes
but follows somewhat later. This peculiarity is suitably
known as a "lag." For these reasons, the selenium cell did
not satisfy the requirements in its original form, although
there are indications that these faults may yet be overcome.

Television was really born with the advent of the photo-
electric cell. This cell consists of two electrodes, one of
which is coated with an oxide or a hydride of an alkali metal
— for example, caesium oxide or potassium hydride. Such
coatings, when illuminated, give off copious supplies of elec-

ESSENTIAL ELEMENTS OF TELEVISION 3

trons, the number of which is a direct function of the light
intensity for any given color. These cells have no apparent
lag nor fatigue although their sensitivity is not yet all that
could be desired. When inserted as part of a circuit they
give an apparent change in resistance under illumination;
although the effect should not be interpreted as a strict
resistance change.

U SENDING STATION

(Courtesy of Bell Telephone Laboratories.)
FlG. i. — A typical television system.

The variations in current given by either of the above
devices are extremely small and must be greatly magnified
to be transmitted over any distance. This is accomplished
by vacuum tube amplifiers, particularly well constructed to
avoid distortion. The amplified output may be sent over
wires or on modified carrier waves, as is done in radio trans-
mission. In the former case it is received as changing cur-
rent,— in the latter, by the usual radio mechanism; so that
if put through a loud speaker it would produce actual chang-
ing sounds. The picture would be "heard," so to speak.

In either case the next stage in the procedure would be
to reconvert the electrical variations into light variations
similar to the original. In the first types of apparatus this
was done by again amplifying the currents at the receiving
end and causing them to actuate an electromagnet. The

4 TELEVISION

latter would vary a slit width or in some other manner
control the amount of light falling upon a sensitive film on
which the picture was to be produced. This method was
adapted only to picture transmission. There was no hint
of television; although if it had been physically possible to
have carried on the operation with sufficient speed the pic-
ture could have been viewed through a translucent screen.

To procure a speed sufficient for television, it is neces-
sary to obtain some method of reproduction free from the
inertia which accompanies a moving mass; a difficulty to
which all of the mechanical schemes were subject. The
introduction of the neon glow-lamp solved this problem.
This lamp glows under a high voltage placed across its
terminals and has so little lag that it can easily follow thou-
sands of fluctuations per second.

It is, of course, apparent that the lines drawn by such
fluctuating lights must always be exactly in step with the
scanning operation, at the sending end. If there is a slight
lag of the one behind the other, the picture will be askew.
This synchronization of the scanning with the reproduction
on the receiving drum, or in the receiving frame, has been
one of the principal experimental difficulties in television.

For the transmission of a moving object, if the image
is to appear continuous, the original must be scanned com-
pletely at least ten, preferably sixteen, times per second.
This likewise is a difficult technical problem.

Here we have the important features both of picture
transmission and of television. The means of accomplishing
each particular step are as varied as the number of experi-
menters. The purpose of the following chapters will be to
consider some of the best known methods in greater detail
and to discuss the accessory apparatus.

CHAPTER II

HISTORICAL BACKGROUND OF THE DEVELOPMENT OF
TELEVISION

ALTHOUGH television is still in its infancy, perhaps not
yet well out of the embryo, we must go back over eighty
years to find the earliest apparatus for picture transmission
by electric currents. Nor should we feel that this early
work was unimportant in the development which has made
possible the systems in use today.

In 1847 Bakewell designed a method for the transmis-
sion of writing or sketches over wires; a system not very
different from one suggested some four years earlier by
Bain, but not developed by him at that time. The apparatus
is illustrated diagrammatically in Fig. 2.

FIG. 2. — Bakewell system.

The metal cylinders (i) and (2), one at each end of a
telegraph line, are revolved synchronously. Over each
cylinder a metal stylus (Sl and S2) describes a spiral path,
similar to that of the needle in the early Edison phonograph,
which used cylindrical records. Note that our problem in
timing would be exactly the same as requiring that two such
phonographs, located at a distance from each other, and
playing the same record, should sound a given note at

6 TELEVISION

identically the same instant. This apparently could be ac-
complished by having the speed of rotation of both cylinders
identical and starting the needle (or stylus) at exactly the
same time.

If now a design be drawn with an insulating material,
such as shellac, on the sending cylinder ( i ) ; the telegraph
circuit will be broken when the stylus passes over this insula-
tion, but closed again when the stylus strikes the uncoated
metal. That is, the current in the line will be intermittent;
only flowing while stylus (Si) is passing over a portion
of its spiral path which is not covered by the design to be
transmitted. The problem then is to make a record on
cylinder 2 which will show when current is flowing in the
line as distinct from when it is not.

This may be accomplished if the receiving cylinder be
covered with a chemically prepared paper such that the
passage of an electric current through the paper will change
its color. For this purpose we might saturate a piece of
porous paper with an aqueous solution of potassium fer-
ricyanide and ammonium nitrate. In this case we will obtain
a dark blue color in portions through which the current has
passed.

Using such a paper, it is clear that if the synchronism
be exact, a negative of the drawing on the transmitter will
be traced on the receiving cylinder. That is to say, the
paper will be turned blue except when the transmitting
stylus is passing over the insulating material. Thus white
portions will correspond to the lines of the drawing or
writing to be transmitted. The received diagram would
then appear like a blueprint made from a line drawing on
tracing cloth.

The movement of each stylus actually describes a helical
path, like the thread of a screw, on the cylinder above
which it moves. When the paper is removed from the
cylinder and opened out flat, however, this path will appear

HISTORICAL BACKGROUND OF THE DEVELOPMENT 7

a b

FIG. 3. a — Shows the letter as it appears on

the sending cylinder;
b — as it is received in the Bakewell
system.

as a series of parallel lines — their spacing depending on
the lead of the stylus control. (See Fig. 3.)

As might be expected, the difficulty in obtaining syn-
chronism in the two cylinders militated against the success
of the Bakewell system. However, the telectograph used
commercially by T. Thorne Baker for transmission of
pictures between London and Paris in 1908 was funda-
mentally the same as the method described above. In the
scheme employed by
Fcrree as recently as
1924, for radio and
wire transmission of
photographs, the prin-
ciples of the early devel-
opment described can
be readily recognized.

Another system
which was tried by early
investigators, notably
the French Post-office
Engineer, Charbon-
nelle, is that of mak-
ing the depth of silver deposit on a photographic film
act as the means of varying the current in our tele-
graph circuit. In order to do this, the gelatine contain-
ing the active silver salt must be laid on a metal sheet
instead of on the usual celluloid or glass backing. On
exposure, the reduction to metallic silver occurs at those
portions where the greatest amount of light is received.
Now place the negative on a cylinder so that the metal
backing forms one terminal of the circuit and the stylus,
pressing lightly on the surface, the other. As the unreduced
silver salt and gelatine are poor conductors, whereas the
silver image is good, it is natural to expect the resistance
of the circuit to vary, being least where the stylus is over

8 TELEVISION

those portions in which the "printing-out" or deposition of
silver is a maximum.

The fluctuating current thus obtained might be em-
ployed, as in the instance previously described, to act on
a chemically prepared paper at the receiving end. Unfor-
tunately, however, experimenters using this scheme seem
to strike serious difficulties. Chief among these difficulties
is the tendency for the flow of current to follow the path
of least electrical resistance rather than the shortest
geometric path between the stylus and the metal sheet.
Hence the current in the circuit at any instant will not be
simply a function of the amount of silver deposit; as would
be necessary for an accurate reproduction.

Failing to obtain satisfactory results in transmission
directly from photographic negatives, it was not surprising
that early experimenters should turn to the half-tone or
process-screen reproductions used in newspaper work. Ex-
amination of such a picture under a magnifying glass will
reveal that it is composed of large numbers of tiny dots
of various shapes and sizes. In the light portion of the
picture these dots will be far apart; in the dark portions
so close together as to merge into one another. The general
effect produced by the ensemble will depend on how fine
grained a structure is used. A picture composed of some
seventeen-thousand dots to the square inch leaves little to
be desired; one with only four-hundred, will be barely pass-
able even when viewed at arms' length; the usual newspaper
production contains four-thousand two-hundred and twenty-
five dots per square inch.

Suppose that we should imagine our photograph placed
on a sheet of fine cross-section paper; so that each square
may be designated by a letter and number as is done in
describing a chess-board; or by two numbers, an abscissa
and ordinate, as is done in plotting charts. We then split
our picture into a number of small parts, each one of which

HISTORICAL BACKGROUND OF THE DEVELOPMENT 9

may be transmitted separately and the whole reassembled
according to the designation of the squares. Such a system
makes possible the simultaneous transmission of different
portions of a picture over separate wires; thus enabling the
entire reproduction to be made more rapidly than would
be the case if only a single wire were used. It then remains
to find some method for describing the appearance of these
squares. We might use a letter of the alphabet to indicate
a certain size and shape of dot as used in the half-tone. In

FIG. 4. — The detail of the reproduction depends upon the number of picture
elements used per unit area, (a) Made through a 6o-screen; e.g., 3600 ele-
ments per square inch, (b) through a ico-screen; e.g., 10,000 elements per
square inch, (c) through a ISO-screen; e.g., 22,500 elements per square inch.

this fashion the square might be coded and transmitted like
a written message.

There are, however, a great many objections to be over-
come. To obtain a good reproduction, a great many sizes
and shapes of dots must be used, making coding a long and
difficult process. H. G. Bartholomew and M. L. D. Mc-
Farlane, in England, have removed the necessity of a human
observer having to assign code letters to the various parts
of the picture. Their device not only automatically codes
the picture; but also perforates a tape which may be run
through the ordinary telegraph or wireless transmitter.
Here the problem is somewhat simplified by using only six
color variations from white to black to describe a given

io TELEVISION

portion of the original. This method, which is known as
the Bartlane process, will be described in more detail in
a later chapter.

L. J. Leishman also has devised a method of trans-
mitting a half-tone without the necessity of coding. The
system is not so very different from that described as un-
satisfactory when applied to the usual latent image in silver.
Here, however, the picture is first photographed through
a process screen, the function of which is to split the original
into dots of varying sizes. A positive is then made from
this negative. The positive is formed on a copper or zinc
plate covered with a mixture of gelatine and ammonium
dichromate. The dichromate is rendered insoluble by the
action of light; so that after washing, only those portions
of the copper plate corresponding to the dark parts of the
original will be covered with the gelatine and dichromate.
After heating, these portions become an excellent thin in-
sulation over the surface of the plate. The reproduction
in Leishman's system is accomplished by a stylus actuated
by electromagnets. Both transmitter and receiver employ
cylinders above which a needle moves much as described
in the Bakewell method. The reproduction, however, may
be made by the mechanical movement of the recording
needle without the medium of a chemically treated paper.
One way in which this can be done is to allow the stylus
to strike a carbon sheet placed over the paper on which
the drawing or photograph is to be received.

The work of Edouard Belin, a French inventor, illus-
trates still another treatment of the problem, which has
been successfully used by not a few experimenters. He
made use of a picture formed in relief on the sending
cylinder. The displacement of a needle passing in a close
spiral path over this irregular surface is made to produce
fluctuations in the line current. In an early form, demon-
strated over a Paris-Lyons telephone line in 1907, this was

HISTORICAL BACKGROUND OF THE DEVELOPMENT n

done by amplifying the movement of the stylus by a lever,
the far end of which moved over a device for varying the
resistance in the circuit. A serious objection to such an
arrangement lies in the fact that to produce such an am-
plified movement a considerable force must be exerted by
the stylus; this causes cutting of the gelatine relief used
for transmission. Later Belin improved the transmission
by connecting the stylus to a microphone in the primary
circuit of a transformer, the secondary of which was in the
transmission line. This is readily recognized as similar to
the ordinary telephone transmitter, except that the move-
ments of the stylus, as it passes over the relief image, have
replaced the condensations and rarefactions of sound waves.

The receiving portion of the Belin apparatus introduces
a method markedly different from any hitherto mentioned.
The varying currents are received on a Blondel oscillograph,
an instrument in which the variation of current is measured
by the displacement of a beam of light reflected from a
mirror rigidly fixed to the suspension. The reflected light
then passes through a wedge-shape aperture, called by Belin
a "scale of tints" ; thence through a condensing lens onto
a sensitized film which is carried by a rotating cylinder
synchronized with the sending cylinder. It will be noted
that here we are using the current in our circuit to vary the
amount of light from a constant source which reaches a
given portion of the film. Thus, the inertia of the receiving
mechanism has been greatly reduced, a major achievement
since speed of reproduction is all-important.

Turning now to the method of Professor Arthur Korn
of the Berlin Technical High School, we find the selenium
cell used at the transmitter. The photographic film is placed
on a glass cylinder which rotates and at the same time
moves along parallel to its axis. Hence by keeping a small
point of light in a fixed position, it may be made to traverse
every portion of the film. The light, having passed through

TELEVISION

the translucent film, is allowed to fall upon a selenium cell,
connected in the usual telegraph circuit. Selenium possesses
the property of becoming more conductive for electricity
under the influence of light. Therefore the resistance of
the circuit at any time will be proportional to the darkness
of that part of the film then under illumination. Clearly,
in order to obtain the desired current variation, the source
of light and the electromotive force acting in the line must
both be constant. At the receiving end, a modification of
the galvanometer scheme employed by Belin is used.

At first glance, it would appear that in this system the
inertia effect has been reduced to a minimum since neither

TIME.

FIG. 5. — Comparison of the fluctuation of light intensity (solid line) and

conductivity of a typical selenium cell (dotted line). Note time lag and

rounding of conductivity curve.

transmitter nor receiver depend upon the movement of a
stylus having mass. Unfortunately, however, the selenium
cell is subject to a distinct lag — it does not respond instantly
to light fluctuation. The accompanying diagram shows
this effect quite clearly. Professor Korn has very in-
geniously corrected this difficulty by the use of two cells
arranged to compensate for each other. Nevertheless, con-
siderable trouble still exists in the selection of suitable cells.
It is for this reason that the selenium cell has been almost
completely replaced by the photoelectric cell, as we shall
consider later.

HISTORICAL BACKGROUND OF THE DEVELOPMENT 13

Up to this point we have considered only the problem
of picture transmission over wires. The wireless or radio
transmission, however, presents the same fundamental prob-
lems— the production of current or potential variations
from the original and their reception so as to give an ac-
curate reproduction. When using a system of the inter-

(Courtesy of Arthur Korn.)

FIG. 6. — Picture of President Fallieres sent by wire from Berlin to Paris in
1907 by Korn system. Time required, 12 minutes. (Note the structure.)

mittent current type as previously discussed, the introduction
of a tuned spark-gap is all that need be considered in order
to understand the early attempts at wireless transmission.
A coherer was employed in the receiving circuit. The prob-
lem of synchronization becomes even more complex with
the lack of direct wire connection between the two stations.

(Courtesy of Arthur Korn.)

FIG. 7. — Examples of pictures transmitted over wires by the Lorenz-Korn

method in 1928. A photoelectric cell is used at the sender and a string

galvanometer at the receiver. Time required, il/2 minutes.

HISTORICAL BACKGROUND OF THE DEVELOPMENT 15

Hans Knudson achieved some success in wireless trans-
mission of pictures over short distances as early as 1908,
despite the inherent difficulties of the spark gap and coherer.

(Courtesy of Arthur Korn.)

FIG. 8. — Examples of wireless transmission by the Lorenz-Korn method, 1928.

Time, i minute.

He used a relief line-process original and transmitted di-
rectly without coding. Professor Korn also adapted the

1 6 TELEVISION

system described briefly above to wireless transmission. This
he demonstrated in 1914. In this case it was found neces-
sary to code the original. A step performed automatically
by a sensitive relay operated by the current fluctuation in
the selenium cell circuit. The perforated tape made by this
relay could be translated into code letters, each one of
which designated a shade (or process dot — in shape and
size) ; and transmitted like any radio message. To facilitate
reproduction a special typewriter was designed in which the
type attached to a given key was a tiny square of the correct
shade (or correct process dot) for that code letter.

A number of other code systems such as the Bartlane,
mentioned previously, are readily adaptable to radio trans-
mission— provided the problem of synchronization can be
solved.

The introduction and development of the three-electrode
vacuum tube resulted in such a simplification, both in the
transmission and reception of radio signals, as to completely
replace the spark-gap and coherer. This is as true in the
field of picture transmission as in sound broadcasting. So
it is that we find the vacuum tube oscillator employed in the
systems of C. F. Jenkins of Washington, D. C, and R. H.
Ranger of the Radio Corporation of America; both of
which will be discussed in later chapters.

The progress made in radio transmission of pictures
since the advent of the vacuum tube has been extremely
rapid. On December 2, 1924, pictures were "radioed"
from London to New York, using Captain Ranger's
method. The system is now established and commercial
service maintained between the Radio House, the Marconi
telegraph station of London, and the R. C. A. in New York
City.

Let us turn now to the problem of transmission of pic-
tures of moving objects, generally referred to as televison.
As the eye will not hold a discrete impression of pictures

HISTORICAL BACKGROUND OF THE DEVELOPMENT 17

presented at the rate of sixteen per second; it follows that,
if a moving object be photographed every sixteenth of a
second and these separate images projected before the eye,
sixteen each second, the impression will be one of continuous
motion. This is then the problem of television.

Little has been said so far about the time required for
transmission. In the Bell Telephone system, one of the most
recent for wire transmission, a 5 x 7 inch photograph in 100
lines to the inch (equivalent to 100,000 dots per square inch)
requires seven minutes for transmission. Any code method
such as those previously described will of necessity require
a much longer time. It becomes apparent that much more
rapid reproduction is imperative. For this reason apparatus
of negligible inertia will be required both for transmission
and reception.

Early experimenters in the field attempted to use the
selenium cell. The first suggestions being to allow the light
from a small section of the original to fall on a cell con-
trolling the current for a light which illuminated the cor-
responding portion of the reception screen. Enough such
circuits must be used to cover the entire object to be trans-
mitted. Although direct enough, this system is certainly
quite complicated for the transmission of any but an ex-
tremely simple picture.

Ruhmer in Germany as early as 1910 accomplished a
remarkable simplification of the elementary method sug-
gested above. He used only twenty-five square sections in his
transmission and reception board, so that only simple geo-
metric figures could be handled. In place of twenty-five dif-
ferent wires from sender to receiver, however, he employed
only one. Each square actuated a separate selenium cell; this
cell controlled a circuit of definite frequency. At the recep-
tion end a relay responsive only to this frequency illuminated
an electric bulb placed behind the corresponding square of
the screen. When a number of squares of the sending board

1 8 TELEVISION

are illuminated a number of different frequency pulses will
be sent over the line without interference and the cor-
responding relays will be actuated, thus lighting the correct
squares on the reception board.

As intimated previously, the lag of the selenium cell
presents a considerable difficulty, hence it has been sup-
planted by the photoelectric cell and the cathode ray oscillo-
graph in more recent developments. Even in 1908, A. A.
Campbell-Swinton suggested, in a letter to "Nature" that
the problem might be solved by the use of the Braun tube,
or cathode ray oscillograph. Several workers have since
followed along these lines; notably Professor Belin and M.
Dauvellier in France.

In America the photoelectric cell seems to have at-
tracted more attention. We find it utilized by C. F. Jenkins
in the transmitter used by him in June, 1925, when he suc-
ceeded in projecting on a small screen, in his laboratory in
Washington, D. C., an image of the rotating arms of a
windmill; the arms of the original were turning nearly five
miles away in Anacostia, Md. At the receiving end a
refinement of the neon tube due to D. MacFarlane Moore
was used.

For a clear understanding of these more recent systems
of television it is essential that one know something of the
construction and characteristics of some of the more im-
portant parts used — the photoelectric cell, the neon lamp,
the cathode ray oscillograph, the scanning disk, optical sys-
tems, etc. The purpose of the following chapters will be
to discuss each of these devices in detail. The theoretical
background necessary for an understanding of the apparatus
will also be treated briefly.

CHAPTER III
OPTICAL SYSTEMS AND THE EYE

IN the study of television one is constantly confronted
with optical systems. There is the optical system which
produces the scanning pencil or which collects the light rays
reflected from the scene at the sending end, and the projec-
tion system for throwing the image on a screen at the re-
ceiving end. Lenses, mirrors, and prisms have a habit of
making themselves useful in what sometimes appears to be
the most unexpected ways, as in the Jenkins scanning disc
for example. These are the tools with which we control
light beams, and as this control is an essential element of
television it is necessary that we know something of it. For
this study we need know nothing of the fundamental nature
of light and this is left for a later chapter. We are here
concerned only with its behavior in relation to optical
systems.

One of the first laws of geometrical optics concerns it-
self with the rectilinear projection of light. This law states
that light travels in straight lines in any homogeneous
medium, that is a medium which is the same throughout.
The second important law is that the intensity of illumina-
tion from an open point source falls off inversely as the
square of the distance from the source. Thus if the distance
of a lamp from an object is doubled the intensity of illumina-
tion of the object is cut to one-fourth of its former value;
if tripled in distance the intensity is cut to one-ninth and
so on. If the source is enclosed, as in a reflector, this law
does not hold and the rate of falling off will then depend

TELEVISION

upon the reflector. It is not true for a source other than
a point, but if the object is removed a distance which is
twenty times the diameter of the source or further, the error
introduced by considering it a point is less than one per cent.
For most practical purposes the law is nearly enough correct
for satisfactory application. In illuminating an object for
television it should not be lost sight of.

Reflection of light is an important point in television.
For a mirror there is a law which states that the angle of
incidence is equal to the angle of reflection. Thus in
Fig. 9, the angle i equals the angle r. This law may

FIG. 9. — For a mirror the angle of incidence (i) of a light beam is equal to
the angle of reflection (r).

be accepted wherever a polished metallic surface is con-
sidered, but it does not hold true for any but well polished
surfaces. A comparatively rough surface which is white,
as a piece of white blotting paper, may reflect more light
than polished nickel and in general will do so: this is also
true of mat white card-board or white cotton, etc. This
reflection however does not obey the law given above, but
rather Lambert's cosine law. The later law is best under-
stood by reference to Fig. 10, where the arrows ending
on the circle represent the amount of light which comes off
at each angle. Such a surface is called a diffusely reflecting
surface, whereas the mirror is called a specularly reflecting
surface. In properly illuminating a subject to be televised

OPTICAL SYSTEMS AND THE EYE 21

the choice of reflectors is important: but, in general, diffusing
screens would be used. These screens are much used in
photograph galleries and their use is no less essential in
television studios.

The fourth fundamental law of geometrical optics is
that of refraction. This has to do with the passage of
light from one medium to another as from air to glass or
from air to water, etc. Dipping a pencil at an incline into
water and viewing it from above will show that it appears
to be bent where it enters the surface. The amount of
bending depends, it has been found, on the relative speed

FIG. 10. — In the case of a diffusing surface reflection follows Lambert's cosine

law. This is shown above where the length of arrow represents the amount

of light reflected in any chosen direction.

of light in the two media considered. It is this property of
refraction which makes lenses possible.

Perhaps the simplest optical device is the plane mirror,
of which it need only be said that the image appears to be
as far behind the mirror as the object is in front. Since no
rays of light actually penetrate the mirror, no rays actually
exist where the image appears to be. Such an image is
called a virtual image.

When we come to curved mirrors, however, we have
quite a different story, and one which is not so simple. If
we consider a concave mirror as shown in Fig. 1 1 ; a ray
marked A striking it as indicated, will be reflected according
to the reflection law, for the ray is so small that the part

22 TELEVISION

of the mirror which it strikes appears flat from its point of
view. The ray B will likewise be reflected as shown.

The point at which the rays cross, F, is called the focus
and if the mirror has been drawn with a compass it will be

A \

FIG. ix. — Diagram of a concave mirror showing path of light rays, the focal
point F, and the center of curvature of the mirror at C.

found that F is halfway between the center of curvature
and the mirror.

Now, the useful images which we will get from mirrors
of this type are of three kinds :

1. The object is to the left of the center of curvature.
Fig. i2a.

2. The object is between the center of curvature and
the focus. Fig. i2b.

3. The object is between the focus and the mirror.
Fig. i2c.

Wherever the image and object are on the same side
of the mirror the light rays actually pass through the image
and it is called a real image. If they are on opposite sides
the image is virtual, as in the plane mirror. Formulas may
be given so that the exact position of image and object may
be located mathematically; but readers are referred to text-
books on optics for this information.

In television, lenses are more important than mirrors

OPTICAL SYSTEMS AND THE EYE

FIG. iza. — The position of the image and its size with respect to the object
when the object is beyond the center of curvature is shown above.

FIG. izb. — When the object is between the center of curvature and the
focus, the image takes a position outside the center of curvature.

^^> -»•*"* A

FlG. 12C. — If the object is between the focus and the mirror, the image is
virtual. It appears to be behind the mirror.

24 TELEVISION

although they behave in much the same manner. Here
parallel rays, instead of being reflected to the focus, are
refracted as shown in Fig. 13. Fig. 14 shows the
relative location of image and object for one position of the

FIG. 13. — The diagram above shows the path of parallel rays which after

refraction by the lens pass through the focus. C is the center of curvature

for one face of the lens.

FIG. 140. — The above is typical of the position of object and image for a

double convex lens when the object is outside the focus. This will be the case

in most television apparatus.

