The Science Notebook
Gilbert Signal Engineering - Part 3

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NOTE:  This book was published in 1920, and while many of the experiments and activities here may be safely done as written, a few of them may not be considered particularly safe today.  If you try anything here, please understand that you do so at your own risk.  See our Terms of Use.   

NOTE # 2:  Some of the codes in this book have changed slightly.  For example, the International Morse Code, known at the time as the "General Service Code is still used today and is as shown in this book.  (See p. 15.)  However, some of the "conventional signals" have changed.  (See p 17.)  Learning to send or receive Morse code by sound, light or semaphore might be a little hard, but it can be a lot of fun, but before you attempt to learn it, be sure you are using the correct code and signals for today.  You should be able to get the current version of the code or sending method you want to use online.  Check out our Downloads and Useful Links pages for additional information.

Pages 51-75


heliograph signals can be read up to forty or fifty miles, and even greater distances are on record. However, the normal range is around twenty-five miles, and to obtain longer distances it is usually necessary to operate from a large hill or mountain peak.

The principal disadvantage to heliographing lies in its dependence upon the sunlight. The advantages are portability of equipment ; great range signals can be exchanged ; the rapidity of sending; and most important of all is the fact that your signals cannot be observed by others unless they happen to be on the line of flash between the sending and receiving station.

Heliograph instruments vary in design according to the organization using them, but all are alike in principle. The chief parts of the equipment for a station consist of one sun mirror, one station mirror, a shutter arrangement of some sort for intercepting the flashes and a device for directing or sighting flashes on receiving station.

The Heliograph and Theory. Every boy has at some time taken a small pocket mirror or bright piece of metal from which he has reflected the sun's rays on a shadowed wall. The result on the wall is a bright sun spot or flash which can be moved at will by slightly shifting the mirror. As this flash is the important factor in heliographing, it must be produced and directed at the receiving station with a great deal of skill.

The mirrors used in heliographs are usually not over 4 or 5 inches square. Two mirrors are made necessary by the position of the sun at time one is sending. When the sun is at right angles to the line joining the two stations, only one mirror is used the sun mirror. With sun at rear of operator, the two mirrors are required.

With one mirror the flash is reflected directly from it to receiving station and with two mirrors the flash is reflected from


the sun mirror to the station mirror and thence to the receiving station. (See Figure 11, A and B.)

How Heliograph Operates With One Mirror. The sun mirror has in center a small peep hole or unsilvered spot about one-quarter inch in diameter. The sighting device is about 6 or 8 inches to front of the mirror. An upright rod is generally employed which can be moved up and down; the rod sets parallel with edge of mirror and has a round disc on an arm which when turned at right angles to rod falls in line with center of mirror.

To direct the flash accurately on distant stations, the operator sights through the peep hole in rear of mirror and adjusts disc so that the peep hole, disc and distant station are on an exact line. Then the sun mirror is adjusted on its horizontal axis only, so that the "shadow spot" cast by peep hole falls exactly on sighting disc. (The shadow spot can be found by placing a piece of paper between mirror and sighting device.) After "shadow spot" is located on disc the flash is visible at receiving station. (See Figure 12.)

How Heliograph Operates With Two Mirrors. The sun mirror is faced towards the sun and the station mirror towards

FIG. 11


FIG. 12

receiving station. The station mirror has a paper disc pasted on its face at the center. The sun mirror is adjusted so that the whole of the station mirror is reflected into it and the unsilvered spot and reflection of paper disc accurately cover each other.

To sight flash on receiving station the reflection of the distant station will be seen in station mirror and, by adjusting this so the disc covers the reflection of distant station, the flash will then be accurately in line.

Intercepting the Flashes. The method of intercepting flashes in heliograph is either with the improved shutter with leaves operated by a key or with a single shutter held in the hand. In either case uniformity of movement should be maintained. Because of the distances it is always advisable to count slowly three times for a dot and six times for a dash.

