|Elliott Sound Products||Morse Code|
The first question that many people will ask is likely to be ' .-- - ..-. ', which is Morse code for 'WTF'. Before you scoff, it should be remembered (or become known to those too young to remember) that Morse code signalled the birth of electronic messaging. The 'electric telegraph' was the first system that allowed people to communicate over long distances, by pressing a key in a sequence of 'dots' and 'dashes' (commonly referred to as 'dit' and dah' respectively).
Earlier systems, such as semaphores (flags used in particular patterns and sequences to pass messages), flag signals (e.g. flaghoist), smoke signals, bonfires or drums all suffered from environmental influence. Visual systems were affected by line-of-sight and weather conditions (fog, rain, etc.) and audible methods were limited by the propagation conditions prevailing at the time (wind, atmospheric 'inversion' layers, background noise, etc.). All had limited range, requiring relay stations at regular intervals where the message could be received and re-transmitted. As you would expect, the requirement for re-transmission could easily introduce errors. The word 'telegraph' was coined in 1792 from the Greek, tele, afar, and graphos, a writer (Concise Oxford Dictionary).
Morse code and the Morse telegraph system were by no means the first methods used for telegraphy. Visual and audible systems existed from ancient Greek times and probably long before, and mechanical semaphore telegraph stations were used in France in the late 1700s. Electrical experiments were conducted as early as 1747 [ 11 ], with a telegraph system developed in 1774 using pith balls, 24 conductors and high voltages. There were many other attempts as well, but it would be folly to even try to list them all. The above reference (amongst many of the others cited here) does cover quite a few of these early attempts, as well as a great deal of historical information.
However, the system devised by Morse and his co-workers eventually defeated the other contenders - partly due to its inherent simplicity, and partly due to intense litigation that saw many competing (or even complementary) technologies disallowed as 'infringing' existing patents by (especially) the US Patent Office. The problems seen by many of today's inventors (and corporations) are certainly nothing new. The once dominant Western Union was the largest provider of messaging (and later 'telegram') services, which were initially all based on Morse code, but adopted new technology such as teletype (TTY) and teleprinter networks when they became available.
Up until the end of the last century, Morse code was a requirement for amateur radio operators, many military personnel and a number of other occupations where communication was involved. Although it's no longer used by most people, it still retains an important place, not only in history. SOS (the international distress call ' ... --- ... ') is still recognisable to this day by a great many people, and will invoke the same reactions now that it did over 150 years ago. Early Nokia phones would beep ' ... -- ... ' when a message was received - Morse code for SMS.
Patented by Samuel Morse, Joseph Henry and Alfred Vail in 1836, the telegraph (using Morse code) was first demonstrated to US Congress in 1844, transmitting the message "What hath God wrought" over a wire from Washington to Baltimore. He later experimented with submarine cable telegraphy, which was to become the first intercontinental messaging system created. However, there is considerable conjecture concerning the real 'who-what-when and why', which is covered very well in the first reference [ 1 ]. However, this short article is intended only to provide some basic background, and to show the importance of the early electro-magnetic signalling schemes.
Most of the very early attempts at telegraphy originated in Europe, but with a few notable exceptions, failed to gain acceptance. A system devised by William Cooke and Charles Wheatstone was in use by the British railways in the 1830s, and was the first electric telegraph system ever to be used to catch a murderer in 1845 [ 15 ]. These telegraph systems used a system of needles which could be deflected left or right, pointing to the desired letters in turn, and were based on an idea first demonstrated by Baron Pawel Schilling (see YouTube video demonstration). The need for multiple wires (up to six) was a significant drawback. Another Wheatstone system used generated pulses to move a pointer to the desired letter of the alphabet on a circular dial (the 'ABC' or 'dial' telegraph). While this was well ahead of Morse code in almost all respects, the equipment would have been far more expensive to produce and maintain, and it failed to gain wide acceptance. Have a look at this YouTube video to see one in action at the Telstra Museum in Sydney (Australia).