FIG. i4#. — For a double concave lens the image is between the lens and
focus when the object is outside the focus. The image is virtual.

object. This is typical as long as the object is not between
the focus and the lens. In the case of convex lenses the
image is real when it is on the opposite side of the lens from
the object and virtual when it is on the same side.

OPTICAL SYSTEMS AND THE EYE 25

As the double convex lens is almost always used in tele-
vision for producing real images, the method of choosing
a lens for this case only will be given. This is based on
the formula:

i. ill

t ~p+q

where / is the focal length, p is the object distance and q is
the image distance. The focal length of any lens can be
quickly found by holding it up toward the sun and finding
where it casts its light spot. This is not accurate, but is good
enough for most purposes. From this formula one can
determine distances and dimensions necessary in the con-
struction of television apparatus.

Several things about the choice of a lens are worth
noting. The greater the diameter of the lens and the
shorter its focus the greater its light collecting ability. A
short focus lens has what is called a flat field. It is said
to have no "depth of focus," and an object has to be moved
only slightly to be thrown in or out of focus. A person's
nose might be sharply in focus but his cheeks blurred in such
an extreme case. The farther we get from this condition
the less light we collect but the more pleasing the view. In
television, light collection is frequently the major considera-
tion, consequently such lens are not uncommon.

When one is using a photoelectric cell, sensitive mainly
in the violet, or when the image is to be photographed, the
available light is increased by use of quartz lenses; for these
transmit the ultra-violet rays of high actinic value.

One must also watch for various lens faults. One of
these is spherical aberration, a fault which causes a blurred
image, as rays from the outside part of the lens focus closer
to it than those from the center. (Fig. 15.) Another is
chromatic aberration, which is the focusing of different colors
at different distances from the lens, the red farthest from

TELEVISION

it and the violet nearest. (Fig. 16.) Some lenses also
give "pin-cushioned" distortion, others, "barrel-shaped"
distortion. Others have astigmatism, the focus across one
axis being different from that across another; they are not
symmetrical . . . and so the number of possible faults
goes on. To correct for such faults good lenses are usually

FIG. 15. — Rays from the outer edge of a simple lens whose surfaces are

spherical focus closer to the lens than those through the center portion.

This gives a blurred image.

made up of two or more pieces each of a different kind of
glass. With these pieces all common faults can be corrected.
While one should be careful in selecting lenses if good
results are expected, it should be remembered that good
selection implies a knowledge of when to use a cheap lens
as well as when to use a good one. It would be foolish, for

FIG. 16. — Different colors will focus different distances from the lens unless it

has been specially corrected for this fault by being made up of different kinds

of glass cemented together.

example, to put an expensive lens in a spot-light, where none
of the faults enumerated above would have any importance.
Before choosing, one should make a careful study of the
needs — one would not choose an expensive limousine to haul
gravel.

OPTICAL SYSTEMS AND THE EYE

The prism is another optical device which may at times
prove useful. This has the ability to bend light as shown
Fig. 17, the red being bent least and the violet most.

FIG. 17. — A glass prism breaks white light up into component colors, the red
being bent least, the violet most, as shown.

The prism has another interesting use, that is as a perfect
mirror. As rays of light come from an object under water,
for example, they are bent more and more toward the
surface as the angle changes, finally being reflected back into

FIG. 18. — A right-angled glass prism may be used as a perfect reflector if
light is allowed to strike it as shown.

the water. Beyond the critical angle the reflection is com-
plete, and is known as internal reflection. If light is sent
in on one side of a right-angle prism, as shown in Fig. 18,

28 TELEVISION

it will strike the oblique side at such an angle as to be wholly
reflected and so the prism acts as a perfect mirror. Prisms
are for this reason much used in binoculars, range-
finders, etc.

One optical instrument which enters into all television
systems is the human eye and since all systems must adapt
themselves to, and may take advantage of its characteristics,
it is essential that we know something about the manner in
which it functions. When the light enters the normal eye
it is focused by means of the eye-lens onto the retina. The
retina is coated with a material known as the visual-purple
in which are imbedded the so-called rods and cones at the
nerve ends. When the light falls upon the visual-purple, a
photoelectric action takes place, according to the generally
accepted Eldridge-Green theory, the electrons being freed
much as they are in a photoelectric cell. These freed elec-
trons set up currents in the visual-purple which are detected
by the rods and cones; these in turn set up currents in the
nerves that carry them to the brain. The detection of the
current in the visual-purple by the rods and cones, and its
production of secondary currents in the nerves, reminds one
of the action of a vacuum tube circuit. The nerves are the
wires which carry the message to the brain. The eye inter-
prets different wave-lengths as color.

There are several features of the eye which cannot be
ignored in planning television apparatus. For best seeing
conditions, the illumination level should not be too high. One
to one-hundred foot-candles are advisable limits. It is
also advisable to keep the contrast between different parts
in the ratio of about ten to one. A ratio of one-hundred
to one will produce a glare. The color for best vision is
in the yellow-green region. When the diameter of an object
being viewed is more than one-twentieth its distance from
the eye, it is too close for easy viewing, and if it is less than
one three-hundredth the distance, it is too far away. A

OPTICAL SYSTEMS AND THE EYE

ratio of diameter of object to distance of one to one-hundred
is most suitable. This should be kept in mind in locating
an object to be viewed before the scanning-disc. At the
receiving end seeing will be easier if all other lights in the
room are dimmed, for stray light will cause the pupil of the
eye to contract needlessly and will thus close off much of
the light from the television screen.

Owing to the fact that the eye does not change at once
in response to any change in light intensity, but has a lag
of about one-tenth of a second, it is possible to produce the
sense of continuous motion by placing an object successively
at slightly different positions and allowing it to be viewed at
tenth-second intervals. This is made use of in the produc-

FIG. 19. — The sensation produced in the eye is not in proportion to the
stimulation. The relation, which is logarithmic, is shown in the figure above.

tion of motion pictures. Without this so called persistence
of vision, television of the modern type, which requires that
parts of the image be sent over in succession, would be
impossible. The speed of scanning is determined by this lag,
which dictates that pictures must be scanned completely at
least ten times per second in order that the picture may
appear as continuous to an observer.

In operating a television receiver it should also be re-
membered that the sensation received by the eye is not in
direct proportion to the intensity of the light, but to a
logarithmic function of this intensity. Thus 1000 foot
candles does not produce an appreciably greater effect than
100 foot candles. This will be seen in Fig. 19.

30 TELEVISION

In operating televison apparatus one should never forget
the fact that every precaution should be made to protect
the eye. If one is projecting the light from a neon lamp
onto a screen at the receiving end, it should be remembered
that increasing the screen size will give a less intense image
and may result in eye strain. At the sending end an intense
source of light should not be used to the discomfort or pos-
sible injury of the subject. By doing so one is only fooling
oneself into the belief that he is making more progress than
is actually the case. Bigger screens, better scanning, etc.,
will only come with improved apparatus and eye injury will
not further the cause but will weaken it.

CHAPTER IV
ELECTROMAGNETIC WAVES

MUCH of our present understanding concerning the be-
havior of light is based on the assumption that we are deal-
ing with a wave motion. Just how close this hypothesis
approaches the truth remains a matter of conjecture. Con-
siderable evidence obtains to discredit such a viewpoint;
yet so many of the common phenomena associated with light
can be readily explained on the basis of a wave theory, that
for pedagogical purposes, at least, this interpretation will
probably remain in vogue for some time to come, At any
rate, such a treatment of the subject will prove helpful in
so far as it concerns us in our study of television.

Everyone is familiar with the action of water waves.
How often have we seen "the angry breakers pile upon the
barren shore"? Rather we should have said "batter
against," not "pile upon." Although there is no doubt of
the force of their impact, as one sometimes finds when surf
bathing; yet considering an extended period of time, no
appreciable volume of \vater is transferred landward despite
the fact that there may have been a continual, apparent
movement of the waves in that direction. Herein lies an
important characteristic of all wave-motion, — energy will
be carried from one point to another but the material car-
rier thereof remains in situ.

In order the better to understand the mechanism by
means of which this is accomplished, let us turn to the
example of a wave form sent along a rope. Suppose the
rope to be held horizontally and a very small section marked

32 TELEVISION

so as to distinguish it from the rest. Let this portion be
viewed through a narrow vertical slit of considerable length.
If now an undulation be sent along the rope by moving one
end up and down rapidly, the mark will be seen to travel
up and down the viewing slit. Thus, in this case, the parts
of the vibrating material move at right angles to the direc-
tion in which the wave form travels; i.e., in which the energy
is transferred. This is typical of the class known as trans-
verse waves — the category to which light radiations belong.
Some of the more important terms used in connection
with vibratory phenomena may be understood by reference
to Fig. 20. A is the amplitude or maximum displace-
ment of a particle from its rest position. This is important
as a measure of the intensity — the loudness of a sound or
the brightness of a light. A (lambda), the distance between
consecutive crests, is called the wave-length. The number
of complete wave-lengths sent out per second is known as
the frequency, generally designated by the Greek letter v
(nu). Suppose a vibrating source sends out ten complete
undulations every second, each two feet long; clearly the
fore of the first disturbance — the wave front — must have
reached a point twenty feet from the source in one second,
if there is to be no overlapping of the wave forms. The
distance traversed by the energy each second we call the
velocity (F) of the disturbance. From the simple numerical
case considered, we are led to an important generalization,
fundamental in the study of all wave phenomena. The
'velocity is equal to the product of the wave-length by the
frequency — in terms of the symbols defined above:

Another important attribute of most, possibly all true,
wave disturbances is best illustrated by a common example.
Let us attend a band concert given in a large stadium. If
our ears are good enough, we shall hear the same tunes

ELECTROMAGNETIC WAVES

whether we sit well forward or near the rear. Since the
various notes played are of different pitch (that is fre-
quency) and loudness (that is amplitude), this must mean
that the velocity of sound is independent of these two
factors; else the time of the music would be effected. One
might argue that sound was not a wave-motion; or if so,
traveled with an infinite velocity. Both contentions, how-
ever, are easily disproven. Under suitable conditions, sound
disturbances may be photographed and shown to behave
much like water waves. As to the velocity of their propaga-
tion, it may be readily measured by the simple expedient

FIG. 20. — This shows a typical wave form, where \ is the wave-length and

A, the amplitude.

of timing the interval between the flash and the report of
a gun, observed from a known distance. More extended
experiments show us that the nature and condition of the
vibrating medium are the only factors of influence in de-
termining the velocity of waves of any given type. In this
last conclusion lies the keynote to one of the most interesting
controversies in the history of physics; one in which the
last word has not yet been said. What are light radiations?
Even before the time of Newton, the idea that light
was an undulatory disturbance had found some favor. That
great genius, however, inclined toward a corpuscular theory;
pointing out the observed fact that "light travels in straight

34 TELEVISION

lines," indeed all the phenomena of geometric optics, could
well be explained by assuming the rays to consist in ex-
tremely small particles projected from the source of
illumination. Later, improved optical instruments and more
careful observations showed definitely that there were in-
stances where light did not follow a straight path — cases
where it actually bent around corners (diffraction). Here
was strong evidence for the wave theory since on this basis
a much more simple explanation could be given.

Whether wave or corpuscular, the disturbance should
have some finite velocity; yet seemingly it travels with in-
finite speed. In the very experiment that was suggested to
determine the velocity of sound, it was tacitly assumed that
no time elapsed between the flash at the gun's muzzle and
the appearance thereof to a distant observer. The truth of
the matter is that the earth is too small a laboratory to
detect a velocity as high as that of light; unless we have
at our command a very accurate means for measuring ex-
tremely small intervals of time. Lacking such a device,
early investigators were unable to obtain conclusive results.
From the vast laboratory of the heavens came the first evi-
dence that light actually did travel at a finite rate of speed,
the work of the Danish astronomer, Romer.

At this point comes a portion of the work of particular
interest in connection with our problem. While the experi-
mental workers, by the use of refined methods where the
light traveled over terrestrial distances, were still endeavor-
ing to verify and improve Romer's determination, Max-
well suggested a way to obtain the result without making
any velocity measurements whatsoever. The determination
of one electrical quantity, for example, the capacity of a
condenser, in two systems of units already fixed was all that
was necessary. Back of the suggestion lay the masterful
discussion forming the basis of the classical theory of elec-
tromagnetic waves.

Em missions -Spectra

Solar
Spectrum

Nitrogen
\Spectrum)

Oxygen
Hydrogen
Barium
Calcium

Strontium

Indium

Thallium

Rubidium
Caesium
Potassium
Lithium

FlG. 22.

ELECTROMAGNETIC WAVES 35

The general picture given by this classical theory, as it
has been filled in to date, is indeed comprehensive. Many
types of phenomena, seemingly unrelated, are included,
y-rays, x-rays, ultra-violet, visible, and infra-red radiations,
and radio waves, all fill the requirements for electromagnetic
disturbances as described by Maxwell — at least to a fair
extent. All have the same velocity — 3 x io10 cms. per
second (where the exponent of io indicates the number of
ciphers to follow the 3). Recalling the relation given
above (F = v\) it will be seen, that if the wave-length be
short, the frequency is high and vice versa. Figure 21
shows how the various kinds of radiations fit into the com-
pleted catalog. Figure 22 shows the visible portion of
this spectrum, as it is called, in more detail.

- I I I I I I *~

7-Rays X-Rays ? Ultra- Visible Infra-Red Radio Waves

Violet

FIG. 21. — Schematic chart of electromagnetic disturbances.

The theory is by no means free from criticism. One
of the most obviously questionable features, is the existence
of a vibratory medium through which the radiations must
pass. In our initial discussion we stressed the fact that in
order to transfer energy from one point to another by a
wave-motion, a material capable of supporting the wave
forms must intervene. The action hinges on the elastic
properties of this intermediary. Great quantities of radiant
energy come to us from the sun; yet we have good reason
to believe that most of the distance between us and that
body is extremely close to a perfect vacuum. Far too low
a concentration of ordinary matter exists there to support
a wave disturbance. To avoid this dilemma it is necessary
to hypothecate some elastic medium, distinct from the usual
chemical substances, a medium which must be conceived to

36 TELEVISION

fill all interstellar space. The name applied to this, so far
intangible something, is uthe ether."

Other discrepancies between theory and experiment are
most pronounced in the short wave-length end of the gamut
as given in Fig. 21. One of these is the well-known case
of dispersion — the breaking of white light into its com-
ponent colors by a prism. The explanation that the short
wave-lengths are more retarded on passing through the
material of the prism than are the long ones, runs contrary
to the idea that radiations of the same type should have the
same velocity in a given medium regardless of their wave-
length. The deviation being greatest for the short wave-
lengths or high frequencies, it is for these that the classical
theory becomes most dubious. What the correct interpreta-
tion of the variation may be is still somewhat hazy. Un-
questionably the electron, that almost infinitesimal unit of
negative electricity, holds the answer. As yet, however,
there is still much to be learned concerning the interrelation
of high frequency radiations and electrons.

In the last paragraph mention was made of the fact
that white light actually consists of many different wave-
lengths or colors. The proportions in which the different
components are mixed depends on the nature of the source
of the illumination. Where this is an incandescent solid,
there is an important relation between its temperature and
the character of the radiation emitted. For qualitative pur-
poses, this may be expressed by saying that the light becomes
more intense (more energy is given off) and more of the
short wave-lengths are included as the temperature of the
emitter increases. See Fig. 23.

The effect may be easily illustrated by connecting a small,
3-4 volt flash-light bulb across a 6 volt storage battery,
through a variable resistance. If considerable resistance
be left in the circuit the filament will emit no light whatso-
ever. Nevertheless the outside of the bulb will become

ELECTROMAGNETIC WAVES

warm to the touch, showing that at this stage the filament
is giving off only low frequency radiations invisible to the
eye but sensible as heat producers. As the resistance is
gradually cut out of the circuit, the filament will first become
dull red, then bright red, then yellow, and finally white in

1450° C

3 4

Wave Length

6 x 1(T4 cms..

FIG. 23. — Relation of maximum wave-length in emitted spectrum to temper-
ature of emitter. Note displacement toward shorter wave-lengths with increase

of temperature.

color — giving off more illumination at each successive stage.
A further decrease in resistance will give so much current
through the filament wire as to cause it to melt; but just
before this happens, the light will become uncomfortably
bright for the eyes and will take on a slightly bluish char-

38 TELEVISION

acter — i.e., shortwave-lengths are beginning to predominate.
As might be surmised from the foregoing experiment, where
higher frequency radiations were produced when larger
amounts of energy were passed through the wire in a given
time; there is a connection between the energy of a radiation
and its frequency. The relation is that the energy is equal
to the product of a constant (known as Planck's constant)
by the frequency. This product, hv, where h is Planck's
constant and v the frequency of a disturbance, is called a
quantum, — a bundle of energy. So it appears that we might
consider light to consist of little packets of energy; yet to
behave like a wave movement. The two ideas are difficult
to correlate; but fortunately in most cases one is of pre-
dominate importance. In general when dealing with high
frequency disturbances the quantum is most helpful; whereas
for low frequencies, the wave serves best.

CHAPTER V
THE SELENIUM CELL

NOT infrequently a consideration of the difficulties aris-
ing under one method of attack upon some problem proves
helpful in understanding the development of another; we
are much more apt to appreciate the new, when we have
seen the disadvantages of the old. For this reason we have
included in this book a brief account of the selenium cell;
even though practically none of the systems now in use for
the transmission of pictures or the television of moving
objects, employ this device.

Prior to 1873 it was known that selenium, after anneal-
ing in the neighborhood of 200° C., became a conductor of
electricity — albeit an extremely poor one. The first intima-
tion that the material became more conductive when
illuminated came in that year as a chance observation of an
attendant in the Atlantic Cable station at Valencia, Ireland,
where the selenium was used to produce high resistances.
The importance of the phenomenon was quickly recognized.
Following close upon the initial observation, we find
numerous publications verifying the fact that the con-
ductivity of selenium increases on exposure to light. The
names of Willoughby Smith,1 Sale, and W. Siemens figure
prominently among the early workers in this field. More
recently other substances have been found to behave simi-
larly, although to a less marked extent: tellurium, thalium

1 Journal of the Society of Telegraph Engineers, 2, 31.

40 TELEVISION

sulphide, stibnite (antimony sulphide),2 and cuprous oxide3
may be mentioned. For further information the follow-
ing books may prove helpful: "Das Selen" by Dr. C. Reis,
"The Moon Element" by Fournier d'Albe, and "Fernphoto-
graphie" by Professor Arthur Korn.

Workers in the field of picture transmission quickly
seized upon the opportunity presented by the use of selenium
to change light impulses into variations of an electric current
that might be carried over wires to some distant recording
device. All that seemed necessary was to place a selenium
resistance, or cell, as it has come to be called, in series with
the usual telegraphic circuit. Light, falling on the cell,
would decrease its resistance : when the illumination was
removed, the resistance would return to its former value.
Thus current surges should pass through the circuit at those
times during which the selenium cell was illuminated. The
method looked promising and attracted much attention.
The problem of telephotography — perhaps, even that of
television — seemed well on the road to solution. Here was
a substance which might be made sensitive to illumination
of as low a value as icr5 foot-candles, about the lowest
intensity capable of affecting the human eye. Could not an
artificial eye — one which would sense the light and shade
of an object as does the human organ — be produced?

The problem proved much more baffling in practice than
it appears on paper. Sensitivity to variation in intensity is
not the only requisite for a light sensory mechanism to be
used for picture or object transmission. It must also be
capable of reacting rapidly and uniformly to successive
changes. In the last requirement, the selenium cell was soon
found to be seriously at fault.

Since the term selenium cell has been so often used, it
is only fair to describe the appearance of a common type

2 Jaeger; Kon. Akad. Amsterdam, 15, 724-30.

3Pfund; Physical Review, 7 (second series), 289 et seq.

THE SELENIUM CELL 41

which might be used in picture transmission, before con-
tinuing the discussion further. The following brief account
applies to the unit made by Giltay, of Delft, Holland; but
may be considered as characteristic of most of the more
recent methods of manufacture.

A rectangular piece of steatite (a high grade insulating
material which is practically non-hygroscopic), some 6x3
cms. in size, is wound with two platinum spirals spaced
about 0.6 mm. apart. These coils form the two terminals
of the cell: the resistance depends on the selenium which
is deposited between the platinum wires. After deposition
in the plastic form, the selenium is annealed at about 200°
C, at which temperature a transformation to grayish, light-
sensitive crystals occurs. For the greatest sensitivity the
deposit should be extremely thin; hence the resistance of the
cell may be extremely high, as much as 250,000 ohms or
more. Finally, the cell should be thoroughly dried and
vacuum sealed.

For such a cell the change in resistance is approximately
proportional to the square-root of the light intensity ab-
sorbed per unit time. For the average commercial type,
the effect is not the same for all colors; but reaches a
maximum for a wave-length in the region of A, = 7 x io~5
cms. in the red. This difficulty may be rectified since it is
possible to construct cells giving a maximum even in the
blue. Hence by suitable combinations a fairly uniform color
sensitivity may be obtained.

A much more serious criticism of the use of the selenium
cell, especially in television, lies in the fact that it does not
respond instantaneously to a change in the intensity of the
light to which it is exposed. Although this inertia is par-
ticularly disastrous in work with moving objects, it tends
to produce distortion even in picture transmission. Con-
sider the scheme of Professor Korn, described in Chapter
II. It will be recalled that here a translucent photographic

TELEVISION

film is placed over a glass cylinder which is so moved that
a beam of light will pass through every portion of the film
in an ordered sequence, thence on to a selenium cell. The
object being to produce in this manner fluctuations in a
telegraph or telephone circuit which in turn operate a device
for reproducing the original on photographic paper. If
the effect of the inertia and lag in the cell were simply to
delay the current variation, the problem might be solved by
suitable retardation of the receiving mechanism.

Exposure

Light cut off

Time

FIG. 24. — Relation of conductivity to exposure time for a typical selenium cell.

Consideration of Fig. 24, however, shows that the
response to light is not linear — the greatest part of the
change occurs during the first half of the exposure time.
Furthermore, when the light is cut off, the conductivity does
not return immediately to its former value; but drops
rapidly at first, then more slowly, never quite reaching the
initial value in any reasonable length of time. This lag in
the cell results in increasing values of the "dark" con-
ductivity after each exposure thus producing serious distor-
tion, as may be noted in Fig. 25 reproduced from Chapter
II for convenience. Even a casual inspection of the diagram
shows that the current plot is not merely a reproduction of
the light intensity curve moved to the right on the time axis;
but is essentially more rounded, failing to image minor
changes in light variation altogether. We would expect the
reproduction, therefore, to be lacking in detail.

As previously noted, Professor Korn has succeeded in

THE SELENIUM CELL

counteracting to a fair degree both the inertia and lag of
the selenium cell by suitable combination of pairs of cells
arranged so that the bad qualities of one will tend to neu-
tralize those of the other. The method involves simul-
taneous illumination of two cells placed in opposite sides
of a Wheatstone bridge, so that the current flowing in the
line will be the difference between that in the two cell cir-
cuits. With correctly chosen cells the current represented

TIME.

FIG. 25. — Comparison of the fluctuation of light intensity (solid line) and

conductivity of a typical selenium cell (dotted line). Note time lag and

rounding of conductivity curve.

by this difference fluctuates in fair synchronism with the light
variations. For a further discussion of the system, the reader
is referred to Professor Korn's book entitled "Fernphoto-
graphie" or to ''Wireless Pictures and Television" by T.
Thorne Baker (pages 28-29).

Other workers have endeavored to devise methods by
which the lag of the selenium cell might be overcome. One
reported by T. Thorne Baker in "Nature," June 19, 1926,
deserves mention. As was stated above, in the normal
circuit the "dark" conductivity of a cell tends to gradually
increase with successive exposures, an effect which Baker
attributed to cumulative ionization of the material. He,
therefore, subjected the cell to high frequency alternating
current. The constant reversal of direction for such a cur-
rent makes a continued migration of ions impossible. The

44 TELEVISION

frequency of the current used was much more rapid than
the variation in light intensities. In this manner Baker re-
ports that he was able to correct the lag of the cell — to
quote : uthe lag is automatically eliminated at each alterna-
tion of the current with the result that the cell responds
with great celerity to changes in illumination and returns to
zero with great swiftness."

A. O. Rankine reported in "Nature," July 3, 1926, on
an interesting series of experiments in which he found that
the light conductance could be increased and the "dark"
conductance decreased very considerably by careful desicca-
tion of the cell before use. For this reason he was led to
attribute the bad effects of "dark" conductivity to a minute
film of moisture between the electrodes, i.e., in parallel with
the selenium. It will be noted that this checks the ioniza-
tion theory, since the migration of ions would naturally take
place in the aqueous skin layer.

R. J. Piersol of the Research Department of the West-
inghouse Electric and Manufacturing Company 4 contributes
still another suggestion for improving the characteristics of
selenium cells to be used for the measurement of light inten-
sities. His work indicated that best results were obtained
when the thickness of the selenium deposited between elec-
trodes was not over 0.0014 cm- The conclusion was that
this represented a maximum depth of light penetration — or
rather, the greatest depth effected directly by variation in
the incident light. Whereas the material still further from
the surface was influenced, the effect was considered to be
secondary, transmitted from the primary surface layer;
hence not entirely under the control of the light variation.
It should be mentioned that Piersol also recorded the fact
that absorbed vapors and moisture tended to increase
"dark" conductivity.

