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.
Then in, the early 1840's, Samuel F. B. Morse used an ELECTROMAGNET to move a pencil and mark a moving strip of paper with short and long marks depending on whether the key was held closed for a short or a long time respectively. He assigned a combination of short and long symbols to each letter in the alphabet to form a "code". 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 called 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.
Sounders continued 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, the morse code was used to encode those messages. 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 land-lines in this country but a
slightly different code called the "Continental" or "International"
code was used in Europe and on the radio waves.
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:
However, over long distances, only 1 wire was used on the telegraph poles with the ground return to complete the circuit being actual earth ground. Since the resistance of the earth was a great deal higher than the resistance of wire, much higher voltages were necessary to produce the same current through the coil.
To get the same 0.5 Amps with a ground resistance of 1000 ohms you would need 500 volts and so on...
Since the ground resistance varied with rain and terrain, the total number of batterys in a series circuit were added or subtracted to compensate for local ground conditions to achieve the needed current through the coil. Sometimes, this had to be done several times a day to compensate for changing soil conditions.
One other factor to consider is that the local wire-only in-house circuits used LOCAL SOUNDERS and LOCAL RELAYS with few turns in their coils and therefore relatively low resistance so fewer batteries were needed... (Typically 2 - 4 batteries) to give the 3-6 volts.
MAIN LINE RELAYS and MAIN LINE SOUNDERS were designed for use in long distance communication that had to cover a great deal of earth terrain. They had many more turns of wire in their coil windings and a higher coil resistance of typically 150 ohms to allow the use of less batteries than would be required with the low resistance LOCAL sounders and relays. With all those additional windings, they required much less current to pull in their armatures so that even with the high ground resistances in very long circuits, they could use lower voltages than the local instruments would have required.
For example, if a so-called MAIN LINE SOUNDER or MAIN LINE RELAY required say 0.05 Amps to pull in it's coil and had a coil resistance of 150 ohms and the ground resistance was 1850 ohms , the voltage needed would be: E = I times R or .05 Amps times (150+1850) Ohms = .05 times 2000 = 100 Volts. This would require that about 50 batteries be connected in series to add up to the required voltage.
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