An Introduction to Information Theory: Symbols, Signals and Noise, by John R. Pierce, says the following:

While linearity is a truly astonishing property of nature, it is by no means a rare one. All circuits made up of the resistors, capacitors, and inductors discussed in Chapter I in connection with network theory are linear, and so are telegraph lines and cables. Indeed, usually electrical circuits are linear, except when they include vacuum tubes, or transistors, or diodes, and sometimes even such circuits are substantially linear.

Because telegraph wires are linear, which is just to say because telegraph wires are such that electrical signals on them behave independently without interacting with one another, two telegraph signals can travel in opposite directions on the same wire at the same time without interfering with one another. However, while linearity is a fairly common phenomenon in electrical circuits, it is by no means a universal natural phenomenon. Two trains can’t travel in opposite directions on the same track without interference. Presumably they could, though, if all the physical phenomena comprised in trains were linear. The reader might speculate on the unhappy lot of a truly linear race of beings.

Thinking about this from a physical perspective, I was wondering how it is that telegraph wires are linear, in the sense that two telegraph signals (in other words, two electric currents) can travel in opposite directions on the same wire, at the same time, without interfering with each other?

I was naively thinking about the wire as a single-lane, two-way road. In this analogy, the cars would be able to travel in either direction, but not at the same time. As I understand it, in solids, movement of electrons produces an electric current, so the electrons would be the cars. Given the author's explanation of linearity, what is going on here with the electrons that allows this concurrent, two-way flow of current?

I didn't find anything on the Wikipedia page for linear circuits that clarifies this physical property of linearity.

I would greatly appreciate it if people could please take the time to clarify this.

P.S. I do not have a background in electrical engineering, so a basically-worded explanation is appreciated.

EDIT: Based on comments from the previous thread, I understand that my analogy would be more accurate if I represent the electrons as double-sided bumper cars, and then imagine the two-way lane that they inhabit as filled with these cars, so that movements in either direction (electric current in either direction) is represented by a sequential "pushing/nudging" motion, like a wave, that is perpetuated by each car "bumping/nudging" into the one in "front" of it (in the direction of the current).

EDIT 2: I see many answers that are telling me that the core of my misunderstanding comes from the fact that I assume that electric current and signal are the same thing. And these answers are correct, I was assuming that electric current and signal are the same thing, because the author keeps implying that they are the same thing in the text (or he fails to clearly differentiate between the two)! See the following excerpts from the same chapter:

While Morse was working with Alfred Vail, the old coding was given up, and what we now know as the Morse code had been devised by 1838. In this code, letters of the alphabet are represented by spaces, dots, and dashes. The space is the absence of an electric current, the dot is an electric current of short duration, and the dash is an electric current of longer duration.


The difficulty which Morse encountered with his underground wire remained an important problem. Different circuits which conduct a steady electric current equally well are not necessarily equally suited to electrical communication. If one sends dots and dashes too fast over an underground or undersea circuit, they are run together at the receiving end. As indicated in Figure II-1, when we send a short burst of current which turns abruptly on and off, we receive at the far end of the circuit a longer, smoothed-out rise and fall of current. This longer flow of current may overlap the current of another symbol sent, for instance, as an absence of current. Thus, as shown in Figure II-2, when a clear and distinct signal is transmitted it may be received as a vaguely wandering rise and fall of current which is difficult to interpret.

enter image description here

Of course, if we make our dots, spaces, and dashes long enough, the current at the far end will follow the current at the sending end better, but this slows the rate of transmission. It is clear that there is somehow associated with a given transmission circuit a limiting speed of transmission for dots and spaces. For submarine cables this speed is so slow as to trouble telegraphers; for wires on poles it is so fast as not to bother telegraphers. Early telegraphists were aware of this limitation, and it, too, lies at the heart of communication theory.

enter image description here

Even in the face of this limitation on speed, various things can be done to increase the number of letters which can be sent over a given circuit in a given period of time. A dash takes three times as long to send as a dot. It was soon appreciated that one could gain by means of double-current telegraphy. We can understand this by imagining that at the receiving end a galvanometer, a device which detects and indicates the direction of flow of small currents, is connected between the telegraph wire and the ground. To indicate a dot, the sender connects the positive terminal of his battery to the wire and the negative terminal to ground, and the needle of the galvanometer moves to the right. To send a dash, the sender connects the negative terminal of his battery to the wire and the positive terminal to theground, and the needle of the galvanometer moves to the left. We say that an electric current in one direction (into the wire) represents a dot and an electric current in the other direction (out of the wire) represents a dash. No current at all (battery disconnected) represents a space. In actual double-current telegraphy, a different sort of receiving instrument is used.

