Telegraphy on Open Wire Lines
Introduction to Types of Telegraph Circuits
(Author’s Note: This is a vast subject and more will be added by this webmaster as time progresses.)
Pioneering telegraphy began with underground–yes, you’ve got that right–underground, dedicated facilities as Samuel F. B. Morse and his pioneering fellows designed the first experimental telegraph with conductors placed in a conduit along railway tracks. The physical science complexities of cable construction eluded the pioneering telegraphers, and their inability to heat up the cable for the first practical application of Morse’s instrument, caused them to back down and resort to open wire as their second–and successful–transmission medium. In the final analysis, the experiment did work with open wire as the intervening facility for transmission, and the rest is history.
In the maturing years of the telegraph industry, there were a variety of telegraph systems in use. By the mid-1920s, for example–using a period where the heyday of the crossarm was rife–in the telegraph industry existed:
- Duplex – Differential Duplex, Bridge Duplex and Half Duplex
- Superimposed – Simplex, Composite, Intermediate Composite
- Printer or Automatic
Commercial telephone companies, railways and telegraph companies put these variously to use in their facilities’ operations and removed their use as time evolved. Let’s briefly touch on each, their operation and then vigorously examine how they fit within open wire application.
Single Circuit Telegraph
By far the simple, most easily understood circuit in telegraphy, consisting of two or more sending and receiving sets in series with a power source. Identified typically as a single conductor series of sets with a ground return. By the use of opening and closing a circuit a pre-determined intervals and having them received by a sounder, or receiving device, the signals are decoded by the receiver and transcribed as a message. The key produces long and short intervals of power interspaced with silence which then are reconstructed as “Morse Code”. It is called a “Single” circuit, because it allows only one directional signal transmission. Railroads used this single wire circuit form where an operating division connected several railroad offices.
Simultaneous Transmission or Duplex Telegraph
More complex, yet more flexible for receiver and sender, is the Duplex System. Here two sets of signals intermingle over one circuit but in opposite directions. There is no interference with each other’s signal. There can be four operators operating the circuit: one sending and one receiving at each end of the line and message capacity is twice that of the Single circuit telegraph system. Unfortunately, cutting the circuit into several intermediate offices is impracticable. And, I might add with duplex operation, two circuits exist:
- Bridge Duplex
Differential Connection of Telegraph
When examining the duplex circuit, we can take the windings of the main line relays and connect them “differentially.” That is, so the transmitted current divides at the sending end relay, allowing a current portion to go through one winding to line and the current remaining to transmit through an “artificial line” to ground. What happens to make this possible is that the artificial line is adjusted so that the electromagnetic field of each winding neutralizes or counteracts the other and does not affect the armature of the device.
At the receiving end, these currents passing through the two relay windings won’t be of values to neutralize each other and will control the armature. Polar type relays are used for the main line and positive and negative impulses form the signaling basis.
Bridge Duplex Telegraph
Have you ever heard of a Wheatstone Bridge. The device is named for the device’s investor, not the inventor’s name, and is similar to the operation of this particular telegraph device: the Bridge Duplex System.
What happens is this: the main line relay is bridged across a retarding coil, and the purpose is important when overlaying telegraph on a telephone open wire line circuit. There is far less interference to telephone speech than the above differential duplex telegraph. They call this circuit a “superposed” one.
We might also mention the “Half Duplex” telegraph circuit design. This is modified form of duplex telegraphy, where should you want to substitute the single circuit in cases where your line has few–if any–intermediate telegraph stations and where single circuit performance fails. Reversals of current are responsible for the Half Duplex system’s sustained successful operation. If you have poor circuits preventing the single circuit telegraph from operating well this would be your alternative.
Four Signal Telegraph: Quadruplex
A single circuit can be asked to perform the transmission of four sets of signals simultaneously over a single circuit–e.g. two in each direction–with limited interference. There can be up to eight telegraph operators, two sending and two receiving at each end of the telegraph line.
Here’s where a two wire pair can perform one or two-wire grounded telegraph return circuitry. This can allow operation without interference between the classes of service. These are called “superposed” circuits. We divide such service into two “classes.” We’ll get more involved with details later.
This system utilizes two conductors of a telephone circuit in parallel as line wires forming a grounded telegraph circuit. The flexibility of this system allows it to operate as a single, duplex or quadruplex circuit. I have heard of phantoming on these circuits by commercial telephone and railroad organizations. The phantom circuit was “simplexed” as a physical circuit by using all four wires as the telegraph line wire. While it was used for short circuits, the leakage problem circumvented its use on long systems.
Compositing means a pair of telephone conductors furnished a telephone and two telegraph circuits when this system was applied. Where a phantom group was arranged on open wire, the two side circuits can be composited which formed four telegraph and three telephone circuits from those four conductors. Railways often operated these superposed circuits as simplex or duplex telegraph system circuits. The drawback to this system is that intermediate stations along the route of this system are not permitted. More on that later . . .
This is a system when telephone sets are required to be installed along a composite telegraph circuit. Usually, this was avoided, due to the transmission losses these sets introduced to the lines.