Backgrounds. Dark backgrounds should be selected when possible for heliographing, as signals can be more readily distinguished.

To locate a distant station when its position is unknown, take


the station mirror and direct it towards the horizon, playing it in slowly from right to left several times. If no response is

FIG. 13
U. S. Marines sending a heliograph message.
Courtesy of U. S. Marine Corps

received, direct it at a point near the home station, and repeat this same process. As a result of this method you will usually locate the station.


If position of each station is known to the other, the station ready first will direct its flash upon the distant station so that that station may be able to adjust its flash to answer the signals.

In heliographing, the sun's movement has to be watched carefully and adjustments made often. In the case of well trained signalmen these adjustments can be made without "breaking" a message. The heliograph is best operated by two men.

Heliograph flashes are sometimes very hard on the eyes; therefore it is always a good plan to smoke the lenses of the telescope a little when its use is necessary.

The Ardois System for night signaling consists of a display of red and white incandescent lamps which indicate the characters of the General Service Code. The lamps are arranged in four units, each unit consisting of a red and white lamp. The units are placed an equal distance apart and usually suspended in a vertical position from a mast, yardarm or staff, in which case characters are read from top downward. When it is necessary to place lamps horizontally they are read by sender from right to left, and in case of receiver from left to right.

A red lamp indicates a dot and a white lamp a dash. The lamps are operated by a keyboard.

The letters of General Service Code are made by a single display; for example, A which is . - would be made in the Ardois System by a display of the red light of the top unit and the white light of the next unit below. The letter B which is  - . . .  would be expressed by a white light from the top unit and the next three units below would be red.

Chart 8 gives alphabet for the Ardois System, also conventional signals and numerals.

The numerals of General Service Code cannot be used in the




Ardois system as the expression is limited to four lamps. Therefore, numerals are made by giving secondary meanings to letters of the alphabet as shown on the chart.

To make a numeral, display the letter by which it is indicated, and blink or pulsate the upper light.

In the case of letters which indicate conventional signals the upper light is pulsated. The letter R is an exception to this. When pulsated it signifies Number 2, when flashed it is the conventional signal for "acknowledge."

The interval is made once to indicate end of word, twice for end of sentence and three times for end of message. When interval is displayed and upper lamp pulsated it is a "Designator" signal.

The general call to attention is a steady display of cornet WWWW. The cornet is not used, however, if call letter of station desired is known. In answer to a call, display call letter of station, the calling station then proceeds with message.

To indicate that an error has been made in the message make "interval," the "error," then "interval" and then begin with word in which error occurred.

The letter R flashed acknowledges the receipt of a message.

When the Ardois System is in use, it is advisable to extinguish all nearby lights which are liable to cause confusion in signals.

The Ardois System is authorized for use by both Army and Navy.


The Very System of night signaling is used by Army and Navy, its use is mainly confined to signals of extreme importance or when distance is great.

The signals are made by firing red and green stars in the air by means of a pistol which has a barrel similar in gauge to


the shotgun. The cartridges firing the stars are like the shells of a shotgun.

This system is based on the dot and dash code, a red star representing a dot and a green star a dash. This system, however, is practical only for use with Army and Navy codes and therefore is not of any service to a boy.


Chapter VI

All of these non-visual forms of signaling are used by the signal corps of every modern army. They are also the common means of communication in everyday commercial life.

In all of the above methods of signaling the use of electrical currents are necessary along with special instruments for receiving and transmitting messages. The theory of electric currents is a study in itself. This subject is covered in the Gilbert Electrical Manual, and any boy wishing to acquire a knowledge of electricity and to apply the theory to his apparatus can do so by consulting Gilbert Manual of Telegraphy or the Gilbert Book on Radio Engineering. The writer will confine himself to the operative side of signaling, which includes the code and proper form of handling messages over these systems.