Morse code was used first in the USA, but Europe and the rest of the world followed quickly, because of its effectiveness and simplicity. The first European line was set up between Hamburg and Cuxhaven in 1847, and many others followed. Soon the need to link countries across oceans and continents was realised. In 1866 a submarine cable link was established between Britain and the USA, and by 1872 a link to Australia was installed. These are remarkable achievements when you look back on the technology of the time, and it's hard to imagine the working conditions of those who did the hard work (and it would have been very hard work indeed).
The international code used (or what used to be used) is slightly different from the original that was developed by Alfred Vail (commonly called 'Morse Code', although it seems likely that Vail did most of the work), but the essential principles are the same. There are no lower case letters - all transmissions are assumed to be UPPER case. The code also provides numbers (0-9) and a limited number of punctuation marks ( . , : ? ' - / ( ) " @ = ). A European variant also exists to allow a few of the European characters to be transmitted.
So-called 'telegrams' (not the app that's currently widely available) were once quite common. A message was sent via Morse code from one place to another, transcribed and delivered to the recipient by a messenger. Prior to the telephone, this was faster than any other method of communication that had ever been available to the public. In Australia and elsewhere, it provided communication services that were widely used for many years, even after telephone services became common. Not every household had a phone, but a telegram message could be delivered to any address world-wide. Eventually, the teleprinter (or teletype) and telephone made the need for telegrams diminish to the point where they are no longer used. SMS (short message service) and email are now responsible for almost all text traffic.
It's also important to understand the limitations of the early telegraph systems. In particular, the extreme lack of privacy - anyone in a given telegraph office could listen to the message being received, and it would have been foolish indeed to send sensitive information. While there were almost certainly laws to prevent telegraph personnel from passing on information to persons other than the intended recipient, this would not actually prevent them from doing so. Encryption wasn't common as far as I can determine, but it was used by some [ 13 ], although it's also claimed that it was banned by law in some jurisdictions. Steganography, the practice of hiding messages within innocent-looking text, was also used to get around any laws prohibiting encryption [ 14 ].
Like many ESP articles, it is hoped that this will inspire people to do some research, and learn some of the fascinating history behind the development of electrically powered systems - particularly in the areas of communications, which was the birth of electronics as we know it. It must be remembered that when the early telegraph experiments were first carried out, knowledge of electricity was almost non-existent for the vast majority of the experimenters and inventors of the time. Strange (to us) 'solutions' came about due to a lack of understanding of the basic principles that we can now learn even in our early school years.
The duration of a dot is considered to be one 'unit', and that of a dash is three 'units'. The space between the components of one character is one 'unit'. The space between characters is three units and between words seven units. To indicate that a mistake has been made and for the receiver to delete the last word, send ........ ('HH' - eight dots). The length of a single 'unit' is usually somewhat variable, especially when a human operator is keying the code. A reasonable unit duration is the time it takes to say "dit", but there appear to be no hard and fast rules, and the duration of a single unit also depends on the transmission speed.
International (ITU) Morse Code A - Z, 1 - 0
In the early days (before radio), the code was sent simply as a voltage on the transmission ('telegraph') line. The operator used a key (a specially designed momentary contact switch) to send dots and dashes. Depressing the key sent a voltage down the telegraph wire, and that operated a sounder or paper tape punch at the other end. Early radio systems used what was known as 'CW' (carrier wave) - a (very) broad-band radio signal that was originally provided by a spark-gap transmitter. This was keyed on and off in the same way as a wired telegraph, providing simple on-off ('digital') modulation.
A spark-gap transmitter literally used an electric arc across a pair of electrodes, and the RF generated by the arc was sent to an antenna by a cable. These generated noise that was detected by various (crude by modern standards) detectors in the receiving apparatus. Nearby stations could not transmit at the same time, because the signal was poorly tuned (or not tuned at all) so provided 'blanket' coverage over a wide frequency range. Tuned systems came later, especially after the advent of 'wireless' valves (vacuum tubes) that could amplify weak signals, and the benefits of tuning became apparent. In particular, a tuned system (if properly aligned) was far more sensitive than early broad band systems.
