Reading Level - Yr 9 to Adult
Describes the technical requirements of long distance transmission across Australia and the development of trunk systems and their dependent networks.
Section 1: The Magic Chain: Linking a Nation
On November 23, 1872, a grand celebratory banquet was held in Sydney to mark the successful opening of telegraphic communication with Europe. The Governor of New South Wales, Sir Hercules Robinson, said in a speech at that banquet: "The Earth has been girdled, as it were, with a magic chain..."
Yet the new link in that "magic chain" was quite unremarkable to look at. A single strand of iron wire, suspended on porcelain insulators on top of a series of roughly-hewn wooden posts, the Overland Telegraph Line would have seemed hardly likely to bring about the beginnings of a communications revolution for Australians. Yet that is what it did.
Unremarkable as a short stretch of it might have seemed to a casual observer, the Overland Telegraph Line was indeed remarkable in many ways. For one thing, that single strand of wire stretched out, not for tens of kilometres, not for hundreds, but for more than three thousand kilometres right across the arid centre of Australia, from Adelaide to Darwin, across country that had barely been explored. At Darwin it met with an undersea cable that had been extended from Java, where it connected with a line going on to Europe and England.
It was remarkable also in its effects, because for the first time in Australia's history it allowed communication to take place with England in a matter of hours. Before the Overland Telegraph Line was completed, news and letters from England took a minimum of 60 days to reach Australia by the fastest clipper ships then at sea.
The actual building of the Line is a story full of heroic endeavour, of clashing personalities, of impossible conditions, and of deadlines missed, but this is not the place to tell that story. The purpose of this kit as a whole is to look at the development of the technology of long-distance telecommunications.
So, how did that single strand of iron wire cause such a communications revolution?
The line was operated using the principles of Morse telegraphy. Samuel Morse set up his first commercial telegraph line in 1844, between Baltimore and Washington in the United States. Messages were sent using a code of his own invention, consisting of dots and dashes (see The Story of Digital Transmission). These were transmitted as short or long pulses of electricity along a conducting wire.
On the Overland Telegraph Line, the electricity was supplied by banks of large batteries enclosed in glass containers, each over 25 cm high and 10 cm in diameter. About eighty of these batteries were needed at each repeater station to provide the necessary voltage for sending telegraph messages on to the next station. They contained zinc and lead electrodes and solutions of copper sulphate and magnesium sulphate, and each produced about 1.5 volts of electricity.
Pulses of electricity were sent by closing a Morse key, or sender. Such keys are still used by ham radio operators. Basically, the key was a spring-loaded switch, which when pressed down by the operator's hand closed a set of contacts and allowed current to flow along the line. By keeping the key down for shorter or longer periods of time, pulses of differing lengths could be sent.
In any electrical circuit, there must be a continuous loop so that current can return to the battery. Yet we have said that the Overland Telegraph Line (in common with all other telegraph lines at that time) consisted of only a single strand of iron wire. How did the current return to the battery? The answer is that the return path was through the earth itself. One terminal of the set of batteries was always carefully grounded to allow this to happen, as of course was one terminal of the receiving equipment.
This use of a ground return had advantages and disadvantages. It certainly made the erection of a line very easy, as just one wire needed to be erected on a simple pole. And apart from the porcelain insulators on each pole, the wire itself could be left bare and uninsulated. Techniques for wrapping or coating wires with an insulating substance all along their length were then at a very primitive stage, and would have increased the cost enormously.
One major disadvantage, however, was that it was vital to prevent the bare wire from becoming 'earthed', that is, from coming into electrical contact with the ground, because this would 'short circuit' the wire and prevent the electrical current reaching the next repeating station. This, of course, was the reason the wire had to be erected on poles, away from contact with the ground. But in wet conditions the branch of a tree touching a wire could create a conducting path to the earth, so trees had to be cleared on either side of the line. Indeed, in rainy conditions, the insulator and pole themselves could become coated with water and so allow current to leak to the earth. Over a long route these small leakages could badly affect the circuit. It was also necessary to put lightning conductors on every second pole, so that lightning would strike the conductor rather than the wire, which otherwise might be broken. Of course, it was vital to prevent the conductor (which ran down into the ground) from coming into contact with the insulator or the wire, or else it would provide a tailor-made short circuit.