An American, Samuel F. Morse, invented the first working telegraph instrument in year of 1835. This instrument was the recording or writing type, that is, it made marks on strips of paper of dots and dashes which could be spelled into a message. The recording instruments are now obsolete and all telegraphy is conducted by sounding instruments, which spell out messages by means of sharp "clicks."


The American Morse Code is used on all land telegraph lines and short cables. It is also the official code of the Army for





electrical signaling on military telegraph lines, short cables and field lines. This code is written on Chart 9. Every signalist should familiarize himself with this code and learn how it differs from the General Service Code.

The beginner should thoroughly commit to memory the signs representing the letters of the alphabet, the numerals and a few of the principal punctuation marks. The remaining characters can be learned afterwards as they are not needed by a beginner.

The Morse Code is composed of seven elements :

(1) The dot; (2) the dash; (3) the long dash; (4) the space; (5) the space between letters ; (6) the space between words and (7) the space between sentences.

The dot is made by pressing the telegraph key down for the smallest fraction of a second and then immediately releasing it. The result on the sounding instrument is a "click-click" very close together. The making of a dot involves time, therefore the dash is equal to two dots and to make this the key is held down accordingly. A "click click" sound results. The long dash is equal to four dots, thus: "click click."

The ordinary space between elements of letters is equal in time to a dot, between the letters themselves it is equal to two dots. The word space is equal to three dots and the sentence space is equal to six dots.

Correct Way of Using the Key. The most successful manner of operating the telegraph key is to let the forearm rest easily upon the table, grasping the key as shown in Figure 14. The wrist should be well above the table, the forefinger curved, but not held rigid. Let the thumb rest on the edge of knob so that a slight control of the upward motion is obtained. The raising spring should assist the upward motion but should never be permitted to control it. Avoid tapping upon the key. The


skilled operator will manipulate it by a muscular action of wrist and ringers.

Elementary Practice of Code. Constant practice of making dots with uniformity and precision must first be acquired, then dashes, then grouping of dots and dashes to form letters and words.

The beginner should commence by making letters slowly, giving proper ratio of time to the elements of each letter. Speed will come in time by persistent drill.

The most difficult letters of the code are C O R Z Y and S, termed the space letters; and if spacing in these letters is not carefully timed they will be readily confused with such letters as H I P and L.

The letters J and K, also numerals 9 and 7, are difficult letters. Care should be given not to separate J into a space which would result in a double N.

The usual tendency is to make an F too long and an L too short.

Practice transmitting from any miscellaneous manuscript at hand. This will always test the skill of an operator.


Receiving is of course more difficult to acquire than sending and is mastered best by having an experienced operator send

FIG. 14


to the beginner slowly, increasing the speed as learner becomes more proficient.

Proper Form of Transmission. A telegraph message like all visual messages must be checked by the sender. All words and figures written in the address, body of message and the signature are counted. Of course To and Sig. are not counted as they are only indicative terms used by operator.

In counting the check of a telegraph message, whether in plain English or code, groups or initial letters are counted as one word.

Abbreviations for names of places, cities, towns and states are counted as one word, as if written in full. This rule applies also to any other abbreviations.

Figures, decimal points, bar of division and affixes to numbers, such as d, st, nd, th and rd will each be counted as a word.


Radio-teiegraphy or wireless, by which term it is more commonly known, was invented by Marconi in 1901. Since that time there has been developed many improvements, which make the transmission of messages by wireless almost as practical as by telegraph lines or cables.

Wireless messages are spelled out by use of the International Morse or General Service Code (see Chart 1), the operator using a key like that used in telegraphing. The result is somewhat different from telegraphing, as wireless instruments have a humming or buzzing sound instead of a "click." The characters of the alphabet are made up of short and long buzzes. The receiving is done through phones.

While wireless is under the control of the Navy in the United States the government does not have exclusive use of it. A great many commercial stations have been established and in


FIG. 15
Boy Scouts of Hartford, Conn., learning the theory of wireless.
Courtesy of Boy Scouts of America

this country alone there are thousands of boy experts using wireless, and enthusiasts are being added to the list daily. Note : For conventional signals other than in General Service Code see Gilbert Book on Radio Engineering.