Tuned transmissions occupied a relatively small bandwidth, allowing transmitters in the same locality to operate without interfering with each other or ruining reception. Each transmitter used its own frequency, so a selective receiver could pick up the frequency of the desired signal source. Unlike today, even relatively low power transmitters were large, complex and expensive, so there would never have been more than a small few in operation at any one time.
As 'wireless' (as it was known at the time) progressed, it became possible to modulate the carrier with a tone. Before that, tuned (single frequency) CW receiving systems generally used a 'BFO' (beat frequency oscillator) that could be adjusted to be around 500-1kHz higher or lower than the transmitted signal. When the transmitter was activated, a 500-1kHz tone could be heard at the receiver. The frequency of the BFO can be adjusted to obtain a signal that is clearly audible, but not annoying to the receiving operator. The first modulation system developed was AM (amplitude modulation), which was used for all early broadcast (voice and music) transmissions.
I still recall being able to tune a 'short wave' receiver across the band an pick up Morse transmissions. At the time, I was not yet a teenager and never bothered to learn Morse code. In hindsight this probably left a gap in my overall education in the world of electronics, but I've never been in a position where it could have been useful so I'm not overly distressed. A transmitter sending amplitude modulated Morse code as a tone provides reception capabilities that are second to none. The tone can be heard and interpreted at levels well below the noise. It's possible (but would require a very low data rate) to detect a tone that's up to 20dB below the peak noise level. This is demonstrated by the Noise plus Morse recording, where the tone is 12dB below the peak noise level (-6dB). The level of the 550Hz Morse code is -18dB.
Morse code was also used for line-of-sight communications, often between ships at sea. They most commonly use a continuous lamp (e.g. Aldis lamp), with a shutter mechanism that blocks the light until it's activated by the operator. The flashes of light can transmit Morse code between vessels, enabling communication during periods of 'radio silence' - usually imposed so that enemy vessels were unable to locate a convoy by using radio direction-finding (RDF). The same thing can be done today using a LED or laser lamp, but the transmitted signal would be high speed digital rather than Morse code. A system that exploited this method was used for digital communication between buildings (Datapoint 'LightLink'), which offered infrared optical transmission up to 3km at data rates of 2.5M bits/second (yes, I used to work on them ).
Until the early 20th century, the primary source of power for the telegraph was primary (non-rechargeable) batteries, typically based on the chemical principles demonstrated by Alessandro Volta in 1800. It seems that the original Morse telegraph used five Grove cells (zinc + sulphuric acid anode, platinum + nitric acid cathode), each producing 1.9V so the total voltage was 9.5V. However, Grove cells generate nitrogen dioxide (NO2) as they discharge. When used in large numbers (such as at a telegraph station), NO2 can lead to lung disease and other ailments.
Note: In case you were wondering, rechargeable batteries could not be used because the telegraph was in constant use well before mains electricity was available anywhere. There was no power source available for charging, and secondary (rechargeable) batteries didn't even exist before 1859 when the lead-acid cell was invented. A web search will provide much interesting history for you to read through.
The wiring between telegraph stations was most commonly iron (or probably what today might be called mild steel). Annealed copper wire is too soft, and is unable to support its own weight across the typical distance between poles, and the idea of 'hard drawn' copper wire as is common today had not been discovered at the time. Copper wire is (and was) also a great deal more expensive than iron, but of course it is a far better electrical conductor. There is some information about rust prevention with iron wire. In the very early systems the wire(s) were coated with tar (presumably coal tar) which would have been a most unpleasant task indeed. In later years the wire was galvanised (coated with zinc), but details are rather sketchy. It appears that in Britain, zinc coating (galvanising) was common, but high sulphur levels in the atmosphere (from burning coal for home heating and industry) caused the zinc coating to degrade quickly.