Because the wire was elevated, it was also rather vulnerable. Breaks were common for a variety of reasons. In one reasonably typical period between 1887 and 1892, the line failed a total of 36 times, interrupting communications for periods ranging from 3 hours to 63 hours in length. Of these interruptions, ten were caused by lightning breaking insulators, despite the lightning conductors; three by storms; two by poles falling over; two by men deliberately cutting the wire in order to obtain assistance from the lineman who would ride out to the break; one by a train pulling down a quarter of a mile of wire; one by a flood; one by a bushfire; and four by short circuits of various kinds, of which by far the most interesting was caused by a frog becoming jammed between the wire and an iron pole! The other breaks were caused by unknown factors.
Still, these interruptions represented less than 2% of the operational time of the line during this period, so the breaks though irritating, did not suffice to really hold up communication. It is important to realise, too, the length of the line, and its remoteness, so that in fact the record was remarkably good. Much credit for this should go to the linemen who rode out on horseback (or in later years, on a bicycle) along the line, looking for the break.
We have mentioned repeater stations several times so far without explaining what they were. Repeater stations were needed because the pulses of electricity sent out along the line by the Morse equipment would eventually weaken and be distorted so that they could no longer be read. It was essential, therefore, to set up repeater stations at regular intervals where the Morse signals could be read, and then sent on again with renewed strength. On the Overland Telegraph Line, there were eleven repeater stations between Port Augusta and Darwin, at an average separation of about 240 kilometres from each other.
Buildings for the repeater stations were constructed during the erection of the line. They were of solid construction, of wood or stone, with a number of necessary outbuildings. Most repeater stations in the interior kept a small herd of bullocks to provide fresh meat, as well as about twenty horses, so in effect each station had to act also as a small farm. Six men worked at each station.
At stations, in addition to the primary electrical circuit and batteries which sent signals along the line, there was a secondary circuit and a second set of batteries used to power the Morse sounder and other equipment. When a pulse of electricity came down the line -representing a dot or a dash - the, by now, weak current operated an electromagnet in a piece of equipment called the line relay. An electro-magnet works just like an ordinary magnet except that it only attracts while electricity is passing through its coil.
When a signal came in, the electro-magnet attracted a piece of metal called an armature. The movement of the armature was enough for it to act as a switch and to allow current from the secondary (local) circuit to flow. In this way, a weak current in the primary circuit could allow a much stronger current to flow in the secondary circuit.
The strong current in the local circuit was used to operate the sounder. This was basically just a large electro-magnet which could pull down a spring-loaded armature with such force that it made a loud sound when it hit a stop, and again when it was released when the current stopped. By listening to the sounder, skilled operators could pick out the Morse Code and write down the letters as they heard them. Occasionally a paper-tape register was used instead, which automatically marked down the dots and dashes onto a continuous strip of paper tape, but skilled telegraphists scorned its use.
Once the message was written out (and any obvious errors corrected), the operator would use the Morse key to send the message on again to the next station down the line. So the telegram passed from station to station, not just along the Overland Telegraph Line, but through stations across Asia and Europe, perhaps finally reaching England within hours of it leaving Australia.
The technology of telegraph transmission remained basically the same for many years, though improvements were made, such as automatic transmitters which worked from a pre-punched paper tape. More details of these developments can be found in The Story of Digital Transmission.
The single strand of iron wire across Australia's arid centre remained a vital link for decades. A second wire was added in 1898, this time of copper ( a much better conductor of electricity). Morse Code continued to be transmitted on the line until after the Second World War, though by then telephone conversations were also carried on the line, and Australia had many other links to the outside world.
Today, telecommunications traffic still follows part of the same route as the old Overland Telegraph Line, but using a technology incomparably more sophisticated, and capable of carrying more information in a minute than the Overland Telegraph Line could have carried in a hundred days!
Material was produced as part of Telstra's Learn-IT program
Copyright Telstra Corporation Limited