The most widely used of all electrical signal systems is the telephone, invented by Alexander G. Bell, an American. The telephone is so common in our everyday life that most of us do not stop to consider the interesting principles involved.

Color Charts



Chart 10 Flags of the International Code
Chart 11 Ship Call Pennants Call Flags
Chart 12 Special Flags of the U. S. Navy
Chart 13 Personal Flags
Chart 14 Weather Signal Flags















In speaking, the vocal cords cause air vibrations, which, falling upon the eardrum are recognized by the auditory nerves as speech. When these vibrations are transmitted into a telephone instrument, they are caught by the sensitive diaphragm, changed into electrical vibrations, carried along the telephone wire to the receiving station and reproduced.

Note: See Gilbert "Sound Experiments" and Manual on Telephone.


When signal stations are connected by telephone, messages are of course sent by this means, it being much more handy.

FIG. 16
U. S. Army Signal Corps field radio station somewhere in France.


The difficulty arising in telephonic messages is the confusion of certain letters of the alphabet having like sounds when spoken by word of mouth.

To provide a ready means of distinguishing similar sounding letters, a code of conventional signals is authorized for military purposes and should be used especially when codes are being sent. These conventional signals are as follows:

A Able
B Boy
C cast
D Dog
E Easy
F Fox
G George
H Have
I Item
J Jig
K King
L Love
M Mike
N Nan
O Oboe
P Pup
Q Quack
R Rush
S Sail
T Tare
U Unit
V Vice
W Watch
X X-ray
Y Yoke
Z Zed

To give an example of the proper use of this code we will suppose an important message is being telephoned to a station and the receiving operator cannot clearly understand certain words such as directory, or the word translation. To make these words clear the operator would spell directory out slowly Dog-Item-Rush-Easy-Cast-Tare-Oboe-Rush-Yoke. The word translation would be spelled out likewise. From this code a clear understanding would undoubtedly result.



The Service Buzzer is a portable piece of signal equipment especially adapted to the needs of the Army Signal Corps. It can be readily attached to either telephone or telegraph lines and used as a telephone or for sending Morse or General Service Code telegraphic signals.

When service buzzer is used in the latter form the signals are received in a telephone receiver in form of a high-pitched hum very similar to wireless signals.

Signals have been exchanged between two buzzer outfits even after wire connecting the stations has been cut in. The instruments were, of course, grounded.

The mechanism of the buzzer is very simple, so simple in fact that any boy can make a practical outfit for Field Service Signaling by following the suggestions given in this book on page 102.


Chapter VII

The Tower of Babel served as a rallying point and in all probabilities was the first signal station. Later examples of old signal towers are those built by the Chinese along the wall of China. Today, however, the modern signalmen do not build such substantial towers, as the up-to-date armies are mobile and consequently when a signal tower is required a portable one or a hand-made affair, which can be erected in a few minutes by field signal troops, is used.

The boy signalist wanting to establish a visual station must first select the site so it is perfectly in view of receiving station and with a uniform background for all signals.  The distant

FIG. 17
Signal tower erected at a boy Scout Camp.
Courtesy of Boy Scouts of America Signals. 


station is the best judge of the proper location and background for the signal tower.

In locating a military station secrecy is of vital importance, and for this reason the tower is usually camouflaged by shrubbery or erected behind foliage so the platform of tower is barely exposed.

The following table gives distances of the visible horizon or how far an object at sea level can be seen. When observer's eye is :

It can readily be seen from the above table that an observer whose eye is 25 feet above sea level can distinguish an object at a distance of 6.1 miles provided the object is at sea level. Now should the object itself be elevated 15 feet its visibility would be increased to 6.1 miles and 4.7 miles, equaling 10.8 miles.

To receive visual signals at the distances given above, a telescope is used.


