BACKGROUND, THEORY, & CONSTRUCTION OF THE "ELECTRIC TELEGRAPH"
Ever since the beginnings of time, people have been trying to communicate over distances greater than the human voice could reach. Early attempts included the use of smoke signals, signal fires, waving flags, and the moving arms of semaphores. Mirrors were also used to flash the image of the sun to distant observers.
After the discovery of electricity, wires were stretched from one point to another and an electric current was either allowed to flow through the wires or broken by a switch called a telegraph key. The electric current was first used to make marks on a paper tape and later, it was used activate a "sounder" which made clicking sounds. The short and long times between the clicks could be decoded into letters from the alphabet.
This revolutionary discovery allowed people to communicate instantly over
distances that had required days or weeks for horse or train-carried
messages. Telegraph stations were set up along railroads first because the
right-of-way had already been cleared and it was easy to set up poles to carry
the telegraph wires. Railroad dispatchers sent messages via telegraph to
control the movement of trains and the wires also began to carry messages
telling of news events and business transactions. It has been said that the
"electric telegraph" was the most significant invention of the 19th century.
At the very end of the 19th century, it became possible to communicate
by telegraph without using wires. This 'wireless' telegraph system
paved the way for all of today's complex wireless communications systems.
HOW LAND-LINE TELEGRAPH WORKS:
A telegraph system is basically an electrical circuit consisting of 3 parts, all hooked together by a WIRE.
A BATTERY supplied the electricity or voltage. A KEY was used to complete or break the circuit. At the distant part of the wire was an electricity detector or ELECTROMAGNET consisting of a coil of wire which pulled on a piece of metal when electricity was passed through it. (More on this "ELECTROMAGNET" in a moment.)
The circuit is shown below: (The lines indicate the wires and the arrowheads show the path of the electrical current as it flows through the wires.)
!--->---->---->------ BATTERY ---->---->---->-----! ! (Supplies the voltage) ! KEY ELECTROMAGNET (Completes or breaks (Pulls on a the electric circuit) piece of metal) ! ! !---<----<----<----<----<----<----<----<----<-----!
The WIRES were usually made of copper because it conducted electricity better than other metals. It was discovered in the 1830's that the second wire could be eliminated by using the earth as an electrical conductor. From that time on, only a single wire was necessary to cover the distance between a key and an electromagnet.
The BATTERY consisted of a glass jar filled with a chemical solution (often Copper Sulfate) with copper and zink electrodes immersed in the solution. A chemical reaction between the electrodes and the solution produced the electrical voltage. The voltage of each cell measured about 1 volt and several cells could be hooked together to produce higher voltages. These batteries produced voltages similar to the dry batteries that we use in flashlights.
The KEY originally consisted of two pieces of brass or copper which could be pressed together to complete the electrical circuit or allowed to spring apart using their natural "springiness" to break the circuit. As people developed the need to send messages more rapidly, the designs of keys changed and the evolution of these different designs of telegraph keys is the focus of my telegraph museum exhibits.
The ELECTROMAGNET consisted of a coil of from 50 to several hundred turns of insulated wire wrapped around an iron core. It pulled on a piece of iron whenever an electric current was passed through it. These devices first caused marks to be made on a paper tape and then, when it was discovered that people could decipher the noises that they made by ear, they developed into the electromagnetically operated "sounders" used from the 1850s to the 1950s.
DIFFERENT TYPES OF "DETECTORS"
First, it was found that the ELECTROMAGNET could move a compass needle and the
"Needle Telegraph" began to be used beginning in the 1830's.
the late 1830s, Cook and Wheatstone in England used electricity to move a
needle and installed the first electric telegraph system to prevent railroad
train crashes on the Blackwall railroad line in England. Inventor Joseph Henry
was also experimenting with electric telegraph systems and installed the first
telgraph system in America to allow him to communicate between his office in
Princeton University and his home.
By the early 1840's, Samuel F. B. Morse had learned about the technology of
the telegraph in discussions with inventor Joseph Henry and Morse's assist
Alfred Vail who helped Morse by inventing devices for switching electrical
voltages and devices for writing changes in these voltages on a moving paper
tape. Then Morse used one of Vail's ELECTROMAGNETs to move a pencil and mark
a moving strip of paper with short and long marks depending on whether the
switching key was held closed for a short or a long time respectively.