Printer or Automatic Telegraph Systems
These high speed data circuits allowed for printers and automatic telegraph transmission to be used. Printing telegraph was used on single wire, simplex or composite telegraph circuitry. Composited circuits usually did not allow for high velocity speeds and were slower than non-composited type circuits.
DC Transmission Line Modelling: Initial Studies on Telegraph Lines
By Tom Hagen
This set of articles is intended to be an introduction to transmission lines and transmission line parameters. I’m hoping that anyone interested in this topic will get a good intuitive feel on why open parallel wire communication systems are built the way they are. This subject can be very technical if you go into the mathematical constructs. It took a number of “first rank” physicists several decades in the 19th Century to get to the point of where the behavior of parallel open wire systems could be definitely modeled, characterized, engineered, and reliably operated in the real world.
I’ll add to this section of Doug’s website as time permits, so check back every few months and I hope to add one or two more articles after the first one.
DC Transmission Line Modelling:
- Theory of capacitance and resistance
- Underground vs. overhead telegraph lines (early work)
- Undersea telegraph cables
- William Thomson’s (Lord Kelvin) efforts
- Thomson’s square law
Traveling Wave Transmission Line Modelling:
The work of the “Maxwellians”
- Comparison of DC and AC transmission line characteristics
- Distributed parameters of the transmission line
- Characteristic Impedance of the transmission line
- Travelling waves on transmission line
- Group delay problems on transmission line
- Distributed inductance to improve telegraph cable speed
- Loading coils on telephone lines
Open Wire Telephone Lines: Application of transmission line characteristics to open wire lines and technology.
The first inklings that long wires behave differently than short ones came about during he early development of the first telegraph systems in the early to middle Nineteenth Century. It was observed that a wire acts one way when it is mounted overhead on poles and insulators and another when it was laid underground. Experiments performed in the 1820s showed that a wire laid underground or in water passes electrical signals more slowly than a wire held overhead in air. Michael Faraday (1791-1867), explained this effect as an effect of the electrical capacitance between the wire and the medium surrounding it. Electrical capacitors were known to scientists by this time because the first electrical charge storage device, the Leyden Jar, was invented in the middle Eighteenth Century.
Referring to the below figures, an electrical capacitor is formed between the ire and the medium. An electrical capacitor stores energy in the form of an electrical field between two conductors in close proximity.
A long wire buried in the ground can take on an electrical charge if you connect a voltage source such as a battery to it and a ground rod driven into the ground. This is similar to giving a balloon a charge of static electricity when you rub it against a cloth.
Under the right conditions, i.e., if the wire is long enough and if the charge leakage to the earth is low enough, you can measure the retained charge that results in a measurable voltage between the wire and the Earth.
In this case, the wire and the Earth form what is called an electrical capacitor.
The concept of electrical capacitance is explained with the use of the diagrams to the right. A parallel plate capacitor is formed by placing two conducting plates close together and not touching. To demonstrate capacitance, the capacitor is connected in series with a battery and a switch. When the switch is closed, current in the form of free electrons in the circuit connecting the plates flows into the plates of the capacitor and charges up the plates.
Electrons are added to one plate from the negative terminal of the battery and are removed from the other plates of the capacitor to the positive terminal of the battery. This separation requires work, or energy, and the stored ene3rgy in the battery supplies the energy to do this.
If the capacitor is disconnected from the battery, the plates remain charged, and an electric field is formed between the plates. This field represents stored energy in the form of electric potential or voltage across the plates.
Since energy flows from the battery into the capacitor at a finite rate, it takes a finite period of time to transfer energy to the capacitor in the form of a charge. If energy flowed instantaneously, then the battery would have to be a power source of infinite power!
Another way to look at the charging process is to compare it to a water bucket filling up from a large reservoir. The large reservoir doesn’t change level in a measurable way when a small amount of water is removed from it, e.g. imagine removing a bucket of water from the Atlantic Ocean and what little difference is noticed from this!
The battery supplying current to the wire is represented by the large reservoir and the capacitor is represented by an empty bucket. When the valve between the reservoir and the bucket is opened, water flows into the bucket and when the water levels are equal, water flow ceases.
The resistance to water flow is set by the diameter of the pipe connecting the reservoir and the bucket. The electrical analogy for this is the internal resistance of the battery and the resistance of the wires connecting the battery to the capacitor.
Electrical capacitance is one of the factors limiting the speed at which signals can be sent over a long wire. If you imagine that the bucket in the above analogy represents the electrical capacitance of buried or submerged wires, then the overhead wire could be represented as seen in this diagram below.
You can see that the water level takes less time to reach equilibrium with less volume (or capacitance) to fill up with a flow of water or electrical charge. Additionally, if you can reduce the resistance to the flow of the water or charge, in both cases, the final water level is reached faster.
Electrical signalling over a wire is done by charging the line voltage up to source voltage to represent a “high” signal and then letting it drop to zero voltage, representing the “low” signal. Analogously, you can send a signal with the water setup by filling the bucket to the top and then draining it to the bottom to represent the “dits” and “dahs” of the Morse Code (or for you modern folks, the binary ones and zeros of a digital signalling system).