Let us review the situation. In selenium we have a

4 Physical Review, 30 (second series) 664.

THE SELENIUM CELL 45

substance capable of transforming light and shadow into
what might be called an electric record. With suitable
preparation the material may be made sensitive to fine
gradations in light intensity. In other words, it may be
made to produce an appreciable change in an electric circuit
for even a small amount of incident illumination. Unfor-
tunately, though, the electric reaction tends to lag behind
the light stimulus. True, this difficulty can be remedied to
a great extent by suitable construction of the cell. Whereas
the results may be satisfactory for the relatively slow speeds
used in picture transmission; television, where the image
must be scanned completely some ten to sixteen times per
second, presents a very much more troublesome case. Hence
it is not surprising that experimenters should begin to look
elsewhere for a solution to the latter problem.

So far in the discussion nothing has been said as to the
cause of the changes observed in the resistance of selenium
when exposed to light. Although the question is somewhat
beyond the scope of a book of this nature, a brief statement
of the principal theories that have been advanced may not
be amiss. One of the early suggestions was that, under the
influence of light, the metal changed from one to another
allotropic form of less specific resistance; then returned to
the first state when the illumination was removed.5 Another
theory, and this appears much more probable in the light
of recent research, is that the action of the light is to free
electrons from their bonds within the selenium atoms, thus
making them available for the conduction of an electric cur-
rent. In this case the removal of the illumination causes
the electrons to be returned to their former "bound" con-
dition. The last concept suggests something akin to the
photoelectric effect — "but this," as Kipling would say, "is
another story," and leads us to the topic to be discussed in
the next chapter, the photoelectric cell.

5Berndt: Physikalische Zeitschrift, 5, 121-4.

CHAPTER VI
THE PHOTOELECTRIC CELL

WHEN one reviews the phenomenal progress that has
been made during the past fifty years in matters technical,
one cannot fail to recognize the importance of the part
played by the great genius of those theorists of the last
century who set the course which has proven so productive
for subsequent investigators. What might now be the con-
dition of our systems of communication, both by wire and
by radio, had the keen mind of Clerk Maxwell not formu-
lated the classical electro-magnetic theory? Certain it is
that Hertz was activated by the desire to confirm this theory
with experiment when he first succeeded in producing and
detecting, what we call today, radio waves. Nevertheless,
his laboratory oscillator, which so beautifully verified Max-
well's generalizations regarding the properties of electro-
magnetic waves — be they radio, heat, or light — also led to
the discovery of a phenomenon that marks one of the prin-
cipal weaknesses in the classical theory. As is so often the
case, the exception has attracted as much attention as the
rule.

In 1887, while using a spark-gap to measure the energy
of the electro-magnetic waves emitted from an oscillating
circuit containing a similar gap, Hertz noted a peculiar
effect.1 His method was to adjust the two circuits to
resonance, then measure the maximum separation of the
points for which sparking could be produced in the receiver.

1 See "Electric Waves" by Hertz.

THE PHOTOELECTRIC CELL 47

Wishing to prevent air currents and hoping thereby to ob-
tain a longer spark, he experimented with a cardboard
chimney placed around the receiving gap. This, however,
produced just the contrary to the desired effect, requiring
the terminals to be brought closer together before discharges
would take place. Surmising that this might be due to the
fact that the light from the sending gap was screened from
the receptor, Hertz used glass to replace the opaque ma-
terial. The result remained the same. Yet when quartz
was used for the screen no diminution of the sparking
distance occurred. Since quartz will transmit ultra-violet
light, whereas cardboard and glass are alike opaque thereto;
Hertz concluded that it must have been this portion of the
luminous discharge from the sending gap which in some
manner facilitated the production of a spark in the receiving
circuit.

It was soon found that ultra-violet radiation from any
source would have a similar effect. Hallwachs,2 in 1888,
made some progress toward a further explanation of the
phenomenon. He allowed the radiation from an iron arc,
rich in ultra-violet, to fall on a negatively charged zinc plate
that was well insulated and connected to an electroscope.
The collapsing of the electroscope leaves showed that under
this illumination the zinc gradually lost its charge. Yet
when a positively charged plate was treated in a similar
manner, no loss in charge was observed. A neutral body
showed a slight tendency to become positively charged. The
suggestion offered was that under the influence of ultra-
violet rays a metal tends to lose negative charge. The next
question naturally is, "What constitutes negative charge and
how is it lost?"

Elster and Geitel 3 found that those metals which are
highest in the electro-motive series — i.e., most chemically

2 Ann. Physik, 33, 301.

3 Ann. Physik, 38, 40, 497.

48 TELEVISION

active — showed the effect under consideration most strongly.
The alkali metals, which top the list, were acted upon by
light of wave-length even in the visible portions of the spec-
trum. They also pointed out that the action took place
with undiminished strength even in the best vacuum obtain-
able; hence was not dependent upon the presence of gas
molecules.

Lenard 4 demonstrated clearly that ions of the negatively
charged material were not involved in the charge transfer,
as is the case for electrolysis. He sealed two electrodes,
one of sodium-amalgam and the other of platinum, into an
evacuated bulb. The circuit was then closed externally
through a battery so as to keep the sodium at a high nega-
tive potential. When the amalgam terminal, the cathode,
was illuminated by ultra-violet light a current flowed in the
circuit. The action was allowed to continue until, had the
sodium ions been the current carriers, enough of them would
have collected on the platinum anode for their presence to
have been readily detected by a standard chemical test. No
such test was given. Ions were apparently not the prime
movers in this phenomenon.

It is to the classical experiments of J. J. Thomson per-
formed in the Cavendish Laboratories, Cambridge, Eng-
land, that we must turn for the answer to our question.
Here about 1897, the existence of the electron, the smallest
particle of negative electricity, was first proven. The ap-
paratus used in this work is so closely analogous to the
cathode-ray oscillograph to be considered in a later chapter
that we shall leave the discussion until that point. For the
present, suffice it to say that we are now in a position to
explain the loss of negative charge without involving either
gas molecules or metal ions. We need simply say that under
the influence of light waves, preferably of the ultra-violet
type, electrons will be liberated from a metallic surface. In

4 Ann. Physik, 2, 359 (1900).

THE PHOTOELECTRIC CELL 49

the experiments of Hertz, these electrons ionized the air
gap, thus rendering it more conductive, easier for the elec-
tric energy to cross in the form of a spark. Such electrons,
once free, are all alike no matter with what atom they may
have been previously associated. So Lenard could detect
none of the attributes of sodium on his anode even though
the carriers had actually come from that metal.

Electrons liberated from an atom under the influence of
light are aptly called "photo-electrons." Just what may be
the mechanism of their release need not concern us; so long
as we recognize that the optimum conditions are obtained
by using a cathode made of one of the most chemically active
metals and by using radiation of short wave-length typical
of the ultra-violet end of the spectrum.

The question in which we are most interested is uHow
may the photoelectric effect best be utilized to solve the
problem of television ?" It will be apparent from the pre-
ceding discussion that we now have another method for
converting light changes into electric currents. What ad-
vantages or disadvantages does it possess when employed
in television?

Stoletow,5 as early as 1890, devised what might be
termed the first photoelectric cell — a device which produced
a photoelectric current when illuminated. Referring to
Fig. 26, which illustrates the apparatus used, C is a zinc
plate connected to the negative terminal of a high voltage
battery, B (what one would call in radio terminology, a
B battery) ; A is a platinum screen connected to the positive
of the battery; G is a galvanometer for measuring the cur-
rent in the circuit. Under normal conditions the air re-
sistance between C and A is so high that no current will
flow. When the cell is illuminated so that ultra-violet radia-
tion will pass through the screen anode on to the zinc
cathode, the latter will emit photo-electrons. These, at-

5 Jour, phys., 9, 486 (1890).

50 TELEVISION

tracted by the high positive potential, cross the air gap to
the anode. From that point the chemical action of the bat-
tery returns them to the cathode, ready to recommence the
circuit. This circulation of electrons constitutes an electric
current, although in accordance with an old convention we
designate the direction of the current
as the reverse of that of electron flow.
Since the electrons have almost neg-
ligible mass, only one two-thousandth
^_^ that of the hydrogen atom, the action

C- — ( o j-^ of the anode field will give them an
I extremely high acceleration. Once
. . J freed from the cathode by the action

i r"V* **!"" Of light, they will bridge the gap to
FIG. 26. — Apparatus as the positive terminal with velocities

used by Stoletow in study- , . , r ,. , . , c r

ing photo-emission. approaching that of light itself. In-
deed, the entire action is as nearly

instantaneous as we could wish. Here is a distinct ad-
vantage over the use of selenium cells, or any of the other
systems requiring the movement of a stylus having mass.

Though the photoelectric cell, so described, may pos-
sess this one very desirable quality — speed of reaction;
nevertheless, it has several very troublesome features. In
the first place, the size of the current which is produced in
such a cell, even by the intense illumination of a carbon arc,
wrill be very small unless ionization occurs in the gas between
the electrodes. Secondly, when such ionization does take
place, the characteristics of the cell become uncertain — its
response no longer being proportional to the intensity of the
incident illumination. Thirdly, if the cathode be zinc, as
above, ultra-violet radiation will be necessary for the
reaction.

The work of Elster and Geitel, mentioned above, sug-
gests a possible remedy for the last two difficulties. If the
electrodes be sealed in a highly evacuated bulb, the troubles

THE PHOTOELECTRIC CELL

arising from gaseous ionization can be eliminated. (The
discussion to be found in Chapter VII on the action of dis-
charge tubes will make this point more clear.) Again, if
we replace the zinc by one of the alkali metals — sodium,
potassium, rubidium, caesium — ordinary white light will
suffice to free photo-electrons from the cathode. This last
is a most distinct improvement since any intense source of
illumination may now be used and the cell may be made of
glass.

There still remains, however, the problem of increasing
the current output. Suitable preparation of the cathode
was found to help somewhat. Elster and
Geitel produced a hydride of potassium
by bombarding the metal with electrons,
in an atmosphere of hydrogen. They also
deposited the metal in a colloidal form.
Both schemes gave increased sensitivity,
the last mentioned being the better, albeit
the more difficult method. The General
Electric Company followed the practice of
silvering the inner surface of the bulb,
later heating a circular portion so as to
drive off the silver; thus leaving an open-
ing to admit illumination. Over the re-
maining silvered part, there was then
deposited an extremely thin layer of dis- photoelectric cell.

• 11 j j- . , . ,. T Note anode, circle of

tilled sodium, potassium or rubidium. In
this type, the anode is a tungsten wire
usually to be found in the form of a
loop at the center of the bulb. (See
Fig. 27.)

These vacuum cells are extremely reliable in their action.
One type,6 now on the market, is claimed to be so free from
the fatigue characteristic of the selenium cell that "it will
G Bulletin No. 271, Dr, R, C, Burr, Pasadena, California,

( Courtesy of Ray-
theon Mfg. Co.]

FIG. 27. — A typical

wire at center of
bulb, and standard
vacuum-tube base,
only two of whose
contacts are used in
the usual circuit.

TELEVISION

record direct sunlight (10,000 foot-candles) all day and
immediately afterward accurately measure the light from
a flashlight bulb at one meter." For a given applied voltage
the output is directly proportional to the intensity of the
incident light, as may be seen by reference to Fig. 28.
Such properties make this form of cell very useful in photo-
metric work. Yet when we consider that a 60 c.p. lamp
only 6 inches distant gives a current of something like

INTENSITY

FIG. 28. — Relation between the intensity of incident illumination and the out-
put of a typical photoelectric cell: a is for a higher applied voltage than b.

i/i 0,000 of a milliampere, it is obvious that a great deal
of amplification is necessary before a picture transmission
or a televison circuit can be operated.

Not until the perfection of the three electrode vacuum
tube was even a partially satisfactory solution of the dif-
ficulty found. This device, so well known for its service
in the radio set, provides a most excellent means for am-

THE PHOTOELECTRIC CELL

plifying the small output of the photo-cell. Figure 29 illus-
trates a typical circuit used for this purpose. At this point, it
should be remarked that the screened grid vacuum tube is
extremely well adapted to this service.

Unfortunately, there are limits to the amount of amplifi-
cation that it is advisable to produce in vacuum tube circuits.
The restraining factor is the trouble introduced by the am-
plification of extraneous circuit noises. In picture transmis-
sion these evince themselves as spots; in television, as
irregular, wavy lines across
the image as viewed
through the scanning-disc
or on the screen. Recent
work of J. B. Johnson of
the Bell Telephone Labora-
tories, New York City,7
seems to show that the
origin of a large propor-
tion of such noises is not
in the vacuum tube itself,
as was originally thought,
but rather in the flow of elec-
trons through the resistances
of the circuit. A photo-
cell designed by Zwory-
kin of the Westinghouse Electric and Manufacturing Com-
pany, tends to reduce this effect to a minimum by the com-
bination of the photoelectric cell and a three or four
element vacuum tube within the same bulb. Of course
in this instance care must be exercised to screen the
photo-active surface from the light given by the vacuum
tube filament. The method employed, is to use the oxide
type of filament which need only glow dull red and to locate
the vacuum tube in the stem, separated by a light-tight

7 Physical Review, 32, No. i, p. 97.

FIG. 29. — Use of a vacuum-tube ampli-
fier in connection with a photoelectric
cell.

54 TELEVISION

diaphragm from the photoelectric cell in the bulb. There
still remains the difficulty in preventing the heating of the
unit by radiation from the vacuum tube filament.

When applied to television, the combination of vacuum
tube amplification with the high vacuum type of photo-
electric cell still leaves much to be desired. In picture trans-
mission, the conditions are not quite so exacting; for we
may use a very intense source of light focused on a small
portion of a specially prepared photographic film which
covers the photoelectric cell. In television, on the other
hand, we must deal with light reflected from the object to
be transmitted. The magnitude of the problem involved
here may be gleaned from the fact that the human face,
even in the lightest portions, reflects only about i/iooo of
the light incident upon it; nor can a source of high intensity
be used because of the discomfort occasioned the subject.

Oddly enough, what appears at present to be a solution
lies in just the action that early workers found vitiated
their photoelectric cells — gaseous ionization. Careful in-
vestigation has shown that the fatigue observed in gas-filled
cells is not a necessary concomitant of ionization. Rather
it appears that contamination of the photo-active surface
is responsible. If, then, an inert gas, such as argon or
helium, be carefully purified before admission to the cell,
we should be able to obtain the increased output occasioned
by the fact that the gas ions give us an increased number
of current carriers; without incurring the irregularities of
the poorly evacuated tube. This is just what is done in
practice. The purification of the gas to be used must be
performed with extreme care, as even the slightest trace
of impurity tends to devitalize the photo-active material.

The action of gaseous ionization may be employed in
a second tube rather than in the photo-cell itself. In this
case, the photoelectric action may be regarded merely as a
trigger which, through the gas-filled relay tube, releases

THE PHOTOELECTRIC CELL

large energy pulses. An example of this may be found in
the Knowles' Relay.8

Another feature of the potassium hydride photoelectric
cell, which causes some trouble in television, is that its sen-
sitivity is by no means the same for all colors. Reference
to the curve given in Fig. 30 shows clearly that the re-
sponse to red light will be only about % of that to violet,
for the same intensity. The usual photographic film pos-
sesses very nearly the same property; possibly for some-
what the same fundamental physical reason. In any case,
we may visualize the defects in the televised image by con-
sideration of the familiar snapshot.

Whereas a red object and blue object may seem of

58ooA

WAVE LENGTH

FIG. 30. — Sensitivity curve for a typical potassium hydride photo-sensitive
surface. Maximum occurs in the upper ultra-violet region, about

equal brightness when viewed directly, when photographed
on the usual film or plate, developed and printed, it is quite
possible that the blue one will appear much the brighter of
the two. It should be recalled that the color of objects,
when illuminated by white light, depends on the wave-length
of the light reflected. Or rather, a material will absorb
certain wave-lengths and reflect others — the net result or
color sum of those reflected is what we call the color of
the object. Hence a red substance does not necessarily

s See Chapter XVI.

56 TELEVISION

reflect only red, but the probabilities are that long wave-
lengths, typical of the red end of the spectrum, predominate
in the reflected radiation. Such an object will have but little
effect on a photoelectric cell; just as it has little effect on
the average light sensitized plate. In other words, red
subjects — or red parts of a subject — when televised will
appear dull on the reception screen, even though bright in
the original. If the neon lamp be used in reception, giving

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FIG. 31. — Sensitivity of caesium type photo-cell.

a red glow to the entire image, this effect becomes even
more unnatural.

Judging from the evidence on hand, this difficulty is
now well on the road to solution. The research laboratories
of the General Electric and of the Westinghouse Electric
companies have succeeded in producing a cell which gives

THE PHOTOELECTRIC CELL

very much better sensitivity in the red than any previous
type. Indeed, it will be noted from Fig. 31 that there
is a distinct peak in the region of 7500 A. The effect
is produced by a specially treated caesium surface. Quite

PHOTO-ELECTRIC

CELL

-III

FIG. 32fl. — The essentials of a photoelectric cell circuit.

likely subsequent research will develop other photo-active
coatings having different color characteristics, so that by
suitable blending, a response to all colors may be obtained
which is comparable to that of the human eye.

PHOTO-ELECTRIC
CELL

AMPLIFIER

AttPLIFIER

(Courtesy of Bell Telephone Laboratories.)

FIG. 32&. — Extension of the circuit of 320, to illustrate function of photo-cell
in a wire television system.

Before closing a chapter on the photoelectric cell, some
mention should be made of the wide variety of uses to
which it may be put, other than in the field of television or
picture transmission. As was stated above, the vacuum
type of cell is most satisfactory for work in which high pre-

58 TELEVISION

cision is required. This type may be used for photometry;
or with a relay, for the operation of various alarm systems
for protection against both fire and theft, the sorting of
materials according to color, the inspection of metals for
rust spots or flaws, the control of artificial illumination, etc.

PHOTO-ELECTRIC

(Courtesy of Bell Telephone Laboratories.}

FIG. 32 c. — Extension of circuit of 32 a, to illustrate function of photo-cell in
a radio television system.

One even wonders whether it has not been credited writh
superhuman intelligence when one finds that Dr. Phillip
Thomas of the Westinghouse Co. has devised a method
of traffic control for outlying districts, the brains of which
is a photoelectric cell!

CHAPTER VII
GLOW LAMPS

IN previous chapters, we have already learned the
fundamentals of television. At the sending end it is neces-
sary to convert varying light intensities into corresponding
electrical variations; then to change the latter biick to vary-
ing light intensities at the receiving end. We must, then,
have some device which reverses the action of the selenium
or photoelectric cell. Electro-magnetic valves are perhaps
the easiest to understand; but where the problem is one of
transmitting an animated object the inertia of the moving
parts of any such device make it impractical.

It is well known that the brightness of any electric lamp
depends upon the current through the filament. Yet in the
case of an incandescent lamp the heat is so great that as
much as a tenth of a second may elapse before a decrease
in current produces a corresponding drop in brightness.
Where changes in light intensity may reach as many as
20,000 variations a second, the absurdity of attempting to
develop a source of this type is obvious.

Fortunately there has been developed a lamp, known
as the glow lamp, which is able to follow variations in cur-
rent as rapidly as 100,000 cycles or changes per second.
This is all that could be desired. One of the best known
of these is the neon lamp developed by D. MacFarlan
Moore. Another the Aeo light, using helium, was developed
at the Case Research Laboratory. The last mentioned
is used extensively in photographing talking movies. Lights
using the glow from neon and from mercury vapor are now

60 TELEVISION

common in advertising signs. They are the tubular signs
which glow amber-red for neon and blue-violet for mercury.
Not infrequently mixtures of inert gases with mercury are
used to produce other colors.

To understand the action of these lights it is necessary
to know something of the fundamental nature of both
matter and electricity. These two are essentially one and
the same thing for it has now been established, beyond a
possibility of a doubt, that all matter is formed of two
kinds of building blocks — positive and negative electricity.
The ninety-two chemical elements differ only in the number
and arrangements of these blocks. From these ninety-two
all compounds may be formed by using the elements in dif-
ferent combinations. The number is almost without limit.
It is also established that, for each element, there is a cen-
tral positive nucleus which consists for the most part of
positive electricity held together by a few negative particles
or electrons, as they are called. Outside this nucleus, re-
volving around it much like the earth revolves around the
sun, are additional electrons to make up a total equal to
the number of positive particles or protons in the nucleus.
At the present time there is a discussion as to the exact
nature of the electrons, whether they may be regarded as
solid particles or as waves of some sort. The outcome of
this, however, will make no appreciable difference in the
picture here presented.

An atom is only in a stable condition when the totrl
number of electrons in it is equal to the total number of
positive particles. If for any reason a negative particle is
knocked out of the atom, another will sooner or later be
acquired. That is, if through some accident an electron is
torn away, the atom becomes at once on the lookout for a
replacement. The desired electron on entering the atom
and falling toward the nucleus loses energy. Since it can
only fall in steps, like a marble rolling down a flight of

GLOW LAMPS 61

stairs, the energy will be given out in pieces. The size of
these will depend upon the distance apart of the possible
orbits in which the electron may pause; just as the kinetic
energy lost by the marble, if it stops on a given step, depends
upon how great was the drop from its last resting place.
The energy which the electron gives out is detected by us
as light whose color is related to the size of jump from one
orbit to another. The drops in the case of the electron, as
for the marble, may be several steps at a time.

If electrons are in some manner set free from the atoms
and are made to flow through a wire, an electric current
is produced. Due to definitions introduced into the study
before the exact nature of electricity was known, the current
is said to flow in a direction which is the reverse of that
taken by the electrons. The current is said to flow from
plus to minus, whereas the electrons flow in the opposite
direction. As the positive particles are nearly two-thousand
times as heavy as the electrons, they are sluggish and hardly
move at all in comparison. This fact that electrons flow
in a direction opposite to the current, frequently leads to
confusion.

Now we know that, whereas electricity will flow through
a conducting wire under a few volts, it requires about
28,000 volts to the inch to make it flow through air. When
it does so, the form taken is a spark resembling a lightning
flash on a small scale. When such a spark is produced
between the ends of two wires or electrodes, it is because
electrons from one of them have been projected by the high
voltage with sufficient velocity to break up atoms of air
which they strike. The flash is due to the light given off
on recombination of electrons with atoms.

If the wires are sealed into the ends of a tube as two
electrodes and the air pressure reduced, the spark will pass
with much greater ease. In this case the electrons sent out
go much farther before striking an atom and so acquire

TELEVISION

enough velocity to break it up even when the voltage is
greatly reduced below the 28,000 volts to the inch value.
If we continue to reduce the pressure the spark becomes
quiet and fattens out, soon to fill the entire tube with a
glow known as the positive column. At this stage the
original electrons disrupt atoms' and the plus and minus
parts of these, joining the stream, in turn break up other
atoms. And so the action goes on. A condition of ioniza-
tion exists, where ions are everywhere in the tube.

The positive column eventually becomes separated from
the negative electrode by a dark space, the electrode itself
becomes covered with a luminous glow which extends over
its surface and is called the negative glow. From this we
go on to the striated condition and eventually to the x-rays,

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FIG. 33. — When a certain stage of rarefaction is reached in a discharge tube
the glow breaks up into striations.

but we need not consider these conditions for the present
problem. The glow in various parts of a partially evacuated
tube is shown in Fig. 33.

The neon glow lamp represents just such a phenomenon
as has been described above. It consists of two electrodes
in a tube of rarefied neon, evacuated to such a point that
the negative or cathode glow is present. The brightness
of this glow is very sensitive to current changes, a fact very
useful in television. As the current varies the number of
ions produced changes; the number of electrons falling back
into atoms changes, hence there is a change in the amount
of light produced. The entire effect takes place so rapidly,
as has been said, that variations as rapid as 100,000 per

GLOW LAMPS

second can be followed. In fact even this may not be the
upper limit.

Just why neon is used instead of some other gas may
be a question. The answer is given in the brightness which
can be produced by very feeble currents in that gas. The
next best is mercury vapor, but this is not practicable since

( Courtesy of Raytheon Mfg. Co. )
FIG. «.

( Courtesy Case Research Laboratory, Inc. )
FIG. 4#.

FIG. 34«. — The Raytheon Kino lamp uses neon as the rare gas. It is intended
primarily for amateur use.

FIG. 34&. — The Aeo light is used largely in making talking motion pictures.
The gas used is helium.

heating is required. Advertising signs are frequently made
with neon and mercury. The neon starts the sign and the
heat from the resulting current is sufficient, in fair weather,
to vaporize the mercury. In cold or windy weather such
signs are frequently streaked with the pink glow of neon.

64 TELEVISION

Neon has another considerable advantage aside from
the brightness obtainable. Being one of the inert groups
it does not readily combine with any materials or impurities
of the electrodes. Furthermore, neon is not subject to as
great absorption by the glass and other parts of the tube
as are other gases. Neither is it occluded to any extent;
later to be evaporated into the tube. As such tubes are
very sensitive to pressure variations all this is of the utmost
importance.

Types of neon tubes are shown in Fig. 34. Some are
so arranged that the negative electrode is in the middle,
for the purpose of concentrating the light for easy focusing
by a lens system. Small lamps of this type use only about
l/io watts. For this reason they are coming into use as pilot
lights to warn the user when the power is on in any elec-
trical device. A single one of these lamps uses so little
power that it will not turn the usual electrical meter.