For transmission, the early keys were simply an on-off momentary contact switch. There were countless designs developed, with special emphasis on ergonomics, with style and design intended to try to sell one maker's unit over the competition. A key that requires minimal travel improves sending speed, and if it's comfortable to use the operator won't tire quickly. The term 'RSI' (repetitive strain injury) didn't exist 150 years ago, but it's unlikely that the condition itself didn't exist for Morse operators who may have done little else during the day. Today, it's a simple matter to have a computer translate ordinary text into Morse code and back again, but of course there's no longer any need to do so.
Figure 1 - Transmitter Key Example
The key shown above is a standard key, and the knob is depressed for the duration of a dot or dash. The spring tension is adjustable, as is the stroke - the distance the key must be depressed to make contact. Individual operators would adjust the key to suit their personal style and preferences. Other keying systems were developed as electronic circuitry became capable of simple logic and timing functions. Early keys had an extra contact that allowed the key contacts to be shorted, and this allowed a single wire with an earth/ ground (literally) return to be used for transmission and reception - but not simultaneously.
In the later years of Morse code, a key system that many found to their liking was a system of 'paddles' that operated from side-to-side rather than vertically, and known as iambic paddles. One paddle produced a train of dashes when pressed (inwards) and the other a series of dots. If both paddles are operated (squeezed together) the electronics would output an alternating sequence of dots and dashes ( .-.-.- ). Timing of the dots and dashes was/is electronic, and is faster and more precise than purely manual operation of a traditional key.
Reception was an altogether different matter. The primary goal was sensitivity, because power sources of the day were generally unimpressive, and for a long range transmission the wire resistance would be considerable. More sensitive receivers needed less current from the telegraph line, and could offer impressive battery savings and/ or longer range. Both were important, because there was no way to amplify the signals, other than by using a repeater - essentially a receiver connected to contacts that could re-transmit the original Morse code with close to a zero error rate. This alone was remarkable !
The earliest receivers were nothing more than a couple of electro-magnets. When a 'dot' or 'dash' was sent from the key operator, the electro-magnet would close for the duration of the signal. This was used to mark a paper strip that was drawn through the system using a clockwork drive.
Figure 2 - Audible Sounder
Another common arrangement was the use of two sets of contacts on the Morse key. When the key wasn't being used, a secondary contact set (placed where the end stop is shown) connected the incoming line and battery supply to the receiver. When the remote key was operated, this would activate the receiver/ recorder unit. Operating the key would close the main contacts, sending power to the remote receiver, via the closed contacts in the remote key. A complete system (albeit greatly simplified) is shown below, including the dual contact key, and a simplistic representation of the receiving system. The paper tape was moved by a clockwork motor.
When the key is at rest, the rear contacts are closed, so the telegraph line connects to the receiver. When the key is operated, it connects the battery to the far end via the line, and to the far end's receiver. This allows two way communication, but only one station can transmit or receive at a time (half-duplex). An end of message code was typically used to indicate that the line was clear, so another operator could send a message.
The very first 'sounders' were simply an electromagnet, as shown in Figure 2. In some early British systems, the deflection of a magnetic compass needle was used as a receiver (developed by Charles Wheatstone and others), but one's eyes are poorly suited to decoding visual cues. Our hearing is far more sensitive to short impulses, and can easily distinguish between a dot and a dash, even when sounded from a simple electromechanical sensor.
However, it is far more convenient to have a permanent record of the message, and the receiver (known as the 'register' in the Morse system) used a paper tape to record the message. The tape's clockwork system could be activated remotely (not details have been found as to how this was done), so an operator could activate the receiver from the far end, then send the message. There did not appear to be a way to stop the tape again though, which had to be done by the operator at the receiving station.
Figure 3 - Complete Simplified Telegraph System (One End)
Some early attempts used pencils, but the stylus was more durable. Later versions used an ink pad or roller. If done today, a thermal transfer paper similar to that used in most labelling systems and cash register receipts would be the easiest and require the least maintenance, but such little luxuries were unknown at the time. Other telegraph systems did attempt to use anything from chemical reactions to primitive (by the standards of today) thermal transfer, but with the general lack of understanding of electricity at the time (and the low speed of both chemical and thermal detectors) they were never implemented in any commercial systems.