Alfred Vail actually invented this system of assigning long and short voltages
to different letters in the alphabet. He chose the length of the combination
of short and long voltages according to the frequency of use of the particular
letters as determined by reading and counting letter frequency in the local
newspapers. For instance, he assigned a single short electrical pulse to the
most frequently used letter in the English language, the letter "E". For the
less frequent letters he chose more complex combinations of short and long
He assigned a specific combination of short and long voltages to each letter
in the alphabet to form a "code" which became known as the "Morse Code"
despite the fact that it had been invented by Alfred Vail. When the key was
closed for a short time and then a longer time, the pencil marked the paper
with a dot followed by a dash and this signified the letter "A". This paper
tape writing device was first called a "portrule" and later became known as a
"REGISTER" and was used well into the 1900's.
In the 1850's telegraph operators began to realize that they could recognize the different sounds made by the register as dots and dashes and a new detector mechanism called a "SOUNDER" was invented. This device used an ELECTROMAGNET to pull on a piece of iron and make a clicking sound. When the ELECTROMAGNET pulled on the iron, it made a more solid and heavy sounding click and when it released the iron, it made a thinner and lighter sounding click. Operators learned to discriminate between these two sounds and to use this ability to tell whether they were hearing a dot or a dash.
A dot was a CLUNK followed, a short time later by a CLICK. A dash was a CLUNK followed, a long time later by a CLICK. This method of copying the code by ear persisted well into the 1950's.
to be improved and the most important improvement was to place them in a small
wooden partial-enclosure called a "RESONATOR" which had
the effect of amplifying the sound by bouncing the echoes of the sounder out
the front of the resonator along with the original sound. Sounders in
resonators became an integral part of every telegraph system.
After it was discovered around 1900 that messages could be sent by radio
waves without using wires, a slightly different version of the Morse Code was
used to encode those messages. Instead of short and long clicking sounds,
this system used short and long beeping sounds to encode and transmit letters
to a radio receiving station. Although voice communications by radio became
possible in the 1920's, the Morse Code continues to be used to the present.
The original "Morse Code" (Also called the "American Morse Code")
was used on the telegraph pole-supported land-lines mostly seen along
railroad lines and in big cities in this country but a slightly different code called the
"Continental" or "International" code was used in Europe and on the radio
Click here for a comparison of the two codes:(2KB)
LOCAL VERSUS LONG DISTANCE TELEGRAPH CIRCUITS:
First, let's consider Sounders and Relays used in LOCAL in-house circuits:
3-6 volts works the LOCAL SOUNDERS and LOCAL RELAYS just fine in a LOCAL
circuit with wire connecting all components.
Ohm's Law is a formula that looks like this:
E = I times R
Where E is voltage, I is current needed to pull in the coil, and R is circuit resistance.
Using standard algebra, this formula converts into: I = E / R
So: In LOCAL circuits, the current (I) in the coil is equal to the voltage (E)
divided by the resistance (R).
For example, the current in a LOCAL SOUNDER or LOCAL RELAY coil can be
calculated as follows: A typical voltage of 5 volts divided by a typical LOCAL
SOUNDER or LOCAL RELAY coil resistance of 9 ohms (and say 1 ohm local wire
circuit resistance) equals 5 Volts / (9+1) Ohms = 5/10 = 0.5 Amps.
LONG DISTANCE "MAIN LINE" circuits:
Over long distances, only 1 wire was used on the telegraph poles with the ground return to complete the circuit being actual earth ground. Although the earth ground circuit remained virtually constant, the resistance of the wire increased with distance producing a weaker signal.
For very long lines a repeater was often inserted into the circuit. The
repeater contained a very sensitive coil that sensed the weak signals coming
in from the line. It used these signals to activate a sensitive relay which
keyed a stronger signal that continued the information path along the
BUILDING A WORKING TELEGRAPH SYSTEM
If you would like to build a simple working LAND-LINE telegraph set,
click on the following link:
How to BUILD a working LAND-LINE telegraph set:(15KB)
If you already have an old telegraph sounder and would like to have
it be operated by audio tones such as those from a code practice
oscillator or short wave receiver, you will find an appropriate
circuit by following this link:
How to BUILD a circuit to allow a telegraph sounder to be operated by audio tones:(15KB)
If you would like to build a simple working WIRELESS telegraph set,
click on the following link:
How to BUILD a working WIRELESS telegraph set:(15KB)
----->> ADDITIONS, CORRECTIONS, and COMMENTS ARE WELCOME ! ! ! ! !
Professor Tom Perera
Montclair State University