Where a large viewing screen is to be shown to an entire
audience thousands of these little lamps would be required.
To avoid this the Bell Telephone Laboratories have devised
a multiple lamp of the same sort. This consists of a long
tubing through the center of which runs a spiral wire. The
wire constitutes one electrode and rectangles of foil pasted
on the outside of the tube at regular intervals act as the
other electrodes. The action takes place through the glass.
As contact is made with a given piece of foil the tube lights
up at that point. Thus we have the equivalent of a great
number of lamps; when, in reality, it is but a single tube.
The tube is bent back on itself a number of times so as to
form a rectangular screen, as will be noted in Fig. 35.
Its use will be better understood when the chapter on the
Bell Telephone system of television is read.

In the Case Aeo light used chiefly in making talking
pictures and shown in Fig. 34^, the gas is helium. The
tube is of glass, or sometimes quartz to allow the emission

GLOW LAMPS 65

of the ultra-violet rays, for their actinic effect is large. It
is a little over an inch wide and about six inches long. The
anode, or positive electrode, is nickel and the cathode, or
negative electrode, platinum coated with a mixture of

(Courtesy of Bell Telephone Laboratories.}

FIG. 35. — The neon lamp shown above is the equivalent of many of the simple
type. The tube, which winds back and forth, has a central electrode and at
frequent intervals there is pasted tin foil strips on the outside. When a con-
tact is made to one of these strips the tube lights up at that point.

alkaline earth oxides. The cathode is "U" shaped and is
activated during manufacture. The lamp operates on about
350 volts and draws ten milliamperes.

It is obvious that if lamps could be produced such that
three, or at least two, complimentary colors could be ob-

66 TELEVISION

tained that we should then have the necessary tools to secure
television in natural colors. Baird was partially successful
in color television in 1928. He used a neon bulb for the
red and a second bulb which combined the greenish-blue
of mercury with the blue of helium. The experiment was
a success in so far as the limitations of the lamps permitted.
A picture was sent first with one color and then with the
other, filters at the sending end being used to correspond
with the lamp colors at the receiving end. A commutator
in synchronism with the filter control threw in first one lamp
and then the other.

It would seem at first thought, that a system using three
lamps would require that the television process be increased
to a speed three times that of the single color type to avoid
flicker. As few objects are of so pure color as to appear
in one picture and not to appear, at least faintly, in another;
a much slower speed than would be supposed was sufficient.

Probably the only satisfactory way of describing the
operating characteristics of a glow lamp is to consider some
special one as an example. For convenience let us choose
the so-called Raytheon Kino lamp. (It should be under-
stood however that neither the authors nor publishers in
any way recommend any particular piece of apparatus. This
lamp is chosen only because considerable data on its opera-
tion is at hand and because its operation is typical of lamps
of this class.)

The Kino lamp does not attempt to reduce the neon
glow to a small spot for focusing, as do some of those
previously described; but rather spreads it out over a large
surface. The intention in this is that the negative glow
should cover an area equal to that of the framed picture
received. As the plates are about one and a half inches
square, this is the size of picture which can be received with
the lamp. The design is intended for amateur use. The
arrangement of the plates in this lamp is such that one may

GLOW LAMPS

view the negative glow on one of the plates without any
obstruction to vision from the other plate. Thus the life
of the tube may be prolonged by reversing the terminals
when one side of the tube has become blackened.

1.5

ST/A/O

D.C.

(Courtesy of Raytheon Mfg. Co.)
FIG. 36.

These lamps are current operated. In order to get
maximum contrast between the light and shade of a picture
it is necessary that their brightness change over a maximum
range with current change. The lower curve, Fig. 36,

TELEVISION

shows the relation between current in milliamperes and the
candle-power. The upper one shows the same relation with
Lamberts. A candle-power is distinguished from a Lam-
bert in that the former is a measure of the luminous inten-

MA/0 L4MP CHARACTERISTICS

20
/ RMS

(Courtesy of Raytheon Mfg. Co.)

FIG. 37. — As the current increases the ratio of maximum brightness to
minimum brightness increases.

sity which leaves the lamp; while the latter is measured by
the brightness of the light producing surface itself.

It will be seen that the more nearly vertical these lines

GLOW LAMPS

the greater the change of light intensity for a given change
in current. This then is an important feature of any such
lamp. Straightness is also desirable in the lines, otherwise
the comparison of light and shade will be distorted. If the

40,

tf/A/O LAMP

» J

( Courtesy of Raytheon Mjg. Co.)

FIG. 38. — The curve above shows the relation of maximum brightness to
minimum brightness from point of view of visual contrast.

curve became horizontal over any portion it is obvious that
here, at least, all variation of light and shade would disap-
pear in the received image.

70 TELEVISION

Furthermore, since the curve is a straight line and since
the eye is less sensitive to variations in light of high intensity
than of low intensity, to run the lamp at a high intensity
defeats the purpose of contrast as well as shortens the life
of the device. It will be seen however from Fig. 37
that as the current goes up the value of maximum bright-
ness to minimum brightness increases. On the other hand
the curve shown in Fig. 38 shows what the relation is
from the point of view of visual contrast; the visual contrast
being roughly proportional to the logarithm of the actual
contrast. From this it will be seen that nothing much is
gained by running the tube above forty miliiamperes.

In the operation of these lamps a background direct
current voltage is used sufficient to give about ten or twenty
miliiamperes through the tube. The alternating voltage
from the receiving set is impressed on this, but would not
be sufficient to light the tube without the assistance of the
d.c. voltage. It is thus the function of the d.c. voltage
to light the tube, whereas the a.c. from the receiver varies
the intensity corresponding to the light and shadows of the
scene. Sometimes, where a dark background is desired,
the d.c. voltage is adjusted just below the starting value,
the additional a.c. being sufficient to operate the lamp.

We have attempted to give a fair picture of the neon
lamp, its characteristics and limitations. In selecting a glow
lamp for television one should consider thoroughly all these
features as they affect the work in hand. The future will
undoubtedly see big advances in the construction of such
lamps. Greater intensity, greater variation with current,
better colors and longer life may be expected. The lack
of suitable glow lamps is at the present time one of the
greatest obstructions in television development.

CHAPTER VIII
OSCILLOGRAPHS

ONE of the most useful relations between electricity and
magnetism is the fact that a current-carrying wire placed
in a magnetic field has a force acting upon it. The direction
of this force is given by the so-called left-hand rule, as
follows: If the left hand is held so that the thumb and two
first fingers are mutually at right angles and if the thumb
points in the direction of the magnetic field (north to south
pole), and if the first finger points in the direction in which
the current is flowing, then the second finger points in the
direction in which the force will cause the wire to move.
Starting from the thumb we
have the directions indicated
as field, current, motion. This
is shown in Fig. 39.

It will help some in mem-
orizing and understanding this

rule if we think of the mag- J > Current

netic field as composed of lines
of magnetic force, the number
of which, for any given cross-
sectional area, depends upon Motlon

the Strength of the magnet, F.IG- 39— The above shows the rela-
' . , tive directions, field, current and

the distance from it and the me- motion,

dium in which it exists. These

lines are thought of as going from the north to the south
magnetic pole. In the case of a current-carrying wire,
similar lines are considered to surround it, and to run in a

72 TELEVISION

helix, proceeding around the wire in a direction like that
of the threads on a right-handed screw. The sense of these
lines would be the same as the direction of motion of a
point on a thread, if the screw were rotated in the direction
of current flow. Now it will be seen, that if the wire is
placed in a magnetic field, the lines from this field and from
the wire will interfere with each other. Figure 40 shows
that at the bottom of the wire, which is carrying current
into the paper, the lines from the magnetic field and from
the current interfere with each other; while the reverse is
true at the top. This will cause the wire to move down, an
effect in accord with the left-hand rule. All oscillographs
make use of this principle.

WIRE

FIG. 40. — If no current were flowing in the wire the magnetic field between
the north and south poles would be nearly uniform. If the wire were out of
the field of these poles its lines would be circular as they are pictured. When
these are brought into the field they result in distortion as shown and there is
a resulting force tending to push the wire out of the field.

The string oscillograph is nothing more than is illus-
trated in Fig. 41. A straight wire is stretched under ten-
sion between the poles of a powerful electro-magnet. As
the current through the wire varies, the latter is caused to
move from one side to the other, depending upon the direc-
tion of the current. The amount of movement depends upon
the value of the current, the magnitude of the magnetic
field and the tension on the wire. To produce a record
photographically, a hole is bored through the pole pieces of
the magnet and a light placed in line therewith. On the
opposite side of the magnet this light casts a shadow of the
wire on the photographic apparatus. If the film stood still

OSCILLOGRAPHS

there would be a blur produced by this shadow; but if it
is pulled through rapidly a curve is traced.

The instrument is extremely sensitive and can be used
for measuring changes in different parts of the body due
to the heart beat. It is used constantly to diagnose heart
troubles. An instrument of this type is properly called an
Einthoven galvanometer; but is frequently called a string
galvanometer; or when used for purposes of examining the
heart, it is called an electro-cardiograph.

FIG. 41. — The arrangement of parts of a string oscillograph is shown above.

The most common type of oscillograph, one which is
more likely to prove useful in television than the string type,
is that which uses a "U" shaped loop of wire strung between
strong magnetic poles. A small light mirror is attached to
these wires and is supported between them. The current
goes down one of these wires and up the other; thus in
operation one wire will tend to be pushed out of the field
in one direction and the other will move the opposite way.

TELEVISION

These combined motions turn the mirror and if a spot of
light is reflected from it the spot will move. Its motion
may be observed visually from a rotating mirror or picked
up on a moving film as in the case of a string galvanometer.
While the moving part is not as light as is the single string
of the Einthoven type, it is sufficiently light to follow vibra-
tions of several thousand per second. For this reason it
will be seen that a pair of such vibrators at right angles
could be used for directing a scanning spot of light and

FIG. 42. — A schematic diagram showing the fundamental parts of an
oscillograph.

could also be used to reproduce the televised picture at the
receiving end. As the inertia of the system to such rapid
motion is large, it can only be used with a small screen and
in general is not as satisfactory as other systems.

The means by which an electric current is carried
through a vacuum, or partial vacuum, was described in the
chapter on the neon lamp. If a vacuum is used a high
voltage is required because in this case the current must be
carried entirely by electrons without the aid of secondary
ions. If the electrode is cold this voltage may be fifty
thousand volts or more. If, on the other hand, the negative

OSCILLOGRAPHS 75

electrode or cathode is heated a relatively low voltage, of
the order of a hundred volts, is sufficient. In either case
the stream of electrons can be sent through a pair of pin

(Courtesy of Bell Telephone Laboratories.}

FIG. 43. — The Du Four oscillograph is used to record transient electrical effects
of frequency as high as one million cycles per second.

holes in line with each other and the beam restricted to a
pencil. These electrons will not spread to any extent due
to their mutual repulsion as they traverse the length of the

76 TELEVISION

tube in too short a time to make this possible when high
voltage is used. With a heated cathode and low voltage
the situation is different.

The electron beam directed through a tube is in every
essential the equivalent of a current in the opposite direc-
tion; consequently the stream of electrons must follow the
left-hand rule. Thus if a magnetic field is placed across
this beam it will be deflected, the amount and direction of
the deflection being dependent upon the value of the field
and the velocity of the electrons, which in turn depends
upon the voltage across the tube. The electron stream may
also be bent by causing it to pass between two condenser
plates which are oppositely charged, the electrons being
repelled by the negative plate and attracted by the
positive.

The first tube of this type was built by Sir J. J. Thomson
for the purpose of measuring the relation of the charge of
an electron to its mass and it is frequently called a Thomson
tube. The first adaptation of this tube as an oscillograph,
the moving part being an electron stream, was in the Braun
tube. This tube had the inner side of one end coated with
a substance which fluoresced under the action of the electron
bombardment so that a spot of light could be seen which
moved back and forth with any changes in the field across the
electron path. As the Braun tube had to be operated with a
very high potential, it required a correspondingly large
change in the magnetic field to bend the stream. Its useful-
ness was consequently limited.

Recently there has been developed a new cathode ray
oscillograph by Dr. J. B. Johnson of the Bell Telephone
Laboratories. This type employs a hot cathode and uses
a voltage of from 300 to 400 volts. While the tube would
operate on a lower voltage, this is used to give the electrons
sufficient velocity to cause a bright fluorescent spot on the
screen. Little is gained in brightness by the use of higher

OSCILLOGRAPHS 77

voltages but at lower voltages the brightness falls too low
to be satisfactory.

As has already been said the beam of electrons has a
tendency to scatter at this low voltage, as the time to
traverse the tube is much larger than at the higher voltages.
This is avoided by having a low pressure of gas in the tube
which is ionized along the electron stream. Both the ioniz-
ing and dislodged electron probably leave the stream; but
there remains a positive ion, which because of its great mass
relative to an electron, is comparatively sluggish. Thus
there is built up in the electron path a pencil of positive
particles and the core of the electron path may be said to
be a positive space-charge. It is estimated that at any one
time during operation that there are as many as four or
more positive ions for each electron along the path. The
electrons move in a tubular form inside of which is a posi-
tive space-charge and outside of which is a blanket of
negative charges thrown out by the ionization. Some elec-
trons returning from the target may also be in this outer
layer. These two space-charges both tend to hold the mov-
ing electrons to a narrow beam, the one by repelling them
inward, the other by attracting them inward. Because of
this anything that will tend to increase the number of ions
will tend to produce better focusing. Raising the tempera-
ture of the filament will send out a greater electron stream
which will produce more ions. Thus the filament control
determines the sharpness of focus. Argon is used in the
tube as argon atoms are the heaviest that can be used of
the inert gases. The lighter ions wander too easily from
the electron track and so focusing is more difficult.

The screen is made up of equal parts of calcium tung-
state and zinc silicate, both of which are specially prepared
for fluorescence. The tungstate gives a deep blue light and
the silicate, a yellow-green. The former is almost thirty
times as effective on a photographic plate as the latter; but

78 TELEVISION

the silicate is many times brighter visually than the tung-
state. A mixture of equal parts gives an excellent all-
purpose screen.

The tube fits into a bayonet type radio socket to which
all connections are made. It is fitted with two pairs of
deflector plates. All variations, which may best be meas-
ured by their voltage effect, may be connected to these. A
magnetic coil outside the glass of the tube may be used for
measurements of current variation.

It is not difficult to see how this tube might be used at
the receiving end of a television system. The electron beam
is limited somewhat in its rate of movement by the possibility
of leaving the positive ions behind at the expense of the
focus. For all practical purposes, however, the effect is nil.
Inertia then is not a problem; but, on the other hand, in-
tensity of illumination is. Perhaps if more intense beams
can be produced this device may become an important factor
in television reception.

A novel use of an oscillograph of this type has been sug-
gested by A. A. Campbell Swinton. At the receiving end
the cathode-ray oscillograph is of the standard type with
heated filament and fluorescent screen; but with two pairs of
magnetic coils, so arranged that their fields are at right
angles to each other. That pair which controls the transverse
motion of the beam has an alternating current through it of
a frequency of about 800 cycles and that which controls the
up and down motion a frequency of about ten cycles. Thus
the beam is made to traverse the screen as in any receiving
mechanism; but it is free from parts having mechanical
inertia.

At the sending end the tube is of a somewhat different
construction; for although the cathode ray beam is con-
trolled in the same manner as that already described, it falls
upon a screen which is composed of a great number of small
cubes. The cubes forming the mosaic are insulated from

OSCILLOGRAPHS

each other and contain some photoelectric substance; such,
for example, as is used in making photoelectric cells. An
image of the scene to be projected is focused on this mosaic,
and parts which are strongly illuminated give off more elec-
trons than those on which the darker parts of the image fall.

FIG. 44. — The circuit shown above is similar to that suggested by A. A.
Campbell Swinton. Light from the object, represented by an arrow, passes
through the lens, through a metal gauze, and then through the vapor chamber
to the photoelectric cell. The cell, in reality, is a mosaic of separate

small cells.

Behind this mosaic is a second chamber which contains
sodium vapor, or any other vapor whose conductivity in-
creases under the action of light. When the cathode-ray
falls upon one of the cubes of the screen the beam passes

8o TELEVISION

through it into the second chamber and across to the final
plate electrode. A current follows this beam from the cube
over to the plate connected to the grid of a vacuum tube
which it actuates in the usual manner. The current passing
depends upon the number of electrons given off from the
cube which, in turn, depends upon the intensity of the
illumination falling upon it.

While this system has never been put into actual practice
it would appear to be one which may eventually solve most
of our present-day television problems. Its chief drawback
is in the lack of a sufficiently intense cathode-ray beam which
can be supplied at a reasonably low voltage.

CHAPTER IX
SCANNING

IN some of the early experiments on television an at-
tempt was made to duplicate the action of the eye, to view
a picture as a whole and to transmit each portion of it
separately. In the attempt to do this, a honeycomb struc-
ture of selenium cells was made and the object placed in
front of it. Each selenium cell was connected by its own
pair of wires to the receiving system which consisted of a
number of lights behind shutters. Light falling upon one
of the selenium cells caused the corresponding shutter to
open and in this way a crude resemblance of the object was
produced. Rignoux and Fournier constructed such a system
in 1906 which consisted of sixty-four cells. Both Ruhmer
and Baird also constructed similar systems.

But the rods and cones which constituted the electrical
detectors in the eye number up into the millions and it is
obvious that to attack television from this angle is rather
hopeless. We should have to produce photoelectric devices
of extremely small size and at the receiving end have a
compact screen of neon lamps of much greater efficiency
than those now in use and of a size so infinitesimal as to be
beyond hope. In addition each of these tiny cells would
have to be connected with a pair of wires to the correspond-
ing lamp. This would require an immense cable of many
thousands of wires.

As a result of these insurmountable difficulties the trend
in television has taken a direction somewhat away from any
attempt to duplicate the human eye. The modern method

TELEVISION

requires that small portions of the picture be sent separately
and in rapid succession over a single pair of wires. The
resultant picture is thus made up of a large number of pieces
which have been separately transmitted.

In picture transmission no great difficulty is encountered
in this method. Either a spot of light is sent through a
transparency or is reflected from an opaque picture onto

a photoelectric cell. If the
picture is placed on a ro-
tating drum, which revolves
on a screw, the spot will
follow a spiral path as the
drum advances. The varia-
tions in the amplified photo-
electric current may be
made to operate a magnetic
shutter, or some form of
oscillograph, to produce
varying light intensity. Fre-
quently a light beam of
varying width is used. This
produces a picture made up
of lines of varying width as
shown in Fig. 45. Varia-
tions of this general scheme
are numerous.

When we come to tele-
vision, however, we have
before us the problem of
greatly increased speed of sending and the difficulty of put-
ting our object on a revolving drum, a method which would
first require photographing. The revolving drum must
accordingly be dispensed with and other means substituted.
Perhaps the simplest scheme is that used by the Bell
Telephone Laboratories which is shown diagrammatically

FIG. 45. — Close examination will show
that this picture is made up of a
number of horizo^aMines of varying

SCANNING 83

in Fig. 46. Light from an intense source is focused by a
lens in such a manner that it illuminates the whole opening
in a frame placed next to the disc. In the figure the source
of light is shown as an arc. The frame should be of such
a size that its length is equal to the distance between holes
in the disc, and its height should be the difference between
the radii of the inner and outer holes. If the wheel is sta-
tionary, light will come through a single hole in the disc
and there will be but one spot of light striking the face of
the subject. This will be true regardless of the position of
the wheel. The holes are arranged on a spiral in such a

Photo-Electric
Cell

Scene

Lamp

FIG. 46. — Above is shown the simple scanning system employed by the Bell
Telephone Laboratories.

relation to the frame that the outermost hole is level with
the top of the frame and they run progressively downward
until the innermost hole, that nearest the axle, is level with
the bottom of the frame. Thus when the disc is turned the
light passing through the holes makes successive strips
across the subject so that in one complete turn every part
of the subject has been passed over by a light spot. The
subject has been completely scanned. As the variations in
intensity of reflected light take place, three photoelectric
cells, only one of which is shown, produce varying current
intensity. A row of holes running diagonally across a con-

84 TELEVISION

tinuous belt would perform the same service; but difficulties
resulting from stretching or slipping would obviously enter
here. As it has no advantage over the wheel except the
compactness which can be procured by suitable pulley
arrangements, it is confined to laboratory and amateur use.
In the case of the circular disc it is advisable to make the
holes radial and not circular, as the latter system is inclined
to emphasize the strips. More light enters through the
centers of the holes, i.e., across their diameters, than enters
at the inner and outer edges of the holes. For this reason
they should be four sided and bounded by radii and con-
centric circles for best results.

When a spot of light is used for scanning it is not neces-
sary that the scene to be scanned should be in darkness.
The entire scene may be illuminated without interfering with
the action of the intense scanning spot. This of course
introduces a background of illumination whose effect on the
photo-electric cell is to produce a constant background of
current upon which the variations due to the scanning spot
are superimposed. It might be suggested that an intense
beam of light traversing the face of a subject would produce
discomfort, but such is not the case. As the spot must scan
the entire face in less than one-tenth of a second, if it is to
be transmitted, and as the eye is not responsive to changes
that take place in less time than this, it follows that the
eye does not recognize the spot of light as such. The spot
rests upon each portion of the scene only about one twenty-
five-hundredth of the complete time for scanning, so that
the eye recognizes only an increase in general illumination
of one twenty-five hundredth that of the beam when
stationary. The scanning process is so rapid that it is not
recognized by the subject as such.

The second system to be described is that due to Dr.
E. F. W. Alexanderson of the General Electric Company.
This consists essentially of a large wheel on whose rim

SCANNING 85

is mounted a number of mirrors each one of which varies
slightly in angle with the next. Thus one mirror will throw
a spot of light at one point on a screen, the next one will
throw it just to one side of this and so on across the screen.
When the wheel revolves a spot will be carried from top to
bottom of the screen by reflection from one mirror. The
next mirror will then come into play and will cover the
succeeding strip and so on. Thus the entire screen is
covered. In principle the procedure is the same as that used
in the disc with spiral holes. In order to adequately cover
a large screen, however, seven spots of light are used in
this system so that the speed of the drum may be reduced
to a reasonable value and the illumination correspondingly
increased.

A third system of scanning by means of a light spot 'is
that devised by M. Dauvillier. In this system two elec-
trically driven tuning forks with their prongs at right angles
are used. One vibrates eight-hundred hmes a second and
the other but ten times. If the high frequency fork produced
the spot alone it would traverse the screen back and forth
800 times a second. If the slower fork were used it would
move over the screen up and down ten times per second. With
both forks vibrating it does both these things, traversing the
screen rapidly and at the same time moving up and down.
Thus the entire screen is scanned ten times per second. In
this system there exists the possibility of using the current
which drives these forks for synchronizing. A pair of
oscillograph systems at right angles may also be used for
scanning.

We now come to a somewhat different system, one in
which the illumination is uniform and is not projected in a
pencil. Here the scanning disc is placed between the scene
and the photoelectric cell, whereas in all those systems so
far described, it was between the source and the scene. The
use of the disc between the cell and scene, as used in early

TELEVISION

experiments, was one of the great drawbacks as an extraor-
dinarily intense illumination was necessary. So much was
this the truth that in Baird's first experiments dummies were
used because of the heat and glare. With improved photo-
electric cells, however, this condition is no longer true.

In the Baird system in place of a spiral of holes, as
previously described, a spiral of lenses was used. This of
course gives greatly increased light collecting ability. Di-

FIG. 47. — Scanning arrangement used in the Baird system.

rectly behind this, revolving at high speed, was placed a
slotted disc which might be called a chopper since it suc-
cessively cuts off the light and allows it to go through. Be-
hind this is another rotating wheel in which a spiral is cut.
The arrangement is shown in Fig. 47.

The first disc, carrying the lenses, rotates at about 800
revolutions per minute and the slotted disc at about 4000

SCANNING

r.p.m. The effect of the slotted disc is to break the light
up into separate light impulses which produce separate elec-
trical impulses in the circuit. This has an advantage where the
changes in intensity are slight or zero; for, in this case, we
would otherwise have the equivalent of direct current am-
plification beyond the photoelectric cell — this is known to
be a difficult problem.

The rotating spiral, as will be seen from the mounting
shown in the figure, is of relatively slow speed. If the

FIG. 480.

FIG. 48 b.

FIG. 48. — As shown in a and b, the rotation of the slotted disc throws various

portions of the scene onto the photoelectric cell. It thus serves to divide it up

into smaller portions than would be the case with the lens disc alone.

lens disc were stationary and this spiral disc were revolved,
it will be seen that different parts of the scene would be
projected through the spiral to the photoelectric cell.
Figure 48 shows the disc in two extreme positions. Figure
48 (a) shows the spiral at its innermost portion which
throws the head of the arrow on the cell. Figure 48 (b)
shows it at its outermost point, so that the tail of the arrow
strikes the cell. As the lens sweeps across the scene the
rotation of this disc has the effect of dividing the image
produced into additional finer strips. With the combination

TELEVISION '

it becomes possible to make the strips so numerous as to be
little noticed.

