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. These were transmitted as short or long pulses of electricity along a conducting wire.

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 required at each repeater station to provide the necessary voltage required 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 1½ 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 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.

The use of a ground return had advantages and disadvantages. It certainly made the erection of a line very easy, as just one wire had to be erected on a simple pole; 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 coming into electrical contact with the ground, because this would 'short-circuit' the wire and prevent the electrical current from reaching the next repeating station. This, of course, was the reason why 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.

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.

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 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, 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 weak current operated an electromagnet in a piece of equipment called the line relay.

When a signal came in, the electromagnet 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 electromagnet 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.

Once the message was written out, 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.

Manual telegraph operators could key messages at rates of up to 30 words per minute. Later, automatic transmitters working from punched tape could transmit at hundreds of words per minute.

Evolution of Telegraphy

In 1819, Hans Ørsted of Denmark had shown that when an electrical current was passing through a wire, it produced a small magnetic field, sufficient to deflect a compass needle. This is the principle of the electromagnet, which, by coiling the electrical wire, directs and increases the magnetic effect, which only operates when electricity is passing through the coil. Later, it was found that a changing magnetic field similarly produces an electrical current in a piece of wire within the field.

Problems with electrical capacity were far greater for telephone transmission than for telegraph transmission because, to give acceptable speech quality, the electrical signals on the telephone line had to be able to alter roughly eight thousand times a second! Compare this rate with the pace of three words a minute over long submarine telephone cables. Of course, for relatively short underground telephone cables, the problem was reduced, but even over only a few kilometres, the capacitance of the cable tended to distort speech.

The provision of metallic circuits reduced this problem somewhat. However, the real breakthrough was when it was found that the lowest capacitance occurred in cables which were not filled solidly with an insulator, as was traditional, but which were left dry or filled only with air, with the wires insulated from each other by being wrapped in paper or cotton.

The development of dry-core cable, using twisted metallic pairs, occurred over a number of years, but was completed by about 1890. The telephone poles were replaced by underground cables.

Dry-core cable reduced the problem of capacitances when relatively short distance telephone cables were used, but over long distances, and particularly for submerged cables, the problem remained for many years. The problem was eventually solved by the invention of loading coils. In 1899, an American professor of mathematics, Michael Pupin, showed that by inserting special coils into a long submarine cable, the phenomenon of capacitance could be cancelled out by a kind of self-induction. If the coils were placed in the cable at the right intervals, the two effects completely cancelled each other out.

Pupin's discovery made long-distance cable telephony possible, but also increased the rate at which telegraph messages could be sent over submarine cables. Even so, when in 1936 a submarine telephone cable was laid between Tasmania and the mainland of Australia, it was then the longest in the world, while the first transatlantic telephone cable was not laid until 1956, using specially-developed submersible repeaters. By then, far more sophisticated telecommunications transmission methods had been developed for use on land.