In single-current telegraphy we have two elements out of which to construct our code: current and no current, which we might call 1 and 0. In double-current telegraphy we really have three elements, which we might characterize as forward current, or current into the wire; no current; backward current, or current out of the wire; or as +1, 0, -1. Here the + or — sign indicates the direction of current flow and the number 1 gives the magnitude or strength of the current, which in this case is equal for current flow in either direction.

In 1874, Thomas Edison went further; in his quadruplex telegraph system he used two intensities of current as well as two directions of current. He used changes in intensity, regardless of changes in direction of current flow to send one message, and changes of direction of current flow regardless of changes in intensity, to send another message. If we assume the currents to differ equally one from the next, we might represent the four different conditions of current flow by means of which the two messages are conveyed over the one circuit simultaneously as +3, +1, -1, -3. The interpretation of these at the receiving end is shown in Table I.

enter image description here

Figure II-3 shows how the dots, dashes, and spaces of two simultaneous, independent messages can be represented by a succession of the four different current values.

enter image description here

Clearly, how much information it is possible to send over a circuit depends not only on how fast one can send successive symbols (successive current values) over the circuit but also on how many different symbols (different current values) one has available to choose among. If we have as symbols only the two currents +1 or 0 or, which is just as effective, the two currents +1 and - 1, we can convey to the receiver only one of two possibilities at a time. We have seen above, however, that if we can choose among any one of four current values (any one of four symbols) at a time, such as +3 or + 1 or - 1 or - 3, we can convey by means of these current values (symbols) two independent pieces of information: whether we mean a 0 or 1 in message 1 and whether we mean a 0 or 1 in message 2. Thus, for a given rate of sending successive symbols, the use of four current values allows us to send two independent messages, each as fast as two current values allow us to send one message. We can send twice as many letters per minute by using four current values as we could using two current values.

And this textbook doesn't assume any prerequisite physics or electrical engineering knowledge, so it seems unlikely that readers would be able to differentiate between signal and electrical current -- especially given the fact that the author seems to constantly imply that they are the same (or fails to, in any clear way, separate the two for people without such a background).

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    \$\begingroup\$ Will have to come back later with an answer class explanation, but essentially the contributions of two senders simply add where they pass each other, the challenge is at the ends. If you know what you are sending you can subtract that and see what the other person sent. The catch is transmission line effects and the possibility of seeing a reflection of your past transmission. If you've sat through the lecture on transmission lines the idea of impulses going in each direction passing through each other is clear, trying to think how to explain that clearly without this. \$\endgroup\$ Commented Dec 22, 2018 at 15:34
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    \$\begingroup\$ First imagine a very large fleet of bumper cars... \$\endgroup\$ Commented Dec 22, 2018 at 16:00
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    \$\begingroup\$ Note that your textbook quote says "two telegraph signals...", whereas your question says "two electric currents..." travelling in opposite directions. But by Ohm's law, $V=IR$, current is proportional to the voltage drop across the wire. So you're never actually going to observe currents >>simultaneously<< flowing in opposite directions. However, like the answers suggest, the waveform represented by a very rapidly varying voltage can encode messages in both directions. \$\endgroup\$
    – John Forkosh
    Commented Dec 23, 2018 at 10:56
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    \$\begingroup\$ Frankly, I don't think the author of that book understands what "linear" means. It certainly doesn't mean what he's describing in the passage you quoted. Capacitors and inductors are decidedly non-linear. @JohnForkosh got it right; you don't have to demonstrate current flowing in both directions to encode two-way signals. In fact, the telegraph circuit that demonstrates duplexing (communication in both directions) is almost absurdly simple. All it requires is a center-tapped coil and a rheostat. See mysite.du.edu/~jcalvert/tel/morse/morse.htm#H1 \$\endgroup\$ Commented Dec 23, 2018 at 17:30
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    \$\begingroup\$ There are two different meanings of "linearity" - the electrical one John Forkosh cites, and another, used in a radio signal context, which this author is using: "The rule of linearity is common among many mathematical and engineering aspects. Plainly, linearity describes that you can describe the effects of a system by separating the input signal into simple parts and using superposition at the output to restore the overall system output." - dspillustrations.com/pages/posts/misc/… \$\endgroup\$
    – James Sutherland
    Commented Dec 24, 2018 at 9:09