A second system of scanning, devised by Baird, is known
as the optical lever. This system has the effect of greatly
increasing the speed of scanning without increasing the speed
of the mechanism over that of other systems. In this system
the transverse scanning of the scene is done by two or more
lens discs rotating in opposite directions; the up and down
movement is provided by a final lens disc. The arrange-
ment is shown in Fig. 49. The image is thrown on a
ground glass between each pair of discs placed as shown
in the picture. It may not at first be evident that rotating

FIG. 49. — This shows the Baird optical lever. The scanned image is thrown

onto a ground glass indicated by the vertical dotted line. This is scanned by

a second disc and so on. The last disc supplies the up and down motion.

the discs in opposite directions will speed up the scanning
process; but considering Fig. 50 should make it clear.

Figure 50 (a) shows what happens to the image of an
object when the lens is moved a short distance. The full
line represents the original position; the dotted line repre-
sents the position after the lens has been moved. Now let
us consider a pair of lenses, A and B, with the ground glass
G between them as shown. (Figure 50 (b) .) The lens A
will throw an image on the ground glass and this in turn
will be picked up and projected by the lens B as shown.
Now suppose we move each lens a distance which we will
call x, and which is identical with the distance the lens was
moved in Fig. 50 (a). The full line and dotted positions
again record the locations before and after the lenses were

SCANNING 89

moved. It will be seen here that the final image has been
moved over a much greater distance than was the case in
Fig. 50 (a). This gives the effect of a greatly increased
speed of scanning; yet the lens discs move at a relatively
low speed. This process can be carried on through addi-
tional stages but it is limited by the rapid diminution of
light as we pass from one lens disc to the next.

FIG. 50. — Moving a single lens will displace the image / as shown in a,
but moving two lenses A and B in opposite directions will give a greater
displacement as shown. B picks up the image from the ground glass screen G.

The Jenkins system uses what is in effect a variable
prism to bend a spot of light from one side of the scene to
the other, and another similar one to move it up and down.
The bending of light by a prism is a familiar phenomenon
and was described in Chapter III. The variable prism is
ground into the edge of a glass disc. The disc is bevelled
off at one point so that it forms a fairly sharp edge on the
glass; as we go along the rim the angle of the bevel gradually
becomes less and less, until, halfway around, the two sides

TELEVISION

of the disc are parallel. As we continue the angle slopes
the other way so that the rim which constitutes the prism
cuts into the glass deeper and deeper. At the completion
of the revolution the rim is almost
severed from the main part of the disc.
In practice one of these discs is used
to traverse the picture and another to
move the light up and down. The two
are so placed that at one point the two
rims are traveling at right angles. The
scene is viewed through the discs at this
point. A cross-section of one of these
discs is shown in Fig. 51 (a) and the
relation of the discs when in use is shown
in Fig. 51 (/?). The disc for trans-
_ _ verse scanning runs at high speed and
the one to produce the up and down
motion runs relatively slow. The sys-
tem apart from the prismatic disc is simi-
lar to the others previously described.
A suggested form of television de-
pends upon a cathode-ray oscillograph

FIG. 51 a. — A cross-sec-
tion of the Jenkins
scanning disc.

Fie. 51 b.

both for transmitting and receiving. As this is a highly
specialized use of a cathode ray-oscillograph and differs very
materially from all other systems of scanning it has already
been described in the chapter dealing with oscillographs.

CHAPTER X
SYNCHRONIZATION

THE process of television, and that of telephotography,
requires that the sending and the reception of the image
or photograph occur in unison. This operation is known
as synchronization — equal timing. The term immediately
suggests something in the nature of a clock control. This,
in essence, is exactly the system at first employed. In order
to understand the difficulties to which it is subject, let us
consider the method in some detail.

In passing, perhaps, some mention should be made of
devices whose speed is controlled by a fly-ball governor —
for example the common phonograph motor. Although
these may be suitable for the motive power of telephoto-
cylinders, they are subject to too much variation in speed
to be used without the checking action of some synchronizing
system.

Timepieces are controlled in two ways: either by a
pendulum or by a hairspring. The latter requires less space
and will function in any position; hence is best adapted to
portable mechanisms, such as watches. The pendulum on the
other hand, must be kept in a vertical plane but is con-
siderably simpler in construction and easier to make reliable;
hence is almost universally used in stationary clocks. Both
methods of regulation are subject to errors produced by
temperature changes. The trouble may be corrected by suit-
able compensation devices, both for the hairspring and the
pendulum; although, in general, automatic compensation is
cumbersome as well as expensive.

92 TELEVISION

Since the pendulum has the merit of great simplicity,
workers in the field of picture transmission early attempted
to employ it for synchronizing their sending and receiving
mechanisms. There are two possible ways to do this. We
may use two pendulums, one at each end of the line; or we
may employ only one pendulum, located at the transmitter
and sending a synchronizing signal, in the form of an electric
current, to the reception apparatus. At first glance, the
former might appear to be the simpler scheme, since no
energy link between the two stations is entailed. Attempts
to put the method into practice were made by a number of
early investigators; but without much success.

To a first approximation, we may say that the period
of a pendulum depends on its length; which, however, is
altered slightly by temperature changes. Where the two
instruments are not used in the same location, it is important
to remember that the acceleration of gravity is also a factor
in determining their periods and that the value of that factor
varies from point to point on the earth's surface. Hence
the difficulty of maintaining accurate unison between two
isolated mechanisms of this type proved well-nigh insur-
mountable.

Turning to the second method, mentioned above, syn-
chronization by a single pendulum, consider the apparatus
described by T. T. Baker.1 "One pendulum has been used
at the transmitting station, the rod being fitted with a spring
contact which strikes a second contact at the end of each
swing. This striking of the contacts throws into circuit a
relay, which actuates an electro-magnet, and thus releases
the cylinder. The receiver is also fitted with a similar electro-
magnet release and relay, and both relays are connected in
series through the telegraph line, the one pendulum thus
operating the synchronizing devices on both instruments. In

1T. T. Baker— "Wireless Pictures and Television," p. 58 et seq.

SYNCHRONIZATION 93

this way any fluctuations in period of swing become im-
material."

Since for commercial telephotography speed is extremely
important, we shall find a tendency to run both cylinders as
rapidly as the receiving and recording operations can be
performed. With the advent of the photoelectric cell and
the neon lamp, both inertia-free, came the possibility of
very much more rapid operation. So that the rotation of
the cylinders, between the synchronizing action of successive
pendulum swings, would be quite appreciable. In other

FIG. 52. — Electrically driven tuning fork.

words it would be possible for them to get considerably out
of step with each other, thus distorting the reproduction.
To avoid this a control is needed which is not only definitely
periodic in nature, but whose period is also very rapid.

The tuning fork answers these qualifications. Its period
depends on the density of the material and shape of the
fork, fluctuates but slightly with temperature and may be
made very much more rapid than that of a pendulum.
Figure 52 shows the way in which a tuning fork can be
electrically driven. The system is not so very different from

TELEVISION

that used in the common electric bell, the electro-magnet
supplies the necessary energy to keep the fork vibrating;
whereas the time at which current flows through the circuit
is determined by the period of the fork. We have, then, a
fixed current frequency which must be utilized to check the
speed of rotation of the cylinders.

A good example of the way in which an electrically
driven tuning fork may be used for timing purposes is seen
in the apparatus of Captain R. H. Ranger, used by the
Radio Corporation of America for picture transmission

FIG. 53. — Schematic diagram of circuit used for synchronization in the Ranger

system. Neon tube, at upper left, is connected to motor shaft, at upper right,

and is used as a stroboscope.

from New York to London. Figure 10.02 gives the details
of the circuit used. In this case the fork is encased in a
constant temperature box to obviate the variations in the
period of the fork produced by the expansion or contraction
of the metal. For simplicity this detail of the apparatus
is omitted in Fig. 53. The general appearance of the
temperature control system is illustrated separately in Fig.
54. The period of the fork is further checked by
an electro-magnetic control operated from an accurate

SYNCHRONIZATION

chronometer. (Note circuit containing crown piece above
prongs of the tuning fork.)

The frequency of the fork is used to check any cumula-
tive variation in speed of the direct-current, shunt field

'*WW

(Courtesy of Radio Corporation of America.

FIG. 54. — Mounting of synchronizing tuning fork in constant temperature box.

Ranger system.

96 TELEVISION

motor which drives the transmission or reception cylinder.
The motor is designed for a speed of 2100 revolutions per
minute, whereas the frequency of the forks is 4200; so that
the controlling action is brought into play twice each revolu-
tion of the motor. It will be noticed from the diagram that
in one position of the tuning fork prongs, the shunt field
of the motor is placed in parallel with the almost negligible
resistance of the auxiliary commutator segments and slip
rings, this will greatly decrease the field current, thereby
tending to speed up the machine. At the next position of
the fork, however, the variable resistance in series with the
field is shorted, so that an increase in field current will
occur; thus tending to retard the motor. By these extremely
rapid fluctuations in field current, the common proclivity of
electric motors to "hunt" — that is change speed cumulatively
due to some slight variation in line current — is pre-
vented.

The neon tube, seen at the left of Fig. 53, is em-
ployed as a method of determining visibly whether the
motor is running at correct speed or not. The tube is
connected mechanically to the end of the motor shaft so as
to revolve at the same speed as the motor. Electrically it
is connected with the tuning fork circuit so that a discharge
is produced for each vibration of the fork. When the motor
is running at correct speed, the tube should light exactly
twice each revolution. That is to say it should be illuminated
at the same two positions every revolution. Since the speed
is too rapid for an observer to receive distinct impressions
of each flash and since each occurs for the same position,
the tube will appear as if stationary. If the motor is turning
too slowly, successive discharges will occur closer together
in the circular path; hence the tube will appear to gradually
rotate backwards. On the other hand when the motor speed
is too high, the tube will seem to rotate slowly in the same
direction as the machine. A device of this type, a strobo-

SYNCHRONIZATION

scope, is helpful in many places where one desires to check
high rotary speeds.

Although the tuning fork represents a decided advance-
ment over the pendulum for a synchronizing control, some-
thing of greater simplicity is desirable for the high speeds
necessary in television. The most common system is to
employ alternating-current synchronous motors. The prin-
ciple of their operation may be understood by reference to
Fig. 55. In this case alternating current is sent through
the stationary electro-magnet, the stator. The drum, or

ROTOR

STATOR

FIG. 55. — Simple phonic drum. This illustrates the fundamental construction
of a synchronous A. C. motor.

rotor, may be made of wood carrying bars of iron on the
circumference. The periodic magnetization of the stator
will cause the iron strips of the rotor to be pulled around
at a speed dependent on the frequency of the current supply.
In practice the arrangement is often changed so that a.c.
is sent through the rotating armature and d.c. is used in
the stationary field. The speed of rotation then depends
on the frequency of the alternating supply and the number
of field coils.

Synchronous motors of this type might conceivably be
used in two ways: either transmitter and receiver could be

TELEVISION

controlled by the same constant frequency generator, or the
transmitter could be made to generate the frequency which
controls the receiver. Both methods require another energy
link between the two stations in addition to that which
carries the image. So that the television receiver, unlike the
radio set, must be designed to receive two distinct signals

simultaneously. To make
this possible, two different
carrier wave bands must be
transmitted — one modu-
lated by the scanning proc-
ess; the other, by the
synchronizing generator.
Since only very small
quantities of energy can
be sent from station to
station by means of radio
wraves, it will be necessary
not only to amplify the
synchronizing signal for
control purposes, but to
supply auxiliary power to
actually drive the receiv-
ing disc. To illustrate,
let us consider a system

FIG. 56. — Scanning-disc motor and speed • • 11 j ^u T7

control. The large unit, near the disc, is Originally due tO the Eng-

the d. c. drive motor; the smaller unit, at lish inventor, Baird.
the base of the same shaft, is the syn- r^, ,. - .

chronous motor used as a speed control. I he scanning disc ot the

sending station is driven by

a d.c. motor to whose drive shaft is coupled a small a.c.
synchronous generator. In this way, any tendency of the
motor to vary in speed will be reflected in a corresponding
variation of the frequency of the current generated by the
a.c. unit which is used to modulate the synchronizing
carrier wave. It will be clear that by reversing the process

SYNCHRONIZATION 99

at the receiving end, we should be able to keep the two
scanning discs turning at the same rate of speed at any
instant; albeit the speed may not be constant. Figure 56
shows a skeleton view of the main drive for the scanning
disc at the receiver and its synchronous control motor. Here
a.c. and d.c. units are again mounted on the same shaft;
although this time both are motors. The d.c. motor sup-
plies the power to drive the scanning disc; but without con-
trol, would be subject to speed variations. However, the
speed of the a.c. unit depends upon the input frequency
which comes from the amplified synchronizing signal as
received from the transmitter. Since both units are con-
nected to the same shaft, it follows that the speed of the
pair will be governed by the synchronous unit. It will be
understood, of course, that the drive motor is brought close
to correct speed by manual control. In short, the scanning
discs at transmitter and receiver will turn in unison.

So far, one important consideration has been neglected.
Though both scanning discs may be turning at the same
rate of speed, analogous parts of the two may not be
opposite the center lines of the respective viewing frames
at the same instant. This will result in a displacement of the
image from the center of the screen, not unlike the effect
sometimes seen in the motion picture theatre. Supposing
the subject to be a human being, we may see the legs at the
top of the screen separated by a dark band from the head
and trunk which appear at the bottom. The picture has
apparently been cut in two and the parts interchanged in the
projector. In television the difficulty can be corrected by
rotating the reception unit, casing and all, without changing
the drive and speed. For this purpose, a ring gear operated
from a hand crank is attached to the outside of the motor
casing. (See Fig. 56.)

The greater the periodicity of the synchronizing current,
the more frequent will be its checking action. It will there-

ioo TELEVISION

fore be advisable to use as high a frequency as circumstances
will permit for this purpose. In the demonstration given
by the Bell Telephone Laboratories during 1927 a frequency
as high as 2125 cycles was used. The main drive current,
on the other hand, may be either a.c. or d.c., whichever
is most convenient.

The foregoing discussion has been designed to give the
principles of the more common methods used for obtaining
synchronization both in telephotography and television. The
development of these concepts may be traced in the descrip-
tions of the various present-day systems to be given in sub-
sequent chapters.

CHAPTER XI
TELEPHOTOGRAPHY

TELEPHOTOGRAPHY, using the word in its broader sense
to mean the transmission of photographs either by wire or
by radio, has now reached the stage of commercialization.
In February, 1929, a number of American newspapers car-
ried reproductions of portions of Einstein's famous five
page manuscript which had been sent across the Atlantic as
radio pictures. Photographs which have been "wired" over
considerable distances are frequently seen alongside the
news report of the event. The larger telegraph offices are
prepared to transmit facsimilies of hand writing as part of
their daily routine. All of which goes to show that photo
transmission has arrived; although to be sure, there is plenty
of room for improvements, in the way of increased speed,
elimination of blurring due to static, and so forth.

In Chapter II an outline of the early experiments in
telephotography was given. For a more detailed description
of this field the reader is referred to one of the following
books: — T. Thorne Baker, "Wireless Pictures and Tele-
vision" ; Korn, "Handbuch der Phototelegraphie und Tel-
autographie" ; Work, "Bildtelegraphie."

In the preparation of this book the authors have felt
that material relating to picture transmission should be in-
cluded only in so far as the main subject, television, was
clarified thereby. For this reason only a few of the more
important present-day systems used in America are discussed.

In Chapter II, it was pointed out that photographs
might be transmitted directly or in code. Since the coding

101

IO2

TELEVISION

operation requires valuable time, we find that most com-
mercial systems are direct. Yet it will be evident that a
code message is less apt to be distorted by extraneous dis-
turbances than one in which the variations are relatively
small and continuous, as is the case in direct transmission.
For example, a Morse-code wireless message is much more
likely to be decipherable through bad static than is a radio
broadcast program. Hence a code method possesses an
advantage where the picture is to be sent over a very con-
siderable distance, such as across the Atlantic. In this
particular instance, there is another reason for the use of

ORIGINAL

TAPE

FIG. 57. — Appearance of original and tape which would be formed for various
sections thereof in Bartlane process.

code. The electrical characteristics of long cables render
them unsuitable for the transmission of modulated currents
such as are produced in any direct photo-scanning system.
On this account we find the Bartlane process, which employs
an extremely rapid, automatic coding of the photograph,
quite frequently used in transoceanic work. This ingenious
scheme is due to Captain M. D. McFarlane and H. G.
Bartholomew, two English inventors.

The first step is the preparation of five special prints

TELEPHOTOGRAPHY 103

made from the photographic negative. These are made on
sensitized zinc plates, each one being given a different ex-
posure so that each contains a different amount of detail.
Suppose, for simplicity, the original photograph had ap-
peared like Fig. 57, in which six shades from white to
black have been represented. Had all the plates been
exposed for the full length of time, they would all appear
like the original. If, however, the exposure times be cut
down in steps, since the prints are made by passing light
through the photographic negative, we may arrange them so
that the longest exposure will be effected by all the original
but part one, the next longest by all but parts i and 2, and
so on; the fifth plate being acted on only in the portion
corresponding bottom section in Fig. 57. The exposure
to light renders the plate coating soluble. So that after
developing and washing, the zinc plates are left bare at those
portions effected as described above; but are covered with
the sensitizing coat, which is a good insulator, at all other
places.

The next step is to mount all five zinc plates on a metal
cylinder, geared so as to revolve and move parallel to its
axis. Over each plate is placed a metal stylus which, before
the operation is completed, will have passed in a close spiral
path, over all points of its particular plate. The electrical
circuit for each stylus is reminiscent of many earlier designs
mentioned in Chapter II. It consists in a battery or other
source of e.m.f., an electro-magnet and is closed through
the metal cylinder to the stylus, provided no insulating
material intervenes between the two. Clearly, then, the
circuit is closed when the stylus passes over a portion of its
respective plate which has been acted on by light. Each
electro-magnet operates an arm designed to perforate a
special tape. Each line across this tape corresponds to one
process spot on the photograph, and as may be gleaned
from the foregoing may contain, anything from none up to

IO4

TELEVISION

five holes. Figure 57 illustrates the relation between the
tone quality of the original and the appearance of the tape.
The motor feed for the tape must of course be timed to
agree with the rotation of the zinc plate cylinder. Since
each spot of the original must be represented by a sufficient
length of tape to record the necessary perforations, it fol-
lows that a very considerable total length will be required.
For the transmission across the Atlantic of a picture of the
Hon. James J. Walker, Mayor of New York, 275 feet of
tape were required.

The first step in reception employs a device similar to
the common automatic telegraph receiver to perforate a

FIG. 58. — Schematic diagram of reception apparatus as used in the Bartlane

process. Note how special lens concentrates light passing through an entire

line of perforations onto one spot of negative.

second tape in exactly the same way as the original. It now
becomes necessary to reconvert the tape message into a
photograph. Figure 58 shows how this is accomplished.
The keystone of the system is the special lens which con-
centrates onto one spot of a photographic film whatever
light passes through a given line of tape perforations. If
there are five holes in one line the exposure of the film for
that part will be five times as great as for another portion
where the tape contains only one perforation. The receiving
film will obviously have to be mounted in a manner analogous
to that used for the zinc plates in transmission. From this

TELEPHOTOGRAPHY

105

point the process merely requires the treatment of the nega-
tive as in the usual type of photography.

Although this method involves coding, the apparatus is
so cleverly devised that a remarkably short time is needed
for the entire operation. When a photograph of the sinking
of the S. S. Antinoe was sent from London to New York,
thirty minutes were required for the preparation of the trans-
mitting tape, some five minutes for cabling, and only 1 1/±
minutes for the reproduction of a four inch by five inch
negative in New York. To be sure, the print was somewhat

FIG. 59.— Schematic diagram of Ranger system of picture transmission by
radio. Note light source inside cylinder which carries photograph. Photo-
electric cell output is amplified by special "condenser accumulator" (seen at
upper left), then applied through relay (center) to radio modulator (right).

lacking in detail but was quite satisfactory for newspaper
work.

Captain R. H. Ranger, of the Radio Corporation of
America, has developed a system of picture transmission
which is now in commercial service over long distances. The
scheme is typical of the direct method, requiring no coding
or special preparation of the negative. This negative is
clipped firmly in place on the outside of a glass cylinder.
A light source within the cylinder is sharply focused by a
lens onto one spot of the negative. That portion of the
illumination which passes through the negative is focused
by a second lens onto a photoelectric cell. (See Fig. 59.)

io6

TELEVISION

The cell output is then made to modulate the carrier wave
used for transmission.

The receiving mechanism of the Ranger system is made
to reproduce the picture in duplicate. One record is made
on paper by an inked pen, the other, on a photographic film
by a light beam. In both instances the material on which

(Courtesy of Radio Corporation of America.}
FIG. 60. — R. C. A. radio picture transmitter of recent design.

the reproduction is made must be placed on a cylinder, whose
rotation is synchronized with that at the sending station.
The synchronization is accomplished by tuning fork control,
a detail already discussed at some length in Chapter X.

The Bell Telephone Laboratories have developed an-
other system of picture transmission over telephone lines,

TELEPHOTOGRAPHY

107

which is in commercial service today. The schematic outline
of the method is well illustrated in Figs. 63 and 64.
The transmitter (Fig. 63) will be seen to differ from
the Ranger device in that the positions of light source and
photoelectric cell are reversed: in the Bell apparatus the
cell is inside and the source outside the cylinder carrying
the photographic negative. In the receiver (Fig. 64)
the amount of light falling on the sensitized paper or film

(Courtesy of Radio Corporation of America.}

FIG. 61. — Photo-radio transmitter and receiver as demonstrated by R. C. A.

recently.

is regulated by a light valve controlled by the received signal.
Tuning forks are used to produce synchronization.

The method for facsimile picture transmission discussed
by V. Zworykin of the Westinghouse Electric and Manu-
facturing Company, at the New York meeting of the In-
stitute of Radio Engineers, January 2, 1929, shows the
present tendency toward simplification of the apparatus and
greater speed of reproduction. To quote, "The chief object
of the design of this system was to produce a simple, rugged

io8

TELEVISION

apparatus for practical usage, which would not require the
attention of a skilled operator. The system does not require
a special preparation of the original, and the receiver
records the copy directly on the photographic paper. . . .

(Courtesy of Radio Corporation of America.)

FIG. 62. — Enlarged reproductions made by radio picture reception apparatus of

R. C. A.

In spite of the simplicity of operation, it is capable of trans-
mitting a five inch by eight inch picture, either in black and
white or in half-tone in forty-eight seconds, or a message

TELEPHOTOGRAPHY

109

at the rate of 630 words per minute — over short dis-
tances."

The great speed attained by this method is in a large
measure attributable to the fact that the original needs no

( Courtesy of the Bell Laboratories.)

FlG. 63. — Schematic diagram of photo-transmitter as used by the Bell
Telephone System.

( Courtesy of the Bell Laboratories.)

FIG. 64. — Schematic diagram of photo-receiver as used by the Bell Telephone

System.

special preparation to adapt it for transmission. The pic-
ture or writing to be handled is mounted directly on the
sending cylinder. Light from a constant source is concen-
trated on a small portion of this original. The reflected
illumination is collected by a parabolic mirror and thrown

110

TELEVISION

against the window of a photoelectric cell. Figure 65

shows the optical system used. The scheme suggests the
scanning methods used in television.

Since the intensity of the light reflected is extremely

(Courtesy oj the Westinghouse E. & M. Co., and of
the Institute of Radio Engineers.}

FIG. 65. — Optical system of the Westinghouse facsimile transmitter.

small, this system will require photoelectric cells of great
sensitivity and highly efficient amplification of their output.
The cell used is of the gas-filled type, the light sensitive
coating being caesium oxide; the gas, argon. The ionization

TO MOOULftTOR

OP TRANSMITTER

(Courtesy of the Westinghouse E. & M. Co., and of
the Institute of Radio Engineers.)

FlG. 66.— Photo-cell amplifier of the Westinghouse facsimile transmitter.

of the argon, when photoelectrons are emitted, greatly in-
creases the output of the cell. Under operating conditions
the photo-cell supplies a current of about ^20 of a micro-
ampere for the white portion of the picture. This must be

TELEPHOTOGRAPHY

greatly magnified before transmission. Figure 66 shows
the vacuum-tube amplifier used. It will be noted that two
screen-grid tubes are used, the third being the usual three-
element amplifier. The voltage output of the last tube
is in the neighborhood of forty volts, sufficient to operate
the modulator of the radio transmitter. The circuit must
be designed so as to be free from any tendency to oscillate
or otherwise distort the photoelectric currents.

For reception of the signals, a standard radio set, em-
ploying one stage of radio-frequency amplification with a

ROTARY S*»TC*T

ON CYLINDER
SHAFT

M. Co., and of
FIG. 67. — Glow-tube control circuit of the Westinghouse facsimile receiver.

(Courtesy of the Westinghousc E.
the Institute of Radio Engineers.}

screen-grid tube, a detector, and two stages of audio-
frequency amplification, is used. The output of this set is
utilized as shown in Fig. 67, to operate a neon glow tube.
The neon tube is designed to expose a small portion of
photographic paper placed on a receiving cylinder, which
must, of course, rotate in unison with the sending device.
Since white portions of the original cause the greatest photo-
cell currents, it follows that they would produce maximum
brightness in the glow tube; hence form the darkest portions
on the photographic paper of the receiver — i.e., the re-

112

TELEVISION

production would be a negative. In order that a positive
may be made directly the process must be reversed either
at the transmitter or receiver. If the reversal be made at
the transmitter, bursts of static would get the same inter-
pretation as dark portions of the original; that is, would
be reproduced as black spots. This is undesirable in the
transmission of material for the most part white, as is usually
the case. For this reason the reversal is made at the receiver.
The way in which the effect is produced is illustrated in Fig.
67. By placing the correct bias on the grids of the two tubes

SHUNT FIELD -

(Courtesy of the Westinghouse E. & M. Co., and of
the Institute of Radio Engineers.)

