BACKGROUND AND HISTORY OF TELEGRAPH AND WIRELESS 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 type of telegraph was called land-line telegraph because the wires crossed the land and used the ground as part of the electric circuit.
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 WIRELESS TELEGRAPH WORKS:
Near the end of the 19th century, it was discovered that an electric spark sent out electrical energy which travelled through the air without wires and could be detected at a distant point. This discovery made it possible to communicate without wires.
HOW TO BUILD A SIMPLE WIRELESS TELEGRAPH SET:
The simplest wireless telegraph set consists of a means of generating and controlling a spark which sends out radio waves into the air, and a receiver or detector to detect the radio waves.
Probably the simplest way to generate and control a spark is to use a switch (called a telegraph key) to turn on and off a buzzer which generates sparks.
The simplest way to receive or detect the radio waves generated by the buzzer is to use an AM radio tuned to a place on the dial where there are no other stations.
Here is the simple wireless transmitter which is basically an electrical circuit consisting of 3 parts, all hooked together by a WIRE.
A BATTERY supplies the electricity or voltage.
A KEY is used to complete or break the circuit.
A BUZZER is used to generate the sparks and the radio waves.
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 BUZZER
(Completes or breaks (Generates sparks
the electric circuit) & radio waves)
! !
!---<----<----<----<----<----<----<----<----<-----!
The WIRES can be virtually any kind of electric wire with the insulation removed from the ends where the connections are made.
The BATTERY can be flashlight or lantern batteries generating about 6-Volts.
The KEY can be any electric switch or a simple piece of metal which can be bent down to make an electrical contact.
The BUZZER can be a door buzzer which can be found in any hardware store. You can also make a buzzer by removing the bell from a doorbell.
Since a buzzer is just an electromagnet which breaks the circuit which is activating it when it is activated, you can make your own buzzer by winding about 100-200 turns of wire around a nail and arranging it so that activating this electromagnet pulls on an armature which opens a set of contacts and breaks the circuit to the electromagnet. As soon as the circuit is broken, a spring returns the armiture to it's original position and the circuit is made again. This cycle of break-the-circuit and make-the-circuit continues and makes the armature vibrate or buzz for as long as a voltage is applied.
After you have built the transmitter, keying the wireless transmitter by closing the switch will send out radio waves. Short key closures are dots and long key closures are dashes. Here is a link to the two types of codes:(2KB)
Here is a picture of a commercially-made key and buzzer telegraph practice set which can also function as a wireless transmitter. Pressing the key causes the buzzer to buzz and create sparks which are broadcast through the air and can be received on an ordinary AM radio. A key and buzzer practice set which can also function as a simple wireless transmitter.
DIFFERENT TYPES OF "DETECTORS"
The simplest detector for your wireless spark transmitter is a common AM radio tuned to a place on the dial where there are no other stations. You will hear a noisy stattic sound every time you press the key. These sounds are exactly like those that people heard during wireless communications one hundred years ago.
CRYSTAL RADIO SETS:
In the early days of wireless, a crystal of galena and a pointed piece of wire called a 'cat's whisker' were brought into contact to make a rudimentary diode detector. This allowed the radio waves to be 'detected' into DC (Direct Current) electricity which could activate earphones or headphones. These early radio sets were called 'crystal radio sets'.
CONSTRUCTION OF A CRYSTAL RADIO SET:
You can make a simple crystal radio detector for your wireless spark transmitter by hooking a wire ANTENNA to a DIODE DETECTOR and a pair of earphones as shown in the following circuit:
--->---->---->---WIRE ANTENNA---->---->---->-----!
(Picks up the radio waves) !
DIODE
DETECTOR
!
!
EARPHONES
!
!
-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-
(GROUND: (Bury wire in the ground))
The WIRE should be at least 10 feet long and fairly close to the buzzer.
The DIODE can be almost any diode such as a 1N4001. It doesn't matter which end is hooked up to the antenna.
The EARPHONES can be almost any sensitive headphones.
The GROUND connection should be a wire, buried in the (preferably moist) ground.
Keying your spark transmitter should produce a buzzing sound in the earphones.
If the signal is not strong enough, you might have to attach an antenna wire to the buzzer contact of your spark transmitter and put it near the antenna of your receiver.
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 by amateur radio operators to the present. It was only in the late 1990's that most governments stopped using the code in favor of voice and satellite communications.
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)
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)
----->> ADDITIONS, CORRECTIONS, and COMMENTS ARE WELCOME ! ! ! ! !
Professor Thomas B. Perera
Montclair State University
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