As mentioned above, it was possible to include a repeater (relay station) in the telegraph line, allowing for much greater distances that could otherwise be achieved. Although the repeater was common fairly early in the development of the systems, it was usually intended to 'amplify' the weak current from the line (influenced by high resistance) to drive the local register (receiving unit). The ability for the repeater circuit to act as a relay gave the name to the device we still know today as a 'relay'. A small current in the coil can switch a much greater current via the contacts. These early relays were the only equivalent to valves or transistors in the 19th century.
The relay station had its own battery supply, so as the signal was received by the relay coil, the contacts closed and delivered a current from the local battery allowing the signal to travel much further than would otherwise be possible. Relay stations would require regular maintenance of course, but this was faster and less error prone than having an operator manually re-transmit the message.
Figure 4 - Telegraph Relay
When current passes through the coil, the steel armature is attracted to the electro-magnet's pole piece, closing the contacts. The armature is prevented from making contact with the contact support by means of an insulator. This was at a time where modern insulation materials were not available, and (apparently) ivory was used in some systems. The relay would generally be adjustable so the sensitivity could be controlled. Almost all of these early systems had adjustments for most of their parameters, because the principles weren't well known - even amongst those building the apparatus. Although Ohm's law was understood (to a degree, by some), this was pioneering work, so much of the equipment in use was barely beyond the stage of an experimental prototype.
This is shown fairly clearly when you look at photos of the original equipment. Today we expect to find closed magnetic circuits, where the iron core not only passes through the centre of the coil, but wraps around so the other pole is also close to the armature (the moving piece). It's not always clear, but most of the gear does use a closed magnetic circuit, generally with two coils on a 'U' shaped polepiece. However, some of the equipment of the mid 19th century seems to have used an open magnetic circuit, which means that more ampere-turns are needed for a given pulling power.
The concept of ampere-turns appears to have been almost unknown to many of those involved, and some believed that to get the best magnetic strength, the wire around the electromagnet had to be as large as possible. This led to some impressively large equipment, with decidedly unimpressive performance. However, coils wound with many turns of fine wire were difficult, because there were no suitable insulating materials for the wire. Most insulation consisted of cotton thread wound around the wire, usually in two or more layers, with each wound in the opposite direction. DCC (double cotton covered) wire is still available (why? - mainly for restoration of antique gear), but the cotton is often also used with insulating enamel, something unavailable to the pioneers of the telegraph.
You may notice from the drawings and schematics that there is no attempt to counteract the back-EMF generated by the receive coils when the current is interrupted. When these systems were devised, there were few people who really understood the concept of back-EMF, and no components existed to reduce it. Today we'd use a diode, but of course these didn't exist at the time. It was many years after the first equipment was developed before even resistors became available, and when they did, they were hand made using cotton covered resistance wire.
In academic literature of the day [ 11 ] the concept of 'induction' (back-EMF) was known, but wasn't understood to the extent that it is today. Measuring systems were minimal, so people relied on the distance that a spark might jump to evaluate the voltage generated by induction. It appears fairly likely that few of those who built or maintained the telegraph would have even been aware of the science of electro-magnetism outside of their own experiments. Much of the material of the day (ca. 1840) indicates that the study of electricity was in its infancy, and it remained poorly understood (even by the likes of Michael Faraday [ 12 ]).
A quick simulation shows that even a 300m length of 50 ohm coaxial cable will cause the back-EMF to be attenuated to a reasonable degree, so the transmission lines of the day would probably have limited the peak back-EMF voltages to a great deal less than you might expect. This is especially true because of the lossy nature of the transmission systems used, but I could not find any information about back-EMF and its effects during the early days of the telegraph. There are reports of linesmen suffering electric shock, but details are scant. In some cases, it would simply have been the result of the use of relatively high voltages, with systems sometimes operating at 100 to 150V to try to extend the range and counteract the line resistance.