FIG. 68. — Synchronizing circuits of the Westinghouse facsimile reproducer.

whose plate circuits are in parallel, an increase of signal
from the receiving set will produce a decrease in the output
of the tube which controls the glow lamp. In other words
a bright part of the original will be recorded by a dimming
of the neon bulb, thus leaving the photographic paper un-
darkened.

Dr. Zworykin employs two electrically driven tuning
forks for synchronization. These are mounted in constant
temperature boxes, as in the Ranger system. The fork

TELEPHOTOGRAPHY 113

controls the speed of the d.c. motor used to drive the
transmission or reception cylinder as the case may be. The
way in which this is done may be seen from Fig. 69. The

(Courtesy of the Westinghouse E. & M. Co. and of the Institute of Radio
Engineers. )

FIG. 69. — Westinghouse facsimile picture transmitter.

period of the oscillations in a vacuum-tube circuit is fixed
by the fork. The oscillations are then amplified and im-
pressed on what may be regarded as an a.c. synchronous

1 14 TELEVISION

motor mounted on the same shaft as the d.c. drive. The
action of this combination has already been mentioned in the
chapter on synchronization. In order to keep the two forks
in unison, a synchronizing signal is sent from the transmitter
every revolution of the picture cylinder. This is sent over

(Courtesy of the Westinghouse Electric and Manufacturing Co.}
FIG. 70 a. — Westinghouse facsimile transmitter.

the same wave-band as the picture signals, but the record
is restricted to the margin to avoid confusion of the two.

In order to insure correct framing of the picture, it is
necessary to make certain that the cylinders not only rotate

TELEPHOTOGRAPHY 1 1 5

in unison, but also that corresponding parts pass under the
projection and reproduction light beam at the same time.

(Courtesy of the Westinghouse Electric and Manufacturing Co.)
FIG. 70 b. — Westinghouse facsimile transmitter, showing photograph to be sent.

This is accomplished by a stroboscopic action. The picture
is held on the transmitter by a black band running the length
of the cylinder. At the starting end this band crosses a

n6 TELEVISION

white strip which runs completely around the cylinc
While the projector beam is exploring this portion, the gl<

cylinder,
low-

(Courtesy of the Westinghouse Electric and Manufacturing Co.)

FIG. 71. — Westinghouse facsimile receiver. Note photographic dark bag over
cylinder on which print is made.

lamp at the receiver should flash once each revolution; that
is, when the projector beam falls on the dark band. At the

TELEPHOTOGRAPHY

117

receiver an interrupter is arranged so as to break the glow
lamp circuit for a length of time equivalent to the trans-
mission of the dark band once every revolution. Conse-
quently, should the two actions occur at the same time, the
glow-tube would not flash. Framing consists in attaining
this condition by "a process equivalent to rotating the frame
of the receiver motor."

Frame

fitti Han
de tends

(Courtesy of the Westinghouse Electric and Manufacturing Co.)

FIG. 72. — Original and reproduction as received by the Westinghouse facsimile

system.

Figure 69 shows a plan of the transmitter; the actual
instrument is seen in Fig. 70. The receiver is illustrated
in Fig. 71. It will be seen that both machines possess the
advantage of compactness. In Fig. 72 "are shown side
by side an original picture and the facsimile transmitted over
a short telephone line and a few miles of radio channel."

CHAPTER XII
THE BAIRD TELEVISOR

MR. J. L. BAIRD, who has for many years been identified
with the development of television apparatus, is generally
credited with having built the first really practical television
system. This he demonstrated before the Royal Institution
in January, 1926. The device, although but three years
old, appears crude in comparison with the improved systems
of the present day. Its crudeness, however, is rather in the
construction than in the principles involved, for no innova-
tion of any consequence has been made since his original
exhibit. Others have followed in much the path taken by
Mr. Baird. The original apparatus is now to be seen in
the South Kensington Science Museum.

The scene to be transmitted is strongly illuminated with
a number of incandescent lamp bulbs, placed in banks. In
the original apparatus this illumination was the source of
so much glare and heat that the system could not be used
to transmit pictures of people. Since then, however, im-
proved more sensitive apparatus has made this possible.

The scene is placed before a large lens-disc which con-
tains thirty-two lenses arranged in a spiral as shown in
Fig. 73. As the lens-disc revolves each lens in turn
scans a strip of the scene and projects the light it receives
from the scene onto a photoelectric cell. Thus the first lens,
farthest out from the center of the disc, projects light from
a horizontal strip across the top of the scene. The varia-
tions in the illumination from the scene along this strip are
projected in rapid succession onto the cell. When the first

118

THE BAIRD TELEVISOR

119

lens has passed across the scene the next one has just reached
it; so that each strip is scanned in turn. As the disc rotates
at a speed of 800 revolutions per minute this means that
the entire scene is scanned 800 times per minute or about
thirteen times a second. The picture is thus completely
reproduced at the receiving end thirteen times per second
and the persistence of vision of the human eye causes us to
interprets the picture as continuous.

FIG. 73. — Baird lens disc showing spiral arrangement by means of which the

scene is scanned.

Directly behind the lens-disc is a second disc, Fig. 74,
which revolves at a thousand revolutions per minute and
which carries sixty-four radial openings. It revolves in a
direction opposite to that of the lens-disc. The radial teeth
in this disc act as a chopper to cut up the continuous light
striking the photoelectric cell into a number of separate im-
pulses. The purpose of this is for better amplification of
the signals in the vacuum-tube amplifier. If, for example,

120

TELEVISION

a strip of the scene being scanned was of uniform brightness
the current produced by the light in the photoelectric cell
would be direct and unvarying if this radial disc were not
there. This would mean amplification of direct current, a
notably difficult problem. With the disc, the current is
started and stopped at a rate too great to be noted by the
eye at the receiving end and such as to make amplification
easy. The disc also gives a definite frequency to the tele-

FIG. 74. — A disc of radial slots is placed behind the lens disc and revolves in
the opposite direction.

vision current which is useful in filtering it from the syn-
chronizing current when both are sent on the same carrier-
wave.

Behind the radial disc is a third disc carrying a spiral
slot, Fig. 75. This disc revolves at a low rate of speed.
If the outer part of the spiral is before the cell, only light
from the bottom part of the strip being scanned enters the

THE BAIRD TELEVISOR 121

cell. If the inner part of the spiral is before the cell only
the lower part of the strip sends light to the cell. This
spiral acts then, to multiply the number of strips scanned
and is the equivalent of placing many more lenses in the
lens-disc. The arrangement of the discs and photoelectric
cell is shown in Fig. 76.

When the varying light strikes the photoelectric cell,
currents are set up through the cell which are proportional

FIG. 75. — A spiral disc, just in front of the photoelectric cell, produces "finer

grained" scanning.

to the light received and these are superimposed upon the
carrier wave of the usual radio transmission apparatus. If
the picture is to be transmitted over wires, the amplified
variations in current of the cell may be placed directly upon
them.

At the receiving end the entering signals are amplified
in the usual manner and if the energy is led into telephones
or a loud speaker it would be heard as sound. If the output

122

TELEVISION

is connected to a neon glow-lamp, as is done in the Baird
system, the brightness of this lamp will vary with the current
passing through it, as the lamp is current operated. From
here on, the system is the reverse of that at the receiving
end, the disc with the spiral slot and the lens-disc being used
to spread the light out on a ground glass viewing screen in
the same manner that the light was originally collected. As

FIG. 76. — Scanning arrangement used In the Baird system.

jjjfej
there is no longer any system of amplification involved the

radial disc is of no service and is accordingly omitted.

Of course, it will be obvious that the beam of light
reproducing the picture must, at any instant, be in a spot cor-
responding exactly to that which is being scanned by the
sending system. They must not only start at the same place
but they must be kept in step. Keeping them in pace is
accomplished by the synchronizing system. On the shaft

THE BAIRD TELEVISOR 123

driving the sending lens-disc there is, besides the driving
motor, an a.c. generator. The a.c. current generated
is sent out by the usual broadcasting system either on a
separate carrier-wave from that used in the television, or
on the same wave, later to be filtered out. In Baird's original
apparatus two sending and two receiving sets were used for
simplicity.

(Courtesy of J. L. Baird.)

FIG. 77. — British scientists at the Glasgow meeting of the British Association
inspecting a Baird receiver, September, 1928.

The a.c. from this generator is supplied after trans-
mission and amplification to a synchronous motor. This
motor is placed on the shaft which drives the receiving lens-
disc but does not itself drive the disc. A driving motor is
brought as nearly as possible to the correct speed by the
usual motor controls and the synchronous motor has just

1 24

TELEVISION

sufficient power to bring this driving motor into exact step
with the system.

The framing, by which is meant centering the picture
on the ground glass screen, can be accomplished by manual
operation of adjustments.

It will be seen that there are a number of limitations
to the system as described. The speed of scanning is limited
by the speed at which the disc can be rotated. The size
of the picture is limited by the light which the glow-lamp

( Courtesy of J. L. Baird.}

FIG. 78.— In the first successful television from London to New York a picture

of Mrs. Howe, at the left of the picture, was sent. This shows a group

assembled around the transmitter.

can supply. The sending signal is limited both by the sen-
sitivity of the photoelectric cell and by the strength of the
illumination of the scene. Mr. Baird, has, however, sug-
gested changes which overcome these difficulties, to some
extent.

THE BAIRD TELEVISOR

125

( Courtesy of J. L. Baird.)

FIG. 79. — A close-up of the first transmitter used in trans-Atlantic television

tests.

(Courtesy of J. L. Baird.)

FIG. 80. — Television receiver in operation on the "Berengaria" when in
mid-Atlantic.

126 TELEVISION

The scanning speed may be greatly increased by use of
a series of oppositely rotating lens-discs, each of which
throws an image onto a ground glass screen from which
it is picked up by the succeeding lens-disc. This system is
described in Chapter IX.

For increasing the amount of light falling on the receiv-
ing screen, he has made several suggestions. One of these
involves what is, in a sense, the placing of one picture
adjacent to another. This may be done, without using
several complete systems, by putting several lens spirals in
the lens-disc, and a corresponding number of spirals on the
radial disc. Each spiral has a different number of radial
teeth. There is a photoelectric cell for each spiral and the
output from each of these passes through a primary coil.
All primaries are coupled to the same secondary coil. At
the receiving end, the output from the different cells is
filtered out by their corresponding frequencies set up by the
radial teeth of the radial-disc. After filtering, the current
is sent to the proper glow-lamp and light therefrom is
projected by a disc similar to that at the transmitting
end. Each part of the picture is projected to its proper
place.

A second system suggested by Baird is to use a screen
which is made up of a number of neon lamps, forming a
mosaic. A motor revolving synchronously with the sending
disc carries a brush which passes over a commutator, thus
connecting one after the other of these lamps into the cir-
cuit. Each row of lamps corresponds to one hole in the
scanning-disc. Thus as one strip is scanned, each lamp in
the row corresponding to that hole will be lighted and the
brightness of the lamp will correspond to the brightness
coming through the hole at the corresponding point of the
scene. If enough of these lamps are used a steady picture
will appear because of the persistence of vision. As each
lamp must have two wires leading to it, an enormous number

THE BAIRD TELEVISOR 127

of wires are necessary for a screen of any size. This is the
chief barrier to a system of this kind.

FIG. 8 1. — Daylight television by the Baird system.

( Courtesy of J. L. Baird.}
FIG. 83.— A group before the daylight transmitter.

128

TELEVISION

FIG. 83. — Party of American and English journalists inspecting a picture being
received by daylight television. Mr. J. L. Baird to the right of the aparatus

is demonstrating.

FIG. 84. — A Baird system whereby a spot of light is projected to the exact
spot of the scene at the moment being scanned, by a double use of the

scanning-disc.

THE BAIRD TELEVISOR

119

FIG. 85. — A part of the color television apparatus used by J. L. Baird.

FIG. 86. — Mr. Baird demonstracting the first color television.

130 TELEVISION

Mr. Baird also suggested a method for overcoming the
difficulty due to the brilliant illumination found necessary
in his first apparatus. He used a light so placed that its
rays passed through a lens in the lens-disc other than that
which was, at the moment, scanning the scene. The result-
ing light spot struck the scene at the point then under the
scanning-lens. This gave a beam of light which fell upon
the exact spot being scanned at the moment. As this light
passed rapidly over the scene it appeared to the eye to give

( Courtesy of J. L. Baird.)

FIG. 87. — A phonovisor, in which the television sounds are recorded on a
cylinder, as in a phonograph, later to be re-created into a picture at any time

desired.

uniform illumination of but low intensity, whereas to the
television apparatus it gave, at any instant, a very intense
spot of light exactly where it was needed; the spot at that
moment being scanned. (Fig. 84.)

In another suggestion a spiral of concave mirrors was
placed on the front of the disc and the light was then on
the side toward the scene. A spot was reflected back at any
instant to the point being scanned.

THE BAIRD TELEVISOR 131

Mr. Baird in addition to being the first to successfully
demonstrate television in a practical manner was also the
first to transmit pictures by short wave radio apparatus
across the Atlantic Ocean. This was accomplished on Feb-
ruary 9, 1928, when pictures were sent from London and

( Courtesy of J. L. Baird. )

FIG. 88. — A recent model of the phonovisor for recording pictures by means

of waves on a cylinder. No picture appears on the cylinder, but when the

sound which it produces, used as a phonograph record, are changed back

into electrical impulses the usual television receiver will create the picture.

successfully received on the American side at Hartsdale,
N. Y., a suburb of New York City. He deserves much
credit for his pioneer work extending over years, for his
successes, and for his many fruitful suggestions.

CHAPTER XIII
THE BELL SYSTEM

ON April 7, 1927, the Bell Telephone Laboratories gave
a most elaborate demonstration of television both by wire
and by radio. The program presented at that time was
made possible by the coordinated research and development
work of the vast staff of technicians of the Bell System. In
describing the demonstration, let us use the words of Dr.
Herbert E. Ives, whose able guidance was in no small
measure responsible for the success attained by the Bell
Laboratories' experiments in television.

". . . In that demonstration television was shown both
by radio and by wire. The wire demonstration consisted
in the transmission of images from Washington, D. C., to
the auditorium of the Bell Telephone Laboratories in New
York, a distance of over 250 miles by wire. In the radio
demonstration images were transmitted from the Bell
Laboratories' experimental station at Whippany, New
Jersey, to New York City, a distance of 22 miles. Recep-
tion was by two forms of apparatus. In one, a small image
approximately two inches by two and one-half inches was
produced, suitable for viewing by a single person; in the
other a large image, approximately two feet by two and
one-half feet, was produced, for viewing by an audience
of considerable size (Fig. 89). The smaller form of
apparatus was primarily intended as an adjunct to the tele-
phone, and by its means individuals in New York were en-
abled to see their friends in Washington with whom they
carried on conversations. The larger form of receiving

132

THE BELL SYSTEM

133

apparatus was designed to serve as a visual adjunct to a
public address system. Images of speakers in Washington
addressing remarks intended for an entire audience, and of
singers and other entertainers at Whippany, were seen by
its use, simultaneously with the reproduction of their voices
by loud speaking equipment." *

The engineers of the Bell System set themselves the

IMAGE BEING RECEIVED IN NEW YORK
FROM DISTANT STATION BY AUDIENCE

BELL TELEPHONE LABORATORIES

b IMAGE BEING RECEIVED IN NEW YORK
"ROM PIS

STANT STATION BY INDIVIDUAL |

(Courtesy of the Bell Telephone Laboratories, Inc.}

FIG. 89. — Picturegram of the demonstration of television given April 7, 1927,

by the Bell system.

primary problem of transmitting the human face in satis-
factory detail, as it was felt that this was the most probable
requirement for a television service to be rendered in con-
junction with the telephone. A consideration of the half-
tone engraving process led to the conclusion that a 50 line
screen (i.e., 2500 dots per square inch) would give sufficient

1 From a paper presented at the Summer Convention of the A. I. E. E.,
Detroit, Michigan, June 20-25, 1927.

134 TELEVISION

detail for this purpose. Fortunately it is possible to transmit
images of this type of a size up to 5 x 7 inches, sixteen per
second, as is necessary in television, without exceeding the
frequency limits of a single communication channel — either
telephone wire or radio wave-band. Accordingly, this was
the structure of reproduction selected, and the operations
of scanning, transmission, screening and synchronization
were adapted thereto.

The arrangement used for scanning is well illustrated in
Fig. 90. Light from a source of high intensity (a 40
ampere Sperry arc, at the right of the photograph) is con-
centrated onto a small portion of the scanning disc. (The
latter may be clearly seen, together with its synchronous
motor drive, at the center of the apparatus table.) This
disc contains 50 small holes, arranged in a spiral near its
periphery. At any instant the illumination will strike several
of these apertures, but by means of a frame placed on the
side of the disc away from the light source, the beam coming
through just one will be selected and focused by a second
lens onto the subject being scanned. As the disc makes
approximately eighteen revolutions per second, the subject
is completely scanned by a very rapidly moving spot of light
that number of times each second. Though the intensity of
illumination is high, its transitory nature, in a system of this
type, prevents discomfort to the person being scanned.

The next step in the process is to pick up the light re-
flected from the portion of the subject being scanned and
convert it into some form of electrical impulse. For this
purpose the Bell System employs three large photoelectric
cells, as seen arranged in an inverted U, just in front
of the subject. Figures 91 and 92 give an idea of
the size and structure of these cells. They are of the potas-
sium-hydride, gas-filled type. The three, arranged as shown,
present an aperture of 120 square inches to collect the light
reflected from the subject. By connecting these cells in

THE BELL SYSTEM

13$

136

TELEVISION

parallel, a current output may be obtained which is above
the noise level of the amplifier system — that is, will not be

(Courtesy of the Bell Telephone Laboratories, Inc.)

FIG. 91. — One of the giant photoelectric cells, which served as the eyes of the
Bell Laboratories' tests.

(Courtesy of the Bell Telephone Laboratories, Inc.]

FlG. 92. — Detail of a photoelectric cell of the type used in the Bell apparatus.
Note large area of photo-sensitive coating.

THE BELL SYSTEM

137

confused with the extraneous circuit noises incident to the
amplifying circuits.

That the problem of rendering the output of the photo-
cel suitable for transmission is no inconsiderable one, will
be recognized from the fact that the power delivered to the
transmission medium is 1,000,000,000,000,000 times the

(Courtesy of the Bell Telephone Laboratories, Inc.]

FIG. 93. — Complete transmitter as developed by the Bell engineers. Note
separate units used for mounting intermediate amplifiers.

power received from the photo-electric cells. The amplifica-
tion must also be uniform over a range of frequencies from
10 to 20,000 cycles if the pictures are to be free from dis-
tortion. In the system employed by the Bell engineers, ten
stages of vacuum tube amplifiers were used to raise the
signal to a point where it would successfully override inter-

138

TELEVISION

ference encountered in transmission. The first two stages
are included in the frame which holds the photo-electric cells ;
the remaining eight are mounted in a special relay rack (see
Fig. 93. Owing to the large amplification and freedom
from distortion which is essential, transformer coupling
between stages was considered unfeasible and the resistance
capacitance type substituted in its stead. Figure 94 is
a schematic diagram of the first two stages of the amplifier.
It should be recalled that the photo-cell output is an
unidirectional current whose magnitude depends on the
general lighting conditions around the object being trans-
mitted; on this the fluctuations due to the light and shade

a SHIELDED AMPLIFIER
H ,

-HH1-

x. — j

(Courtesy of the Bell Telephone Laboratories^ Inc.]

FlG. 94. — Schematic diagram of the first two stages of the vacuum-tube
amplifier used with the photoelectric cells in the Bell equipment.

of the various portions of the object itself are superimposed.
Now, satisfactory amplification of a direct current presents
very considerable difficulties. So much so, that it was de-
cided to introduce this background current arbitrarily at the
receiver, making no attempt to transmit it either by wire
or radio. The results obtained by this system were quite
satisfactory and the amplifier characteristics could be
specifically designed to handle the alternating component of
the cell current.

THE BELL SYSTEM 139

In the determination of the electrical characteristics of
the amplifier, attention was given to the possibility of cor-
recting distortion produced in scanning. Since the scanning
spot has finite dimensions, its response to an abrupt change
in the surface being viewed will be less sharply defined than
the original. For example, take a surface such as illustrated
in Fig. 95. At the time the scanning spot crosses the
white to black boundary the cell output should drop abruptly.
As a matter of fact, there will always be a finite area
illuminated by the light spot (e.g., dotted circle) ; hence the
illumination received by the cell will depend on the total
amount reflected by this area. Clearly, then, the current
in the photo-cell circuit is related to the average coloration
of the area covered by the light spot; so that in the case

FIG. 95. — Circle denotes area covered by scanning-spot.

considered no sharp drop will be produced, but rather a
gradual decline as the proportion of dark surface under
illumination increases. This apparent sluggishness can be
greatly reduced by sharp definition in the scanning spot. In
fact for objects of soft tonal quality such as the human face,
little difficulty is encountered from this quarter. For more
extreme cases, however, such as black and white designs,
it was found possible to obtain markedly improved trans-
mission by suitable design of the electrical circuits used in
amplification. An explanation of the method used would
entail a somewhat involved discussion of electrical circuits
which the authors have felt beyond the scope of this book.
The interested reader is referred to Section II of a paper
entitled "The Production and Utilization of Television
Signals" by Frank Gray, J. W. Horton, and R. C. Mattes,

140 TELEVISION

to be found in the Bell System Technical Journal, for
October, 1927.

Considering next the actual transmission of the television
signals: two systems were used — wire and radio. Wire
facilities capable of transmitting a wide range of frequencies
were available between New York and Washington. The
characteristics of these channels were so well known that the
problem of adapting them to television requirements was
solved almost entirely in the laboratory. When the final
tests were made, the character of the images sent from
Washington to New York was not inferior to that attained
in short transmissions in the laboratory. For the April
7th demonstration two circuits were provided for picture
transmission, one being a spare for use in case of trouble;
a third circuit was used for transmitting the synchronizing
signal, which will be discussed later; a fourth for the speech
transmission; and a fifth for operating orders and so forth.

The problem of radio transmission proved more trouble-
some than the wire case, because of the severe crowding
of the "air" in the New York area. The difficulty is es-
pecially pronounced where television signals are to be sent, on
account of the great width of the frequency band needed.
Preliminary tests made with the available channels led to
the selection of a 1575 kilocycle band for picture transmis-
sion, a 1450 kilocycle band for speech and one of 185 kilo-
cycles for synchronization. Of the three, as would be ex-
pected, the picture signals gave the most trouble in trans-
mission. The portion of the station at Whippany which
contained the photo-electric cell circuits was completely
copper-shielded from antennae radiation; this was con-
sidered necessary because of the very great amplification
used in those circuits. A Western Electric 5-B Radio Broad-
casting Transmitter was modified so as to suit the special
requirements of television, under which conditions it gave
approximately a one-half kilowatt output. Tests made with

THE BELL SYSTEM

141

142

TELEVISION

THE BELL SYSTEM

the equipment indicated that the daytime was pre/erable for
transmission. Fading began with the sunset period and be-
came more pronounced as evening advanced. Coincident
with the fading of the desired image, an appearance of
"ghosts" was noted. These were readily seen to be similar
to the principal reproduction but incorrectly framed. The

( Courtesy of the Bell Telephone Laboratories, Inc.]
FIG. 98. — Television transmitting apparatus in the studio at Whippany.

effect was attributed to reception of signals which had
traveled over paths of different length, hence had required
different times for transit and were consequently out of
phase. The probability was that the main image was pro-
duced by energy coming by the most direct route; whereas
the "ghosts" represented energy which had traveled from

1 44

TELEVISION

THE BELL SYSTEM

the transmitter upward to the Heaviside, or conductive layer
of the earth's atmosphere, whence it was reflected to the
receiver. Calculation of the interval between the two signals
verified this conclusion, giving as a height of the reflection
surface about 60 miles — a value close to
that generally stated for the height of the
Heaviside layer.

The radio reception apparatus con-
sisted of a specially designed superhetero-
dyne receiver for the television signals,
one of standard design for the speech, and
a third receiver for the synchronizing
channel. In the television receiver a sys-
tem of triple detection was employed in
order to pass the wide frequency band
used, without too great a loss of selectivity.

The received television signal, after
amplification was impressed across the
electrodes of a neon discharge tube. The
tube, or glow-lamps, were made in two
very different forms; one for small pic-
tures to be viewed by a single person and
one for large projections, large enough to
be seen by a fair-sized audience.

Considering the apparatus for individ-
ual screening first, Fig. 100 shows the
neon tube, and Fig. 101 the way in which
it is mounted for viewing. The electrodes
of the tube are two metal plates placed
about one millimeter apart. The gas pres-
sure is so regulated that the glow discharge
develops on the outer surface of the nega-
tive plate, or cathode. The luminous sur-
face of this plate is viewed by the observer
through the holes of a disc similar to the

( Courtesy Bell. Tel.
Lab., Inc.]