14 Answers 14


The physics explanation is that waveguides (including the free space) have orthogonal modes for the two propagation directions. This means that the two signals traveling in opposite directions will not interfere. (This is not an approximation, there will be no interference).

The device which separates "transmitted" and "received" signal is a circulator. It exists also in the optical domain and can be used to implement duplex communication over a single optical fiber. In the RF domain it can be used to implement separation of transmit and receive signals over a single antenna (at same time and at same frequency of course). Practically one often uses different frequencies for transmit and receive, mainly due to technical reasons. The circulator has not perfect isolation and the separation does not work so well for very weak received signals. But if one had a perfect circulator device the arrangement would work.

In the old analog telephone system there was only a single wire pair, yet it was possible to speak and hear at the same time.

TL/DR: A very elementary explanation is that one has voltage and current in a wire and that can be used to carry separate information in two directions. Consider the following:

On one side of the wire there is a controllable voltage source, and the to-be-transmitted information is the instantaneous voltage. On the other side of the wire there is a controllable current source (or better "sink"). The to-be-transmitted information here is the instantaneous current. Clearly, station 1 (the one with the voltage source) can read the signal from source 2 by just measuring the current through the wire. Station 2 can also receive the signal from station 1 by measuring the voltage at the terminals of its current source. So this proves you can transmit information in two directions simultaneously over a single wire pair. And if you doubt that it might not be possible to connect a current source/sink to a voltage source. This is perfectly possible, one can think of the current source/sink just as a special form of an adjustable load resistor.

EDIT: There is also a elementary explanation for waves: A free space wave has an oscillating electric and magnetic (E and H) field. They are oriented with 90° angle in space and have a temporal phase shift of 90°. It is +90° for forward and -90° for backward propagating direction (it may be vice versa depending on the choice of the coordinate system or sign of phase). Also the ratio the magnetic and electric field amplitude is fixed to the wave impedance of the medium (which is 377 Ohm for vacuum). If we now have a back and forward propagating wave we will have the superposition of the electric and magnetic fields everywhere in space and time. However a ideal separation of both waves is possible. Simply speaking: The electric fields will add while the magnetic fields will subtract (due to the total phase shift of 180°). Since the amplitudes of E and H field of each component is have a fixed ratio we can substitute the H for E field (or vice versa) and solve for the two E field amplitudes of the forth and back propagating waves. That demonstrates that ideal separation of the two propagation directions is possible.

And the very abstract physics explanation behind this, is - as I wrote earlier - that the modes corresponding to the two propagation directions are always orthogonal and signals do not interfere.

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    \$\begingroup\$ In the old analog telephone system there was only a single wire pair, yet it was possible to speak and hear at the same time. -- Yes, but that's because the two voice signals were mixed, the same phenomenon that makes it possible to put multiple instruments into a song using a mixer. \$\endgroup\$ Commented Dec 23, 2018 at 16:55
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    \$\begingroup\$ @RobertHarvey no. Each end hears the other end in their speaker, without hearing their own voice (or at least, hearing a greatly attenuated version of it; mismatches in the system always cause a little bit of reflected signal). \$\endgroup\$
    – hobbs
    Commented Dec 23, 2018 at 18:28
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    \$\begingroup\$ @hobbs Your comment doesn't completely match my experience. I definitely hear my own voice when talking on a land line, loudly and clearly, and even on a line with no dial tone but battery (48 V supplied by the phone company), I can hear myself breathing into the test phone. That's how I know there's battery on the line. That last point does highlight the way that I agree with your comment: hearing yourself on a landline is not because of hearing your own signal on the line, it's the phone itself that is mixing the signal from your phone's microphone and the signal from the line. \$\endgroup\$ Commented Dec 23, 2018 at 20:18
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    \$\begingroup\$ See also "sidetone" \$\endgroup\$
    – Nemo
    Commented Dec 23, 2018 at 22:57
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    \$\begingroup\$ @kostas "multiple inaccuracies": could you be more specific? Had you read the beginning of the answer ("- as I wrote earlier - "), you would have noticed that I referred to "modes" of propagation. But true, the last sentence in this form was inaccurate. I edited it to be precise and to match the body of the answer. \$\endgroup\$
    – Andreas H.
    Commented Dec 25, 2018 at 18:36