One fascinating quote [ 11 ] that highlights the issues faced ... "More damage is often done to the telegraph in a second by a thunder storm, than by all the mischievous acts of malicious persons in a whole year." Lightning arrestors and other protective measures were developed to minimise damage to equipment and operators. This was especially important in America, because violent thunderstorms are far more common there than in Europe, so it's no surprise that many of the lightning protection systems were developed in the US.
Prior to the discovery that gutta-percha (still used for root canal therapy in dentistry) made a good insulator, underwater services weren't possible because sea water is highly conductive. A rigid natural latex produced from the sap of various trees of the genus Palaquium, gutta-percha was commercialised in the mid 1800s. Underwater telegraph cables became possible after British suppliers started producing underwater cables that were immune from attack by marine creatures (plant or animal). The first trans-Atlantic telegraph cable started service in 1858 and used gutta-percha insulation (amongst other protective coverings). This cable subsequently failed due (it's claimed) to high voltages being applied. It's not clear if this was due to inappropriate testing methods or an attempt to improve the transmission speed. Both claims exist, and it appears impossible to determine which is right.
By the beginning of the 20th century, people had a much greater understanding of the behaviour of electrical signals in a long transmission line. Speed improved from a claimed 2 minutes to transmit a single character (0.1 WPM - words per minute) to 8 WPM by 1866 or thereabouts. This was partly due to improved cable construction. By the early 1900s, transmission speeds improved to around 120 WPM as engineers discovered that electrical loading systems (coils, capacitors and resistors) could be applied to ensure that the sending and receiving systems matched the impedance of the cable itself. For more information on this particular topic, see Coaxial Cable.
Even the commonly used single suspended wire with earth return forms a transmission line once it's long enough. At the transmission speeds used at the time, the effects were minimal, but undersea cables had far greater capacitance per unit length than an above-ground system, and the effects of this weren't understood at the time. Today we know that a transmission line terminated with its characteristic impedance is close to flawless even at very high frequencies, but these concepts were unknown at the time. Experimentation was the only tool available.
Once radio (wireless) became mainstream, the growth of the telegraph became an unstoppable force. The very early systems were extremely limited, using spark gap transmitters and receivers consisting of 'coherers'. The coherer was a primitive detector, relying on fine conductive particles in a sealed glass tube aligning themselves (cohering) to provide a low resistance path upon reception of a radio signal. A mechanical means of 'de-cohering' the device was required, typically a small 'clapper' as may be used by an electric bell. When the coherer became low resistance due to a wireless signal being received, this activated a solenoid. This activated an arm that tapped the tube, restoring the non-coherent state of the particles within to await the next signal.
As expected, coherers were slow, and were never a truly satisfactory means of reception. Detection and distinction between dots and dashes of Morse code would have been a specialised skill, by listening to the sound produced by the decoherer as it constantly reset the coherer while a radio signal was present. There is little information available on this particular topic, so we must imagine that the Morse signals would have been heard as bursts of 'noise' from the decoherer resetting the device as the message was received.
Once John Fleming invented/ discovered the electron 'valve' (vacuum tube), detection became easier, but it wasn't until the invention of the first amplifying valve (the Audion) by Lee De Forest in 1906 followed by true (high-vacuum) triodes in 1913 that wireless became really viable. The Audion and high-vacuum triode created a flurry of activity that hasn't abated to this day. By the 1920s, wireless was well understood and broadcasts of popular music and news were becoming common.
Despite this, Morse code was still very much alive, especially for military applications. It was likely possible to operate an AM (amplitude modulated) transmitter in the field in 1918 or thereabouts, but the size and complexity of the equipment needed to transmit and receive the transmissions was such that it would have been non-sensible to try. This changed when miniature valves first appeared in 1938. Even during WWII, Morse code was widely used, with one of the most notorious schemes ever seen to appear in the late 1930s - the German Enigma encryption system.