FIG. 100. — Neon re-
ceiving lamp used
in Bell Laboratories'
experiments. The
rectangular cathode
is covered by a
uniform glow
slightly larger than
the field of view on
a television disc.

146 TELEVISION

scanner used at the transmitting station. This viewing disc
must revolve synchronously with the scanning disc; so that
at any instant, the observer is viewing a portion of the
luminous plate which corresponds in position to that part
of the image then being scanned. Since the brightness of
the glow discharge depends on the current passing through
the tube and this, in turn, depends on the output of the

(Courtesy of the Bell Telephone Laboratories, Inc.]

FlG. 101. — The individual receiving equipment as used in the Bell system

demonstration. Note mounting of neon lamp behind scanning-disc and

synchronous drive motor.

photoelectric cell at the sending station, the observer ac-
tually gets a series of rapid glimpses of a surface illuminated
proportionally to the corresponding parts of the object being
transmitted. So rapidly does the motion occur, 17.7 com-
plete transitions of the entire viewing screen (i.e., the dis-
charge tube cathode) each second, that the eye is conscious
of no discontinuity, unless it be a slight horizontal line-tex-
ture of the image. This may be corrected by allowing the

THE BELL SYSTEM 147

paths described by successive holes, as they pass the viewing
frame, to overlap slightly.

Whereas the image produced by the apparatus described
in the last paragraph was about 2 inches by 2^2 inches in

( Courtesy of the Bell Telephone Laboratories, Inc.)

FIG. 102. — Complete disc receiver apparatus of the Bell system. The observer

looks through the shielding \vindow at a picture some 2l/z inches square.

The 36-inch scanning disc is used.

size, another system was used that gave an image nearly
12 times that size. In the latter case the large neon tube
seen in Figs. 103, 104 and 105 was used. The tube is
bent back and forth, so as to give fifty parallel sections.
Each section contains fifty exterior electrodes, cemented on
the back side of the tubing. In this fashion 2500 picture
elements are produced, just as in the case of the small view-
ing frame. The operation of the large grid receiver is con-

148 TELEVISION

trolled by a 2500 wire distributor (Fig. 106) which plays
the same part as the viewing disc previously discussed.
Through the center of the tube runs a single spiral electrode
(Fig. 104), connected permanently to one output terminal
of the receiving set. Contact is made through the revolving

(Courtesy of the Bell Telephone Laboratories, Inc.)

FIG. 103. — This large grid, formed by a continuous neon tube bent back and

forth, is the electro-optical element of the receiving equipment used by the Bell

Laboratories for large audiences.

arm of the distributor between the second output terminal
of the receiver and the successive external electrodes of the
large tube. As contact is made to a given external electrode
a discharge will occur between it and the central electrode.
Due to the high frequency of the voltage used (500,000

THE BELL SYSTEM 149

kilocycles) the current will actually flow, by a capacity effect,
through the glass and luminescence will be seen on the inside
of the tube. It will be seen that if the distributor arm
revolves synchronously with the scanning-disc at the trans-
mitter, we may build up the enlarged image in this grid
receiver exactly as was done on the smaller screen.

( Courtesy of the Bell Telephone Laboratories, Inc.]

FIG. 104. — Detail of Bell grid-receiver. Note continuous spiral electrode run-
ning through center and external electrode elements placed at intervals on

back side of tube.

In the transmission of the television signal it will be
recalled that the direct current component of the photo-
electric output was not utilized, hence it becomes necessary
to introduce this background illumination at the receiver.
This is done by placing a suitable bias across the neon tube
so as to produce a steady state of current therein, on which
the alternations from the sending station are superimposed.

150

TELEVISION

With the large grid receiver, it was found that when a con-
siderable interval elapsed between discharges at a given elec-
trode, there would be a lag between the application of the
potential and the appearance of illumination at that point.
To correct this difficulty pilot electrodes were placed at the
bends of the neon tube thereby irradiating each branch and

(Courtesy of the Bell Telephone Laboratories, Inc.)

FIG. 105. — Final mounting of Bell grid-receiver. Note framework which
covers pilot electrodes.

keeping it in what might be called a sensitive state. These
pilot electrodes were hidden from the audience by the frame-
work placed around the receiver. (Fig. 105.)

Turning now to the method used for synchronization,
we find a system not unlike that previously described as due
to the English inventor, Baird. The Bell engineers took

THE BELL SYSTEM

as a standard for synchronization the requirement that the
sending and receiving discs should be not more than one-
half of the width of a picture element apart. Since there
were 50 elements in the entire width of the picture, which

( Courtesy of the Bell Telephone Laboratories, Inc.]

FIG. 106. — Distributor whose function is to send high frequency current to
each of the 2500 external electrodes of the Bell grid-receiver at the proper

time.

corresponds to the separation between each of the 50 holes
of the disc, it follows that the requirement set meant that
the two discs should not be more than % x %0 x %o of a

TELEVISION

revolution, i.e., .072 degree apart. The ordinary two-pole
synchronous motor will not approach this degree of preci-
sion. But increasing the number of poles improves the speed

"I'l'l'l'l'l'r—

( Courtesy of the Bell Telephone Laboratories, Inc.)
FIG. 107. — Circuits used for neon tube control by Bell engineers.

characteristics of such motors, and it was found that a
machine with 120 pairs of poles could, under favorable con-
ditions, be expected to satisfy these requirements.

(Courtesy of the Bell Telephone Laboratories, Inc.)
FlG. 108. — Assembly of scanning-disc motor used by Bell apparatus.

The speed is set for these motors by the necessity of
producing 17.7 complete pictures per second, as noted

THE BELL SYSTEM 153

previously. Each picture corresponds to one revolution of
the disc, hence the motor speed must be 17.7 r.p.s. or 1062
r.p.m. For 120 pairs of poles this gives a frequency of
17.7 x 1 20 or 2124 cycles per second. As a matter of fact
the machines were made of the variable reluctance type,
with 1 20 teeth on the rotor (see Fig. 108) in which case
only eight coils are needed. The tendency of these motors

FIG. 109. — Mounting of scanning

^--^y

( Courtesy of the Bell Telephone Laboratories, Inc.}
ig of scanning-disc drive as used by Bell apparatus.

to "hunt" (i.e., vary their speed of rotation) was checked
by a series condenser placed in the circuit which feeds energy
between them.

There still remains the question of the framing of the
picture, which implies that both discs be in the same relative
position at the same time. This may be accomplished by

( Courtesy of the Bell Telephone Laboratories, Inc.]
FlG. no. — Synchronizing circuit as used by Bell Laboratories in short distance

transmission over wires.

FIG.

. , . :. . .. • : • • £83 >*»

( Courtesy of the Bell Telephone Laboratories, Inc.)

in. — Large scanning-disc motor used in completed form of Bell
apparatus.

i54

THE BELL SYSTEM

155

manual rotation of the entire motor (note crank seen in
Fig. 109). However, it was found that too rapid rotation
of this control would throw the motor out of step. For this
reason the d.c. drive motors were fitted with a pair of slip
rings tapped to opposite commutator bars, so that, at 1062
r.p.m., they generated a 17.7 cycle current. With a two-
pole machine of this type there will be only one angular

SPEECH SIGNALS
(50-5000~)

TELEVISION SIGNALS
(I0-20000~)

INPUT TO RADIO TRANSMITTER
/ 10-20000- \
V25000-35000~/

30 -SUPPLEMENTARY CARRIER (30000-v)

20 25

OUTPUT OF RADIO TRANSMITTER
/I540-I550KC\
1555-1595 KC
\I600-I6IO KC/i^ RADIO
1545 CARRIER

1605
.1^

1540 1550 1555 1575 1595 1600 1610
( Courtesy of the Bell Telephone Laboratories, Inc.}

FIG. 112. — Diagrammatical representation of frequency conversions in
multiplex radio system.

position at which synchronization will occur. For a schematic
diagram of the arrangement see Fig. no.

Where the synchronizing signal must be sent over a
long distance either by wire or radio, large quantities of
energy cannot be transferred from the sending generator to
the receiving motor. In this case the energy must be at-
tenuated at the transmitter and amplified before use at the

iS6

TELEVISION

receiver. The circuits necessary to accomplish this energy
transfer may be found in a paper entitled "Synchronization

THE BELL SYSTEMS

157

of Television," by H. Stoller and E. R. Morton, published
in the Bell System Technical Journal, October, 1927.

158 TELEVISION

In the summer of 1928 the engineers of the Bell Labora-
tories announced a decided advance in their television trans-
mitter. Whereas, the earlier apparatus required that the
object be illuminated by light from a powerful electric arc,
the new development made possible the transmission of
scenes illuminated only by daylight. The system employs a
large lens which forms an image of the scene and it is this
image that is scanned much as the object was in the former
system. In this manner moving persons and objects at a
considerable distance from the lens could be successfully
scanned.

In concluding this chapter it is well to emphasize the
fact that the methods herein described represent a super
refinement of television now possible only with the vast tech-
nical equipment and trained staff of an organization like
the Bell System. In the words of Walter S. Gifford, Presi-
dent of the American Telephone and Telegraph Company,
at the opening of the demonstration, April 7, 1927: "The
elaborateness of the equipment required by the very nature
of the undertaking precludes any present possibility of tele-
vison being available in homes and offices generally."

CHAPTER XIV
THE JENKINS SYSTEM

THE lone experimenter whose lack of equipment is a
constant goad to his ingenuity and whose endeavor is en-
tirely of his own choosing, has played, in the past, no incon-
siderable part in the development of new fields of endeavor.
Nor is the research laboratory of our titanic industrial cor-
poration ever likely to entirely replace him. The inventive
mind is sine qua non a free lance; only with difficulty can
it be caged in the toils of a large organization. Though the
superb coordination of the engineering and research staffs
of the Bell Telephone Laboratories has given us the most
elaborate demonstration of television; the individual efforts
of J. L. Baird in England and C. F. Jenkins in America
have come much closer to realizing the ideal of home enter-
tainment by radiovision.

C. Francis Jenkins has long been a worker in the field of
optics. To him is generally accredited the invention of the
motion picture machine. He is also the holder of an Elliot
Cresson gold medal, awarded by the Franklin Institute of
America for original contributions to the field of motion-
picture mechanics.

As early as 1923, he was able to give an official demon-
stration of the transmission of pictures by radio over a
distance of about seven miles. The most interesting part
of the apparatus then used was the prismatic ring, a device
original with Mr. Jenkins. In describing it he says: "the
prismatic ring is equivalent to a glass prism which changes
the angle between its faces, and in rotation gives to a beam

i6o

TELEVISION

of light having a fixed axis on one side, a hinged or os-
cillating axis on the other." The device can be visualized
by imagining a glass wheel with its edge ground to the form
of a triangular prism; but at no two points on the circum-
ference of the wheel would the angle between the faces of
the prism be the same. For half a revolution, the base of
the prism is toward the center of the wheel and for half a
revolution it is toward the periphery : the angle between the
faces varies continually.

The result of this construction is that a beam of light
traveling parallel to the axis of the disc and focused at a
point near the periphery will be refracted a varying amount

as the wheel is rotated.
Assuming the wrheel to
be spinning about a hori-
zontal axis, the light
beam, after passing
through the top of the
prismatic disc, will move
up and down in a vertical
FIG. n5.— Mounting of Jenkins' prismatic plane perpendicular to

the face of the wheel.

Now if the light beam should pass through the side of a
similar disc, it would be made to move back and forth in a
horizontal plane, also perpendicular to the face of the wheel.
By the combination of these two actions with two discs
mounted as shown in Fig. 115, an object may be scanned
completely by the emergent light beam. In practice the
horizontal displacing disc rotates one hundred times as fast
as the one which gives the vertical displacement; hence the
object surface would be covered in one hundred horizontal
strips.

More recently, the name of C. F. Jenkins has been
associated with a very definite attempt to bring radio movies
into the home. It must be understood that this is not yet

THE JENKINS SYSTEM

161

true television, for only silhouettes from a motion picture
reel can be received. However, the advance represented by
the simplification of the receiving apparatus so that an equip-
ment suitable for viewing by a group of five or six people
is as compact as the average radio set, and costs but little
more, is certainly worth of mention.

The device by which this is made possible is another
ingenious optical piece original with Mr. Jenkins — the drum-
scanner. The great merit of the construction used in this
drum is that it makes possible a picture of good intensity
with low current input to the glow-lamp. In scanning
through a disc the area of the cathode of the neon lamp

( Courtesy of Jenkins Laboratories.)
FIG. 116. — Jenkins' drum-scanner. Note quartz spokes.

must be a little larger than the total size of the picture
reproduced and the entire plate must be illuminated; al-
though at any instant, only a very small portion is being
viewed. Whereas in the drum method used by Jenkins, the
cathode of the tube is, in effect, divided into four parts and
only that part being viewed is illuminated. Furthermore,
the light from the lamp cathode is concentrated onto the
viewing aperture by a quartz rod. By the combination of
small plate area and effective use of illumination, it is claimed
that a picture of sufficient size to be seen across a room
can be produced with as low as 5 milliamperes input to the

\6i TELEVISION

glow lamps; as compared to some 50 milliamperes required
for a 2l/2 inch square picture formed with a disc-scanner.

Another decided advantage of the drum-scanner is its
compactness. Where a disc is used for scanning, the
minimum separation of the apertures determines the width
of the picture and the offset of the ends of the spiral fixes
the height. If we employ a 48-line image and make suitable
allowance for framing and size of the apertures (Vk inch),
we find that a disc of 36-inch diameter is necessary even to
produce a picture 2 inches square. Where a drum is used,
the aperture spiral may be divided into a number of parts
spaced along the axis of the drum, so that the number of
holes per revolution is decreased and the periphery need not
be so large. We may say that the four-turn drum used
by Jenkins in his radio-movies receiver need only be one-
quarter the diameter of a disc which would produce the
same size image. And here we have neglected the magnifica-
tion made possible by the intensification of the image in the
Jenkins' drum. Nor is there any fundamental objection to
increasing the number of parts into which the aperture spiral
is separated.

A detailed description of the drum as used in the radio-
movies receiver will probably serve to clarify the preceding
discussion. The drum is a hollow cylinder about 7 inches
in diameter and 3 inches long. On its surface are punched
48 holes about %4 inches in diameter and arranged as if
on a screw thread which makes four complete turns in 2
inches along the cylinder (i.e., has a pitch of y2 inch). A
quartz spoke connects each one of these elemental picture
areas with the hub of the drum. This hub connects, at one
end, with the motor drive shaft, but for the length of the
drum is hollow and about i y2 inches in diameter. Inside
the drum-hub, but not attached thereto, a glow-lamp is
mounted. The lamp is of special design, having four
cathode plates about %G by % inch in size, one under each

THE JENKINS SYSTEM

163

turn of the quartz rods; but only one anode, running the
entire length of the under side of the tube.

(Courtesy of Jenkins Laboratories.)
FIG. 117. — Mounting of Jenkins' drum-scanner.

(Courtesy of Jenkins Laboratories.)

FIG. 118. — Jenkins' radio-movies receiver. Note magnifying lens and mirror
on top. The drum and its drive are inside the box.

With such a device the picture is built up, one-quarter
for each complete revolution of the drum. The drum shaft

164 TELEVISION

is equipped with a commutator so that the negative input
from the radio amplifier is connected to each of the four
cathode plates of the neon tube in turn. The connection
remains on a given plate during that revolution of the drum
in which the picture is being constructed by the turn of the
quartz rods over that particular plate. By the reduction
of the glow area in this fashion, considerable brightness
may be produced by a small current. This illumination is
most effectively utilized by employing a quartz path for the
light from the tube to the scanning aperture. In this case

(Courtesy of Jenkins Laboratories.)
FlG. 119. — Complete radio and radio-movies equipment. Note comparative size.

the internal reflection at the walls of the quartz tube tends
to reinforce the light within the tube cross-section.

The drum described gives a picture two inches square.
It will be noted from the accompanying photographs, how-
ever, that the picture is viewed through a magnifying glass.
The resulting image appears about 6 inches square. The
entire outfit is very compactly arranged; the motor with its
controls and the drum being in a neat box on which is
mounted the viewing lens. Since the drum shaft is hori-
zontal, a mirror inclined at 45° is used to change the light
from a vertical to a horizontal path.

THE JENKINS SYSTEM 165

For synchronization the Jenkins' "radiovisor" set relies
on the standard 60 cycle alternating current supplied by
the power lines. He claims to have had no difficulty with
synchronous motor drives of this type even when trans-
mitting from Washington to New York.

CHAPTER XV
ALEXANDERSON SYSTEM

THE Alexanderson system of television has its chief
value in the receiving and projecting equipment rather than
in the transmitter. In fact a transmitter of the type to be
suggested has a number of distinct disadvantages. For this
reason let us examine the projector in detail and leave the
remaining part of the equipment for brief discussion at the
end of the chapter.

The projector consists of a drum on the periphery of
which is mounted a number of mirrors. In Alexanderson's
first apparatus there were twenty-four of these, each one
being eight by four inches in size. They are mounted on
the rim so that they are normal to a radial line of the drum.
In other words their position corresponds to the tread of
an automobile tire. Each mirror is set at a slight angle to
its neighbor. Thus if a reflected ray from the first of these
mirrors falls upon one side of a large viewing screen the
same ray reflected from the last one will fall on the opposite
side of the screen. The intermediate mirrors will cast the
ray to intermediate positions and thus the entire screen is
covered as the wheel revolves. Each mirror sweeps the ray
from top to bottom of the screen as it passes the incident
beam. These rays are referred to in Alexanderson's original
paper as the "paint brushes" which paint the picture on the
screen. As this is done over and over again with sufficient
speed to take advantage of the persistence of vision the
scene painted appears to be continuous.

The advantage of this system of projection over others

166

ALEXANDERSON SYSTEM 167

is that without any complication it can be used with a mul-
tiple spot. Alexanderson used, in fact, seven light spots.
The multiplicity of spots has several advantages. Perhaps
it is best to describe these in his own words as given before
the St. Louis section of the American Institute of Electrical
Engineers, December 15, 1927.

"When the drum revolves, the spot of light passes across

(Courtesy General Electric Co.)
FIG. 120. — The Alexander projection drum.

the screen. Then as a new mirror which is set at slightly
different angle comes into line, the light spot passes over
the screen again on a track adjacent to the first and so on
until the whole screen is covered. If we expect to paint a
light picture of fair quality, the least that we can be satisfied
with is ten thousand separate strokes of the brush. This
may mean that the spot of light should pass over the screen

168 TELEVISION

in one hundred parallel paths and that it should be capable
of making one hundred separate impressions of light and
darkness in each path. If we now repeat this process of
painting the picture over and over again sixteen times in a
second it means that we require 160,000 independent strokes
of the brush of light in one second. To work at such a
speed seems at first inconceivable; moreover, a good picture
requires really a scanning process with more than 100 lines.
This brings the speed requirements up to something like
300,000 picture units per second.

"Besides having the theoretical possibility of employing
waves capable of high speed of signalling, we must have a
light of such brilliancy that it will illuminate the screen
effectively, although it stays in one spot only one-three
hundred thousandths of a second. This was one of the
serious difficulties because even if we take the most brilliant
arc light we know of, and no matter how we design the
optical system, we cannot figure out sufficient brilliancy to
illuminate a large screen with a single spot of light. The
model television projector was built in order to study this
problem and to demonstrate the practicability of a new sys-
tem which promises to give a solution to this difficulty.

"The result of this study is briefly that, if we employ
seven spots of light instead of one, we will get 49 times as
much useful illumination. Offhand, it is not so easy to see
why we gain in light by the square of the number of light
spots used, but this can be explained with reference to the
model. The drum has twenty-four mirrors and, in one
revolution of the drum one light spot passes over the screen
twenty-four times; and when we use seven sources of light
and seven light spots we have a total of 170 light spot
passages over the screen during one revolution of the drum.

"The gain in using seven beams of light in multiple is
twofold. In the first place we get the direct increase of
illumination of 7 to I but we have the further advantage

ALEXANDERSON SYSTEM

169

that the speed at which each light beam must travel on the
screen has been reduced at a rate of 7 to i, because each
light spot has only 24 tracks to cover instead of 170. While
the light itself may travel at any conceivable speed there
are limitations of the speed at which we can operate a
mirror drum or any other optical device and the drum with
24 mirrors has already been designed for the maximum
permissible speed. A higher rate of the light spot can

(Courtesy General Electric Co.)
FIG. 121. — A broadcast studio sending a drama by television and radio.

therefore be attained only by making the mirrors correspond-
ingly smaller and a mirror one-seventh as large will reflect
only one-seventh as much light. The brilliancy of the light
spot would therefore be only one-seventh of what we realize
by the multiple beam system, which gives seven light spots
seven times as bright or forty-nine times as much total light.
"There is another advantage in the use of the multiple
light beam. Each light beam needs to move only one-

170 TELEVISION

seventh as fast and therefore needs to give only 43,000
instead of 300,000 independent impressions per second. A
modulation speed of 43,000 per second is high with our
present radio practice but yet within reason, being only ten
times as high as we use in broadcasting.

"The significance of the use of multiple light beams may
be explained from another point of view.

"It is easy enough to design a television system with
something like 40,000 picture units per second, but the
images so obtained are so crude that they would have very
little practical value. Our work on radio photography has
shown us that an operating speed of 300,000 picture units
per second will be needed to give pleasing results in tele-
vision. This speeding up of the process is unfortunately one
of those cases where the difficulties increase by the square
of the speed. At the root of this difficulty is the fact that
we have to depend upon moving mechanical parts.

"If we know of any way of sweeping a ray of light back
and forth without the use of mechanical motion, the answer
might be different. Perhaps some such way will be dis-
covered, but we are not willing to wait for a discovery that
may never come. A cathode ray can be deflected by purely
electromagnetic means, and the use of the cathode ray
oscillograph for televisions has been suggested. If, how-
ever, we confine our attention to the problem as first stated
of projecting a picture on a fair sized screen, we know of
no way except by the use of mechanical motion. If we also
insist upon a good image, we must reduce the dimensions so
that we will have only one-forty-ninth as much light. Our
solution to this difficulty is, not to attempt to speed up the
mechanical process, but to paint seven crude pictures simul-
taneously on the screen and interlace them optically so that
the combination effect is that of a good picture.

"Tests have been made with this model television
projector to demonstrate the method of scanning the screen

ALEXANDERSON SYSTEM

TELEVISION

with seven beams of light working in parallel simultaneously.
The seven spots of light may be seen on the screen as a
cluster. When the drum is revolved, these light spots trace
seven lines on the screen simultaneously, and then pass over
another adjacent track of seven lines until the whole screen
is covered. A complete television system requires an inde-
pendent control of the seven light spots. For this purpose

(Courtesy General Electric Co.}

FIG. 123. — In directing a television drama, the director uses a receiver to
enable him to see the effect as his audience will see it.

seven photoelectric cells are located in a cluster at the trans-
mitting machine and control a multiplex radio system with
seven channels. A Hammond multiplex radio system may
be used with seven intermediate carrier waves which are
scrambled and sent out by a single transmitter and then
unscrambled at the receiving station so that each controls
one of the seven light beams.

ALEXANDERSON SYSTEM 173

"Seven television carrier waves may thus be spaced 100
kilocycles apart and a complete television wave band should
be 700 kilocycles wide. Such a radio channel might occupy
the waves between 20 and 21 meters. If such use of this
wave band will enable us to see across the ocean, I think all
will agree that this space in the ether is assigned for a good
and worthy purpose.

(Courtesy General Electric Co.)
FlG. 124. — A scanning apparatus as used in Alexanderson's later experiments.

"How long it will take to attain this, I do not venture
to say. Our work has, however, already proven that the
expectation of television is not unreasonable and that it may
be accomplished with the means that are in our possession
at the present day."

The Alexanderson drum does not lend itself to use at
the transmitting end as it does at the receiving end. Here

TELEVISION

the method would be to project an image of the scene onto
the mirrors by means of a lens. It would require that the
scene be illuminated as a whole, and that the mirrors be
scanned by seven apertures which, by suitable lens systems,
cast the rays received by them into their respective photo-
electric cells. The intensity of illumination necessary on the
scene would be large, an objection which applied to the
original Baird system. It is possible of course that an ex-
ploring spot, or spots, could be used, provided that at any
instant their position on the screen corresponded exactly with
those being explored on the mirror image. This would
introduce the synchronization problem and its added tech-
nical difficulties. One synchronization system between trans-
mitter and receiver is all that most people care to be troubled
with.

On the other hand there is no objection to the use of
a multiple spiral disc such as that suggested by Baird. This,
used at the transmitting end in conjunction with the Alex-
anderson drum at the receiving end, would make an ad-
mirable combination. Since, in later apparatus, Alexander-
son used a spiral disc at the transmitting end, it is likely
that such an arrangement will constitute the essentials of any
system which he may eventually develop.

CHAPTER XVI
RELAYS

FREQUENTLY, in electrical work, there is available only
a weak current from the primary source; and yet it is re-
quired that this current operate apparatus requiring con-
siderable power. This is the case in long-distance trans-
mission, or in radio transmission, where the conditions are
such that only feeble currents reach the receiving apparatus.
In such cases it is necessary to use a relay; a device, which
controlled by the primary current, in turn controls a
secondary current capable of operating the machines or ap-
paratus being used.

