Today I am showcasing a fantastic website:
This website covers the history of wired telecommunications from 1843-2020.
Filled with articles and photos covering telegraphy, telephones, physical systems, personnel, memoirs, and has technical articles, historic photos and more.
This is a site well worth the perusal if you are interested in how these systems were built, maintained, and operated.
Below in its entirety is an article, the first in a series, on the analysis of telegraph lines, just to give you a taste for what this site contains!
Enjoy
73
Ciao
KJ
DC Transmission Line Modelling: Initial Studies on Telegraph Lines
By Tom Hagen
Introduction
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.
Sections:
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
Practical Examples:
- 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.
Some Basics
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.
Conclusion
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.”
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