Messages were first written, then encoded using the Enigma machine. The coded message was transmitted using 'Morse' code - albeit a modified version that suited the German alphabet. The encoded message made no sense to anyone who intercepted it - even if they had an Enigma machine themselves! If they didn't know which set of rotors were being used, and the initial setting for each individual rotor, the message could not be deciphered. The rotors were set at the beginning of each day to a pattern described in a code book, and each time a key was pressed, the coding changed. Do a web search if you want to know more - it is a fascinating (albeit very complex) topic, and doubly so if you look into the procedures used to break the code. I don't propose to cover this in any greater detail, but one thing that made the job at Bletchley Park easier (this is where Enigma was fully broken and decrypted) was the simple fact that the Enigma was unable to assign a plaintext (not encrypted) character to itself. For example the letter 'A' could become any letter in the alphabet in the ciphertext - except 'A', and likewise for all other characters. In modern cryptology, this is considered an epic fail.
Other than for its entertainment value for enthusiasts, there is almost no Morse code used any more. For anyone wanting to learn, there are countless websites that have pre-recorded Morse samples that you can practice with, and the chart shown below makes it easy to get started with slow (typically no more than around 5 words per minute) Morse code. The chart should be printed out to make it easier to use
Figure 5 - Morse Code Learning Aid
The aid shown above is easy to use, and can help you to learn Morse code. To use it, when you hear a dash ("dah") you move to the left and down, another dash means you move left again. A dot means that you move right. As each segment is heard, you simply move left or right, so ' -..- ' takes you left, then right, right again, then left. The letter is 'X'. There are a few slightly different versions of this chart, and I have tried to make this one as clear as possible. The full stop (period) and hyphen have been added, as has the bracket (parenthesis) - there is only one in the code, and it's up to the operator to decide which way it goes.
I've also shown a short example of code, along with the relative spacings of dots, dashes and spaces between characters and words. As noted earlier, a dot is 1 unit, a dash is 3 units, the space between characters is 3 units and between words it's 7 units. The length of a 'unit' depends on the transmission speed.
There are also a number of 'prosigns' (procedural signals) used. These are mostly two letter codes that are sent without the normal character space, so are transmitted as if they were a single letter. The last two ('C L' and 'B K') may be transmitted as separate letters, with the normal inter-character space (the length of three dots) between them. Some references show them as being sent as a single stream, while others show them as two characters.
Prosign Code Meaning AA .-.- New line (carriage-return + line-feed) AR .-.-. New Page AS .-... Wait BT -...- New paragraph CT -.-.- Attention (important message) HH ........ Error (delete last word) KN -.--. Invite a specific station to transmit SK ...-.- End of transmission SN ...-. Understood (also VE) SOS ...---... International distress message C L -.-. .-.. Going off the air (clear) B K -... -.- Break (back to you)
The recommendation from nearly everyone is that you learn Morse code by sound, and not as a written sequence of dots and dashes. Although there is no longer any requirement for anyone to learn Morse code, there will undoubtedly be those who want to learn just for the fun of it. There are countless applications (even today) where it could be useful, and this is especially true if you happen to think that Armageddon is on its way someday soon .
Almost all messaging is now digital, and this includes the land-line telephone - it's analogue only as far as the local exchange (central office), and from the far-end exchange to the home. Most businesses with more than a couple of phone lines connect to the network digitally, and the conversion to analogue may not take place until it reaches the telephone itself. Many 'cordless' home phones now use DECT (digitally enhanced cordless telecommunications), another digital protocol that has far greater security than earlier analogue cordless phones, and other (often proprietary) digital protocols are used by various manufacturers.
Mobile ('cell') phones with SMS provide greater connectivity than ever before, and there is no longer any need to use Morse code. However, it's an important part of history, and as such it has to be preserved. There is a good case for museums in particular to utilise some of today's technology to enable simple demonstrations of the technological triumphs of the past, with interactive displays rather than a few pieces of yesterday on a shelf behind glass, doing nothing.