So, in our water “signalling” system, we can send data faster with the smaller bucket than we can with the larger bucket. In other words, it take less time to fill and drain the smaller bucket than the larger bucket, thus we can signal at a faster rate. This is analogous to the speed-limiting effect of electrical capacitance.
A simple model for a telegraph line is seen in the diagram below. A basic telegraph setup may use one wire on poles with glass insulators attached to the pole. All electrical circuits must have an outgoing path and an incoming path to the power source (e.g. a battery). The circuit is considered complete because the current can flow from the negative (-) terminal of the battery to the positive (+) terminal. The current loop is completed to the battery through the Earth (Yes, the Earth is a good conductor because the cross-section of such a conductor is enormous).
When the key on the left is closed, current flows through the line and into the receive relay coil. The magnetism of the oil pulls the relay contacts closed and lights up the bulb. The capacitance of the line to ground is represented by the capacitor symbols and the leakage currents to ground are represented by the resistor symbols at the insulators on the poles.
This simple model illustrates the behavior of a telegraph line in terms of charge moving into and out of the line in a direct current (D.C.) model. Three D.C. characteristics are modelled:
- Series resistance of the wire
- Line capacitance
- Line leakage
This model is adequate for solving the problems of very long undersea telegraph cables.
Undersea Telegraph Cables
About twenty years before the invention of the telephone, the first telegraph cable was laid across the Atlantic Ocean between Ireland and Newfoundland. It was completed in 1858, after a couple of failures, beginning in 1857. The combination of crude detection techniques and signal delay on this cable limited its data rate to about one word every ten minutes! Unfortunately., this cable was worked for only a few months before its insulation failed due to an ill advised attempt to increase the signalling voltage to thousands of volts. When the first successful trans-Atlantic cable was laid in 1866, the data rate was up to a whopping eight words a minute. This did, at least, allow the cable to be a viable business proposition.
A number of undersea cables of shorter length had been operated before this time, starting between the British Isles and Continental Europe. Signal delay on these shorter cables had already been observed, and the word-per-minute rate on a given cable was set according to the time delay of that cable.
Undersea telegraph cables are made of a single insulated conductor surrounded by an outer jacket of steel wire armored wrappings. For signalling, the center conductor and outer jacket are connected to a battery. Current flows and charges the entire length of the cable to battery voltage. One polarity represents a “high” or “dah” Morse Code signal. The battery connection to the cable is reversed, the cable discharges to zero volts and then charges to the opposite polarity. The opposite polarity represents a “low” or “dit” Morse Code signal.
The return path for the current is provided by the surrounding armor jacket and the adjacent seawater. The seawater and and armored jacket create one side of the capacitance of the cable and the center conductor is the other side. This capacitance, that is, the current leakage between centered and shielded conductors, and the resistance of the center conductor, were the effects studied by the engineers involved with designing and laying cables.
William Thomson (Lord Kelvin)
William Thomson, later Lord Kelvin (of the temperature scale fame), solved most of the problems to get the first trans-Atlantic cable working successfully. In addition to inventing very sensitive receive instruments, Thomson applied principles of heat transfer and Fourier mathematics to the problem of signal delay in developing a square law stating signal retardation (delay) on a cable is proportional to the square of its length. This model is analogous to heat transfer in a heat-conducting rod. For example: a cable two miles long will have four times the signal delay as a cable one mile in length. This delay is dependent upon the resistance of the center conductor, leakage between center and jacket conductors, and the capacitance of the center conductor to the steel jacket. Since not much can be done to reduce the capacitance of the cable, Thomson focused on minimizing leakage and center conductor resistance. The center conductor was made of copper as large in diameter as possible and of the highest purity (conductivity) possible. The purity of the gutta percha insulation around the center conductor was carefully controlled. The center conductor was also kept at the exact center of the insulation to minimize current leakage.
After Thomson had solved the major cable problems, he was able to signal across the Atlantic with a battery the size of his finger! This is mostly a testament to the sensitivity of his mirror galvanometer and later, the siphon recorder. But this fet would not have been possible without first understanding, and then later correcting, the D. C. issues involved with long undersea cables.
The first problems with long telegraph lines became very apparent with the development of the first undersea telegraph cables. After many trials and false starts, the science of long distance telegraphy was worked out, primarily through the efforts of William Thomson.
The initial work was based on a direct current model for a long cable. It involved understanding how electric charges behave in a system with resistance, capacitance, and current leakage between the conductors. It is analogous to the rate of heat transfer lengthwise in a conductive rod.
The next steps in modelling transmission line behavior require electromagnetic field theory to explain how traveling waves move on the conducting guides formed by pairs of parallel wires. This effort began in 1861, when James Clerk Maxwell published his electromagnetic theory and continued through the 1890s by the group of esteemed physicists known as the “Maxwellians.”
See also Tom Hagen’s article in: the-electric-orphanage.com/loading-open-wire-lines/