The height necessary for building a signal tower should be calculated according to distance between points of communica-


FIG. 18
A natural point of vantage for signaling.
Courtesy of Boy Scouts of America

tion. When possible, natural points of vantage should be used, such as the roof of a building or a platform built in a tree. Sometimes several trees can be found close together which can be connected by stringers and a platform laid around, to which a rail can be added. A ladder would lead up to the staging to complete the arrangement.

Where no natural supports can be found, it will be necessary to build a tower. A substantial tower can be erected by using either three or four uprights for supporting the platform. The uprights can be made of finished lumber, using 2x4 pieces or heavier ones, depending on height of tower wanted.

The drawing on page 71 suggests a tower made of three selected trees cut to lengths of 18 feet. These are placed in the ground about 1  l /2 or 2 feet, the arrangement of placing being triangular and 8 feet apart. The uprights are leaned in at the top and tied 4 feet apart, on which a platform is laid which will accommodate two signalmen. The platform can be made of


FIG. 19
Illustrating method of constructing signal tower


boards or else straight limbs of trees spiked to the cross girders. At a distance of 5 feet apart cross ties should be spiked to make the tower as rigid as possible.

A row of cleats nailed to one of the uprights does very well for the ladder.


Both the Army and Navy have their code books, which are especially adapted to military needs and insure both secrecy and economy of words in signaling. These codes are confidential except to those in service of the Government.
The land telegraph and cable companies also issue code books from time to time to their customers, not so much for the reason

FIG. 20


FIG. 21

of secrecy, as for economical benefit of the messages. These code books can sometimes be obtained and will serve very well for all classes of signals.

So the boy signalman will not be handicapped for the want of a secret code, the writer will suggest the cipher disc which is used by the signal corps and another improvised method of using the cipher.


The cipher disc used by the signal corps is a simple but ingenious device pictured in drawing on opposite page. It consists of two circles of cardboard, one smaller than the other. These are joined at center so as to revolve. The inner circle is lettered around the edge with small letters and the outer circle with capital letters of the alphabet.

The alphabet reads from right to left on outer circle and left to right on the inner circle.

The letter A on inner circle indicated by the arrow is the key letter to the cipher. The purpose of this cipher is only to transpose one letter of the alphabet for another, thus the message WE BREAK CAMP AT SUNRISE when read from the disc


pictured would be sent and received as: JB EOBFV DFTQ FM NLSOXNB. It is of course understood by reader that the letter F would have been agreed upon by the sending and receiving stations prior to time this message was sent.

Any letter can be agreed upon between sending and receiving station and then the key letter A is set opposite on disc to encipher the message.

All numbers are spelled out when sent in a cipher message.

It is apparent to the reader that this method is not absolutely unreadable to any one who would take the time to figure out the key; however, when used in connection with a code, it can be made much more complicated to any one desiring to read your message.

The above method of sending cipher could be used with the General Service Code which was in existence prior to the adoption of the International Code of dots and dashes.

The old General Service Code is written as follows :


The foregoing code is used with the various signal systems as follows :

Wigwag or single flag - one would be to right and two to left, three would be expressed by the front motion.

Ardois System - one would be red light and two white light, a space would be made for end of word, etc.

Sound System, by whistle, bell and foghorn - one would be indicated by a short blast or taps and three likewise.

Telegraph, Wireless and Flash Light System - one would be made by one click, buzz or flash, two by two clicks, buzzes or flashes and three made in same way using three.


A practical, yet simple cipher can be made by first obtaining several pieces of ordinary flat picture moulding like those shown in Figure 21. These pieces will slide parallel to each other; one piece should be at least 14 or 15 inches in length, while the other can be just half that length.

Next take white ruled paper and paste along flat surfaces of moulding, the ruling or lines of paper should be about 1/4 inch apart.

Fifty-two spaces are necessary for the long piece and the alphabet is written twice, backwards, in small letters, starting from top as shown in Figure 22, these letters are numbered from 1 to 52.

FIG. 22

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