The descriptions above do not include bipolar signalling (positive and negative voltages applied to the telegraph line or cable), nor the many variations of senders and receivers that were in common use. This is a short introduction only, and was never intended to be a complete reference work. The basic sender (key), receiver and relay are fairly detailed because it's necessary to show just how they worked. The 'register' (recording receiver) is included because it was such an integral part of the system.
There is a surprising amount of information on the Net covering Morse code, the various adaptations used for specific countries and the history of telegraphy. I encourage anyone who is interested to do a search, as some of the equipment used is of great historical interest, as are the inevitable arguments (and legal challenges) as to who did what and when. During the early days of electronics (because this really is the beginning of electronics as we know it), there were some epic battles between the various people involved. Some were very well known, but others not so much so.
This short article is intended as an introduction, and as a recommendation for others to look into the subject. As with many of the early inventions and discoveries, it's inevitable that if they hadn't been invented by the people we know now, someone else would have done so. In many cases, someone else did invent things that are routinely attributed to others - after all, history is written by the victors in any altercation, but that doesn't make it real. Reference 1 is a long article, but it goes into some detail about the 'disagreements' between the protagonists, and also has a truly impressive list of references. Reference 6 has many photos of early Morse keys, sounders and receivers.
There is little doubt that Morse code and the equipment developed to transport messages signalled the beginning of the 'information age'. Although it's not often acknowledged, the communications industry was responsible for the vast majority of the things that we take for granted today. Once the telephone became popular, 'phone companies pushed the boundaries of what was possible. The transistor was the result of research at AT&T's Bell Laboratories - after that, electronics became a part of our lives that becomes ever more entrenched as we rely on better, faster and more ubiquitous technology. Communication still rules as one of the primary drivers of our advanced technologies.
It is educational to read the words of the 'ancients' (as it were) from the early days, just to learn how they perceived and understood (or failed to understand) principles that are considered to be basic knowledge by anyone even remotely connected with electrical equipment today. Many of the texts from the 1800s are available as a free ebook or PDF download thanks to Google's efforts at digitising this material. We have free access to knowledge and research material that was difficult or impossible for most people to get at the time.
As you look into the history of written telecommunications, you find references to Baudot code, patented by Émile Baudot in 1874 (5 bit), which was the precursor of EBCDIC (Extended Binary Coded Decimal Interchange Code - IBM) and ASCII (American Standard Code for Information Interchange). The latter two are 8 bit encoding schemes, and ASCII is still used as the basis for most human-readable text used in computers and on the Net. The term 'baud' for serial communications speed came from Baudot.
Remember that the readily available knowledge that we now expect at our fingertips had its beginnings in the printing press (invented in 1440), but instant communication came from the electric telegraph, as the first method ever devised by humans to transmit information over thousands of kilometres in just a few minutes. So much has been achieved in such a short time, thanks to the efforts of early pioneers who didn't know a fraction of the information that we expect to find at a moment's notice today. One wonders what they would think of the Internet !
These references are in no particular order, so the first may be referenced towards the end of the article or vice versa. The reference numbers you'll find scattered through the article do point to the specific reference below. Some may not be referenced in the text at all, indicating that they have either simply been used as verification, or snippets of the info have been used in multiple places. I have tried to include all of the main reference material here, but it's also probable that some have been missed. If so, I apologise in advance.
To see some of the truly vast amount of information available on-line, do a search for 'electric telegraph'. This will lead you into some of the basics, but as you widen your search you'll discover just how much you never knew about telegraphy in general. In its day, the telegraph was a far greater leap into the unknown than the internet, because the latter was based on so many discoveries from the past.
Please Note: There are countless references that were used to double-check the validity of many claims made, and to extract a few finer points about the systems and how they worked. Not all have been included above, as the reference list could easily become unwieldy. For those interested, the list above is a good starting point, but it's surprisingly easy to look at ten different sites (and/ or books) and get ten different answers. It's up to the reader to determine what looks as if it might be real and what is obviously (or not so obviously) bogus. Historical information such as this can be notoriously difficult to verify. Much of the very early material was based on conjecture, because the principles of electricity (as we know them today) were still mysterious.
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