Relays are naturally of many varieties to meet the great
number of requirements of every-day practice. In general,
they operate by passing the primary current through coils
of an electro-magnet which pulls over the contact to com-
plete the secondary current. The usual telegraph relay is
of this type. The principle of the electro-magnet is too well
known to require explanation.

The simple telegraph key is one of the class of non-
polarized instruments. It matters not what direction the
primary current has; the secondary contact will always re-
act in the same way. It will close the circuit in the secondary
when a current flows in the primary.

For many purposes this is not sufficient, and so polarized
relays have been developed. These are made to close one
circuit when the flow of current is in one direction and to
close another when the current reverses. It will be seen that
an adaptation of the usual moving-coil galvanometer is

175

176 TELEVISION '

capable of doing this. The moving-coil of such an instru-
ment hangs between a pair of permanent magnetic poles.
The magnetic polarity of the coil will depend upon which
direction the primary current flows through it. (Fig. 125.)
Its face will turn either toward the north or south mag-
netic pole, depending upon the direction of the primary cur-
rent. The electrical contact for the completion of the
secondary current may be made by a pointer attached to
the moving coil being carried over to a fixed contact on
either side of its swing. In actual practice, however, the
torque which turns the coil is not sufficient to hold the con-

tacts firmly together. On this ac-
count the apparatus is frequently
built so that a pointer carried by
the coil moves between the two
pairs of jaws, one pair on either
side. These jaws open and close
periodically but never make elec-
FIG. 125.— A moving coil gal- trical contact except when the

vanometer may be used as a inter attached to the COll IS
polarized relay. A pair of J .

jaws on either side of its swing caught between the jaws. I he con-

o the °ther is

tween a pair then the circuit then made through the material of
is completed on that side. ^ po;nter Th;s offers & means

of periodically adjusting a current in the secondary to
a change in direction of current in the primary. It may
also be arranged so that the pointer is normally on
one side. Thus for a weak current it will remain there,
but for a stronger one, depending upon the torsional value
of the coil suspension, it will go over to the other side. This
offers a means of controlling secondary currents by a fluc-
tuating value of the primary current. Instruments built
upon this principle and carrying a recording pen are used
for recording temperature changes, light changes, etc.

An alternative method of construction is to use a per-

RELAYS

177

manent magnet, or a bar of iron magnetized by the inductive
effect of a nearby permanent magnet, as the moving part.
The incoming current is now passed through the magnetizing
coils of the fixed electro-magnetic pole pieces. As a change
in direction of current changes the polarity of the fixed pole
pieces the moving magnet may be attracted to the one side
or the other, depending upon the direction of this current.
The rod supporting the permanent magnet, or magnets, also
carries a contact point which may close either one circuit

FIG. 126. — In this type of polarized relay a pair of vane-like magnets are
fastened to a stem. These are magnetized by a pair of large magnets not
shown. Change in direction of current in the primary cause these vanes to
turn one direction or the other and contact is made by a contact point shown
at the top of the figure.

or another, depending upon the direction of swing. The
arrangement will be obvious from Fig. 126.

While it is customary where different frequencies are
sent over the same circuit, or on the same carrier wave, to
separate them by means of electrical filters, it is not impos-
sible to do this directly by means of tuned relays. One relay
of this sort which has been suggested has a number of
tuned reeds arranged much after the manner of an har-
monica. These reeds are placed next to an electro-magnet
operated by the current received or by a corresponding

178

TELEVISION

amplified current. The vibration of the tuned reed causes
the current to vary in a coil an amount depending upon the
extent of the reed vibration. As this vibration depends upon
resonance with the received variations in current, the
secondary will only be actuated when the frequency of the
reed is equal to or nearly equal to the fre-
quency of the received signal.

Perhaps the most unusual type of relay
is the Knowles device. This relay has an
appearance resembling the radio vacuum-
tube. It has three electrodes arranged as
shown in Fig. 127. A rare gas, usually
neon, is present at a pressure of about one
centimeter of mercury. When a voltage is
applied across terminals P and TV, a positive
space charge is soon built up around the P
terminal which is itself positive. This space

FIG. 127.— The

Knowles relay.

charge prevents any further flow of current. If, however,
the hooked terminal G is grounded or made negative the
space charge is dissipated and current is allowed to flow.
Such a tube has an amplification factor of about a million
to one. It may be readily actuated by the feeble effect of a
photoelectric cell and in this respect acts as a very positive
control; much more so than is the case with the usual
vacuum-tube amplifying arrangement.

CHAPTER XVII
AMATEUR EQUIPMENT

ALTHOUGH most of the manufacturers of radio parts
are prepared to supply the amateur with scanning discs, neon
lamps and other special requisites for a television receiver,
when the outfit is assembled there is little guarantee of any
definite entertainment to be obtained therefrom. At the
time of writing, there is a discouraging lack of uniformity
in the spasmodic attempts which are being made by the
various broadcasting stations to transmit television pro-
grams. Too much emphasis can hardly be placed on the
fact that television is still in the experimental stages of
development.

On the other hand, the adventurer who gropes his way
into new realms is certain to be rewarded with thrills never
experienced by the man who trods only well-beaten paths.
There are those who derived keen enjoyment, some ten
years ago, in adjusting the "cat's whisker" of their crystal
detector so that, with almost superhuman auditory acute-
ness, they might hear a phonograph record being played in
a broadcasting station some ten miles away; albeit the same
record lay in their own rack not ten feet distant. To such
as these, television now offers a fruitful field of endeavor.
The disappointments are apt to be many; but the explorer
does not go without reward.

From the point of view of the development of the art,
there is nothing more beneficial than widespread amateur
interest. Not only does it act as a stimulus to organized
commercial progress in the field, but often leads to the dis-

179

i8o TELEVISION

covery of new ideas and new talent. Few will deny that
amateur enthusiasm over radio at the close of the World
War contributed more than anything else to the perfection of
the present-day broadcast receiver. Television, today, is
sorely in need of just such a boost.

The authors feel that amateur television equipment is
at present in such a state of flux as to make the discussion
of any particular "hook-up" inadvisable in this book. How-
ever, the interested reader will find no difficulty in obtaining
detailed information of this character from the various
manufacturers of radio parts, photoelectric cells, neon
lamps and so forth. The following brief discussion con-
siders the problem only in the most general terms.

Recent action of the Federal Radio Commission indi-
cates that television signals are likely to be restricted to
the short wave-length bands, below those generally used in
speech and music broadcasting. For this reason the tele-
vision set must be adapted to short wave reception. There
are a number of standard kit sets of this type on the market.
The use of a 222, or screen-grid tube, ahead of the re-
generative detector has the advantage of increasing the sen-
sitivity of the receiver and preventing radiation from the
set. Aside from the usual electrical shielding, which good
practice dictates in all set construction, it is well to remember
that mechanical vibration from the scanning-disc motor must
be guarded against when building a television receiver. For
this reason, all parts should be securely fastened in place
and the set, as a whole, cushion mounted.

In the discussion of television signals given in the chapter
on the Bell system, it was pointed out that to produce a
good half-tone image a wide frequency range was necessary.
This means that transformer coupled amplification can
scarcely be used without introducing distortion. Only re-
cently have transformers been developed whose reproduction
was faithful over the normal range of audible frequencies

AMATEUR EQUIPMENT 181

(16-5000 cycles). For the "audio" amplification in a tele-
vision set, we would expect uniform results over a range
of from 1 8 to 20,000 cycles. To be sure, a recognizable
picture may be obtained when standard transformer coupling
is used, and for initial experiments such a "hook up" may
be good enough. Where the best results are desired a
resistance coupled amplifier is probably much to be pre-
ferred. One manufacturer recommends three stages of this
type of amplification between a regenerative detector and
the neon tube, employing a 240, a II2A and 171 A tube
in the order named and with the usual B and C potentials
applied. The tube characteristics dictate the constants of
this portion of the circuit as in any other radio amplifier.

The exact nature of the neon tube circuit will depend
on the nature of tube used. In general a background direct
current of about 20 milliamperes is desirable. To produce
this current a permanent voltage must be supplied across
the tube; but a high resistance must be inserted in series
with the tube because of the tendency of ionization, which
occurs when a current flows through the gas, to decrease the
resistance of the tube itself. This series resistance should
consist in a high permanent portion, for safe-guarding the
lamp, and a variable portion, for controlling the current to
the lowest satisfactory value.

The scanning-disc and its motor drive, as previously
noted, should be so placed that vibrations from them will
not be introduced into the tube circuits. These synchronous
vibrations evince themselves as horizontal wavy lines ap-
pearing across the picture. To avoid this trouble, the best
policy is to mount the disc and motor drive in a unit entirely
separate from the receiver and amplifier circuit. The motor
will probably be a 60 cycle a.c. type of about % horse
power rating, fitted with a rheostat for speed control. A
good method to employ here is to have one resistor which
may be varied for general speed control, and another, of

182 TELEVISION

smaller value, which may be shunted, when the need arises,
with a push button held in the operator's hand. Unfor-
tunately, programs requiring 24, 36 and 48 hole discs are
all "on the air," which means that the experimenter will
probably wish to have the facilities to receive them. Either
the three types must be at hand so that a change can be
made when needed, or a special combination scanner must
be used. The latter, as produced by one manufacturer,

(Courtesy of the Westing house EL and Mfg. Co.)

FIG. 128.— Dr. Frank Conrad, Assistant Chief Engineer, Westinghouse Electric
and Manufacturing Co., adjusting his television motion picture equipment.

merely requires a shift in the neon lamp position in order
to make the shift from one to the other class of program.
In the discussion of scanning, it was pointed out that radial
holes lead to less "lining" of the image; discs of this kind
are available.

The successful operation of a television receiver is
naturally an art which takes some time to acquire. A few

AMATEUR EQUIPMENT

183

pointers may prove nelpful. The tuning of the receiver is
not very different from a similar operation in the usual radio

184 TELEVISION

set. The signal may be heard by attaching a loud-speaker
(with a microfarad condenser in series, for protection)
across the input terminals of the neon lamp circuit. "If
you are getting a good television signal, it will sound very
much like a slowly revolving circular saw which is slightly
off center. In other words, you hear a high pitched note
which might correspond to the tooth frequency and this is
broken up into groups whose frequency corresponds to the
rate at which the saw (the disc) rotates." a

The framing of the image is an operation requiring
some skill in handling the motor speed control. Where the
image persists in appearing inverted, however, the lamp
plate is obviously being scanned from bottom to top, instead
of from top to bottom. The fault can be corrected by re-
versing the direction of rotation of the disc, or by turning
it so that the side formerly toward the lamp is now toward
the viewing frame. Where the image appears right side
up but transposed horizontally (that is, viewed from right
to left instead of from left to right) the correction is more
troublesome. The direction of rotation of the disc and also
the side facing the lamp must be changed. Where the image
obtained is a negative of the original, reversal of the input
leads to the neon tube will correct the difficulty.

The Jenkins, the Westinghouse, and the von Mihaly
systems of radio-movies all promise good "picking" for the
amateur. But, to date, the authors have insufficient material
at hand to give any helpful information, other than what
has already been given in previous chapters.

1 Bulletin TS-io, Raytheon Manufacturing Company, Cambridge, Mass.

CHAPTER XVIII
THE FUTURE OF TELEVISION

TEN years ago one of the authors of this book was
extremely skeptical of the possible success of television. The
speed required to successfully accomplish television, the state
of photoelectric cells, the lack of suitable sources, seemed
insurmountable difficulties. He is now writing the final
chapter to the first American book on television.

With this in mind the situation is a difficult one. For-
tunately his ideas of ten years ago are not in print. Ten
years from now the situation will be different. Shall we
go to the limit in our predictions and line up with the "Jules
Vernes" of the day or even with those wild spectacular
writers who out-Verne Verne in some of our present daily
publications? Or shall we line up with the rank and file
of humdrum, unimaginative engineers who still almost deny
the existence of even the steam locomotive; perhaps, because
of the fact that one is considered a good scientist by scientists
if he is ultra skeptical of future developments?

Undoubtedly it will be wise to steer a middle course;
or better yet, to predict what television will be like a hun-
dred, rather than ten years from now. In that case the
remarks are likely to be forgotten in suitable time. Let us,
however, discuss the attitude taken by leading men of the
present day.

Mr. M. H. Aylesworth, President of the National
Broadcasting Company, tells us that television is coming in
our homes. But he advises that this should not stop us

185

1 86 TELEVISION

from buying a present-day radio set. We shall have time
to buy three or four before we are able to buy a combined
radio and television set.

In opening the demonstration of television given by the
Bell Telephone Laboratories, April 7, 1927, Mr. W. S.
Gifford, President of the American Telephone and Tele-
graph Company remarked: ". . . The elaborateness of the
equipment required by the very nature of the undertaking
precludes any present possibility of television being available
in homes and offices generally."

The advance in the next ten years cannot help but be
astounding. The invention is in its infancy and it is at this
stage that most rapid growth takes place. Let us take the
case of the modern automobile; let us assume that the
progress of television is as far advanced. Could we expect
much change in ten years? Dr. Charles F. Kittering,
Director of Research for the General Motors Company,
sums this up in an article in Nation's Business. His remarks
are as follows :

"A few weeks back I was sitting with a group of execu-
tives. All were admiring a new model.

' 'It is absolutely the best automobile that can be made,'
enthused one. I objected to that statement.

" 'Let's take this automobile which you say, is the "best
that can be made" and put it into a glass showcase,' I said.
'Let's put it in there — seal it so no person can possibly touch
it. Just before we seal it in the case let us mark the price
in big letters inside the case.

" 'Let us do that and come back here a year from today.
After looking at it and appraising it, we will mark a price
on the outside of the glass. It will be a price something
less than what we think the car is worth today. Probably
$200 less. Then, let's come back once every year for ten
years, look through the glass, and mark a new price. At
the end of ten years we won't be able to put down enough

THE FUTURE OF TELEVISION 187

ciphers to indicate what we think of the car. That is, of
course, eliminating its value as junk.

" 'In those ten years, no one could possibly have touched
the car. There could be no lessened value through handling.
The paint would be just as good as new; the crank case just
as good; the rear axle just as good; and the motor just as
good as ever.

'What, then, has happened to this car?

' 'People's minds will have been changed; improvements
will have come in other cars; new styles will have come.
What you have here today, a car that you call "the best
that can be made," will then be useless. So it isn't the best
that can be made. It may be the best you have made and,
if that is what you meant, I have no quarrel with what
you said.' '

If this is true of the automobile, how much more is it
true of television? How true has it been of radio in the
last ten years? Less than ten years ago I tuned in my first
home-made radio; a crystal, a few coils on a breakfast-food
box, and some telephones. I heard WWJ from a point
about forty miles from Detroit. It was remarkable! A
parade of neighbors filled the house each evening. A face
would light up with "Yes, I can hear it now." Today we
expect an almost exact reproduction of the studio rendition.
I was astonished when it was announced in our local paper
that on such and such a night at such and such a time our
electrical goods dealer would have on exhibition a loud
speaker in operation. How could he be sure the set would
work at that time? In fact what he got was simply a lot
of squeals with a little music coming through. People were
still working in the laboratories trying to send music over
by a heterodyne system. A violin came across fairly well.
A saxophone was not bad. But the two together sounded
about as bad as one could imagine.

When we look at this situation is it possible to believe

i88 TELEVISION

that we can predict too much? The fact that wave-bands
are now being set aside by the Radio Commission for tele-
vision broadcasting shows how seriously the subject is being
taken. There can be no doubt that television of moving
pictures will soon come about. This brings up the problem
of illumination only at the receiving end, instead of at both
ends. But one broadcast station can serve many receivers
so that expense need not be spared at the transmitting end.
With searchlights now in use in aviation with beam candle-
power up into the millions we need hardly worry about
this point. From movies to actual dramas will be but a
short jump.

The chief difficulty at present is that television requires
a rather broad band of wave-lengths. Had television come
ten years ago this would have presented no difficulty. As
matters stand now, however, with a broadcast station
crowded into every possible space, the introduction of tele-
vision will of necessity crowd some of these out. With their
enormous commercial possibility, none are willing to drop
out for the general good of the future of television. Here,
perhaps, lies television's greatest obstacle. It is probably
greater than the various technical obstacles which have been
presented in this book. In the meantime, the fact that there
is no public demand for television magnifies this difficulty.
If the public knew that it wanted television, if there would
arise a vast army of enthusiasts such as those who built one
home-made radio set after another a few years ago, then
television would at least be given a hearing. But now a
factory-made set is so much superior to one fabricated at
home that most of these so-called fans have disappeared.
As it is now we are waiting for a good factory-built tele-
vision receiver. But will this come without public demand?
We are met with the problem of public demand on both
sides and it appears that this will only come as a result of
press reports of laboratory demonstrations. It will be a

THE FUTURE OF TELEVISION 189

rather slow process. Television cannot win its way foot
by foot; it must come as a more or less finished product.

Julius Weinberger of the Radio Corporation of Amer-
ica, speaking before the Federal Radio Commission, recently
said:

"If the public is interested in purchasing picture or tele-
vision receivers, and if commercial interests are desirous of
setting up a service along these lines, it will be possible to
set up and develop a better class of service with far less
interference with the present sound broadcasting art if visual
broadcasting service is placed in those bands above 1500
kilocycles. If this is done the necessary elements of
standardization can be worked out at a reasonable and
thoughtful pace and will develop so as to be of the greatest
general public service."

Other speakers before the Commission were reported
by the New York Herald Tribune of February 17, 1929
as follows :

UM. B. Sleeper, of the Sleeper Research Corporation,
told the commission that television is no more annoying than
any other program and the public is privileged to tune off
any program it dislikes. He favored television programs
in the broadcast band, and stated his belief that if sets were
on the market the public would buy them. Under the
present conditions, he said, most of those who have sets had
to make them.

"Declaring that engineers developed all the great inven-
tions and that statements made on television other than by
engineers are of little value, C. W. Horn, manager of radio
operations of the Westinghouse Electric and Manufacturing
Company, told the commission that television will have no
right on the broadcast bands until it has developed so that
a moving picture can be shown. Television is now in the
laboratories, he said, and not ready for the market, intense
research work still being necessary.

190 TELEVISION

"Oswald Schuette, executive secretary of the Radio Pro-
tective Association, said the commission could do everything
possible to encourage the development of television. He
opposed the standardization by certain groups and asked
that the independent manufacturer, inventor and others be
given a free hand in the development of television. Colonel
Manton Davis, vice-president and general counsel of the
Radio Corporation of America, agreed with Mr. Schuette
that 'development of the art should not be cramped.' 'Let
us, if we can avoid it, not develop one radio art at the
expense of another,' Colonel Davis said."

This probably gives us a fair picture of the present atti-
tude toward television by those capable of passing judgment
upon it.

Another difficulty comes in the lack of standardization.
If one transmitter is working with a scanning-disc of forty-
eight holes, another with thirty-six, two receiving discs
would be needed. To shift from one station to another
we should have to change the discs or make some equivalent
adjustment. This is but one of several problems which lack
of standardization presents. On the other hand, standard-
ization at the present stage is dangerous. It is extremely
difficult to change a standard, however undesirable it may
prove, after the public has invested thousands of dollars
in equipment.

But development goes on, and will go on. There is no
question but that the technical difficulties will be overcome.
This in turn will overcome the other difficulties which have
been outlined. There is little question but that ten years
from now we shall receive television broadcasts as readily
as we receive radio programs today. And they will be rela-
tively as satisfactory.

INDEX

Aberration, chromatic, 25

spherical, 25
Aeo light, 59, 63, 65
Alexanderson, E. F. W., 84
Alexanderson projector, 166
Alexanderson system, 166
Amateur equipment, 179
Amplifier, Bell system, 138
Amplitude, 32
Atomic structure, 60

Baird, 66, 81, 86

color television, 129

daylight television, 127

J. L., 118

optical lever, 88

synchronization, 98
Baker, 101

T. Thome, 7, 43, 92
Bakewell, 5

Bartholomew, H. G., 9, 102
Bartlane, 10, 102
Belin, Edouard, 10, 18
Bell, synchronization, 100
Bell System, 132

neon tube, 145

Blondel oscillograph (fig. 42), 74
Braun tube, 76
Burt, Dr. R. C., 52

Caesium photoelectric cell, 56
Campbell-Swinton, A. A., 18
Cathode-ray oscillograph, 70, 76
Charbonnelle, 7

Chemical reproduction, 6
Chromatic aberration, 25
Circuit, neon tube control, 152

synchronizing, Bell, 154
Code system, 102
Color-television, 66

Baird, 129
Cosine law, 20
Critical angle, 27

Dauvellier, M., 18

scanning, 85
Daylight television, 58
Daylight transmitter, Baird, 127
Disc— Baird lens, 119

Baird radial, 120

Baird spiral, 121

prismatic, 160
Disc receiver, Bell, 147
Discharge tube, 62
Dispersion, 27
Distributor, Bell, 151
Drum scanner, 161
DuFour oscillograph, 75

Einthoven galvanometer, 73
Eldridge-Green theory, 28
Electromagnetic waves, 31
Elster and Geitel, 50
Equipment, amateur, 179
Ether, 36
Eye, 19

Fading, 143

191

192

INDEX

Ferree, 7

Fluorescent screens, 77
Fournier, 81
Framing, 99, 184

Galvanometer, Einthoven, 73

moving coil, 175

moving magnet, 177
Geometrical optics, laws of, 19
"Ghosts," 143
Giltay, 41
Glow lamps, 59
Governor, fly-ball, 91
Gray, Frank, 139
Grid receiver, Bell, 148

Half-tone, 8
Hallwachs, 47
Heaviside layer, 145
Hertz, 46
Historical, 5
Horton, J. W., 139
"Hunting" of an electric motor,
96

Inverse square law, 19
Ives, Herbert E., 132

Jenkins, 89

Jenkins, C. Francis, 18, 159

Jenkins, drum, 160

Radio-movies, 162

synchronization, 165
Jenkins system, 159
Johnson, J. B,, 53, 76

Kino lamp, 63, etc.
Knowles relay, 178
Knudson, Hans, 15
Korn, 101
Korn, Prof. Arthur, 11, 41

Lag-selenium cell, 42
Lambert's cosine law, 20
Left-hand rule, 71
Leishman, L. J., 10
Lenard, 48
Lens disc, 119
Lens, convex, 24

faults, 25

formula, 25
Light, velocity of, 34
Lorenz-Korn, 15

McFarlane, Capt. M. D., 102
MacFarlane Moore, D, 18, 59
McFarlane, M. L. D., 9
Mattes, R. C., 139
Maxwell-Clerk, 35
Mercury vapor tube, 63
Mirror, concave, 21
Mirror-drum, Alexanderson, 167
Moving coil galvanometer, 175
Moving magnet galvanometer,

177

Morton, E. R., 157
Mounting, amateur equipment,

131
Multiplex radio system, 155

Neon tube, 63

Bell type, 145

Neon tube control circuit, 152
Non-polarized relays, 175

Optical lever, Baird, 88
Optical systems, 19
Oscillograph-cathode ray, 76, 90

Du Four, 75

string, 73
Oscillographs, 71

Pendulum, 91

INDEX

193

Phonic drum, 97
Phonovisor, 130
Photoelectric cell, 46

caesium, 56

circuits, 57

Photoelectric cells, Bell, 136
Picture transmitter, Bell system,

109

Piersol, R. J., 44
Planck, 38
Polarized relays, 175
Prism, total reflecting, 27
Prismatic disc, 90
Prismatic ring, 159
Process screen, 8
Projector, Alexanderson, 166

Quantum, 38

Radial disc, 120

Westinghouse, 182
Radio-movies, Jenkins, 162
Radiovisor, 165
Ranger, R. H., 16, 94, 105

receiver, 106
Rankine, A. O., 44
Real image, 22
Reflection, internal, 27

law of, 20

Refraction, law of, 21
Relay, Knowles', 178
Relays, 173

non-polarized, 175

polarized, 175

telegraph, 175

tuned, 177
Retina, 28
Rignoux, 81
Romer, 34
Ruhmer, 17, 81

Scanner, drum, 161
Scanning, 81

Scanning-disc motor, Bell, 152
Scanning spot distortion, 139
Scanning system, Alexanderson,
84

Baird, 86, 122

Dauvillier, 85

Jenkins, 89

oscillograph, 85

system, simple, 83

tuning fork, 85
Selenium cell, 39

lag of, 12

Selenium, lag of, 42
Sensation, relation to stimula-
tion, 29

Seven-spot projector, 167
Spectrum, chart of, 35
Spherical aberration, 25
Spiral disc, 121
Stoletow, 49
Stoller, H., 157
String oscillograph, 73
Stroboscope, 96
Swinton, A. A. Campbell, 78
Synchronization, 91

Baird, 98

Bell, 100, 150

Jenkins, 165

Synchronizing circuit, Bell, 154
Sychronizing system, Baird, 123
Synchronous motors, 97

Telectograph, 7
Telephotography, 101
Thomson, J. J. 48, 76
Transatlantic transmission, 131
Tuned relays, 177
Tuning fork, electrical, 93

194

INDEX

Virtual image, 21
Visual contrast, 70
Visual-purple, 28

Wave formula, 32
Wave-length, 32
Waves, 31
transverse, 32

Westinghouse facsimile trans-
mitter, 107

Westinghouse radio-movies, 182
Wire television, 140
Work, 101

Zworykin, 53, 107, 112

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