in the sense that two telegraph signals (in other words, two electric currents) can travel in opposite directions on the same wire, at the same time, without interfering with each other

This is the point. A telegraph wire is not just a wire that transports a direct current. It transports a signal, which in essence is a voltage or current wave. The current oscillates around a value of \$0~V\$ (or \$0~A\$).

If two current waves travel in the opposite direction, the waves have no trouble passing through each other, just as two sound waves are able to travel in opposite directions in the same medium.

enter image description here

(Here, the blue travels to the left, green to the right, and the red wave is the resulting superposition of them. The red wave is the current/voltage distribution that is measured in the wire over time.)

Instead, the value that you would measure at any point corresponds to the linear sum of the amplitudes of both waves - that is what is called the Principle of Superposition. Waves obey this principle if their underlying equations are linear, i.e. the wave equation doesn't contain higher powers (\$x^2\$, \$x^3\$, etc) of the variable that is "waving". That would be the voltage \$U(x, t)\$ or the current \$I(x,t)\$ in the telegraph case.

Specifically, the equations describing waves in a transmission line even have a name: the Telegrapher's Equation. $$\frac{\partial}{\partial x} U(x,t) = -L \frac{\partial}{\partial t} I(x,t) - R I(x,t)$$ $$\frac{\partial}{\partial x} I(x,t) = -C \frac{\partial}{\partial t} U(x,t) - G U(x,t)$$

\$L, C, G \$ and \$R\$ are properties of the transmission line (inductance, capacitance, conductance and resistance) and determine how the shape of the signal is distorted, attenuated and slowed down while it is traveling down the line. The rest describes how an excitation of the voltage and current propagate through this transmission line along the position \$x\$ and through time \$t\$.

As you can see, all of the quantities of interest (voltage \$U\$ and current \$I\$) appear only in the first power. That has the consequence that if you find two solutions to the wave equation, then also their sum will be a solution to the same equation (even when multiplying them by a factor). So if \$U_1(x,t)\$ and \$U_2(x,t)\$ are solutions to the wave equation, then also $$U(x,t) = \alpha \cdot U_1(x,t) + \beta \cdot U_2(x,t)$$ is a solution, where \$\alpha\$ and \$\beta\$ are factors.

Side note about DC:

Having two currents flow in opposite directions would cancel out their contributions and result in no current. Alternatively, you can convince yourself that a direct current (DC) cannot flow in both directions at once, simply by Ohm's Law:

Say you have a wire with a certain resistance \$R\$. According to Ohm's Law, we need a potential difference of \$U = \varphi_2 - \varphi_1\$ between the ends to cause a current \$I = \frac{U}{R}\$ to flow. The current (composed to moving positive charges) moves from the more positive potential to the more negative potential.

We can now flip the potentials: $$U' = \varphi_1 - \varphi_2 = -U.$$ But since the resistance stays the same, this also flips the current: $$I' = \frac{U'}{R} = \frac{-U}{R} = -I.$$

If we make both potentials equal, there will be no difference and the current will be zero.

The only way to have a current come out at both ends is to have a source in the middle, which is not really interesting.

  • \$\begingroup\$ Strictly speaking, waves can propagate also in the same direction without affecting each other, for example waves of different frequencies. Some similar equations (although not this one, i think) can allow for waves of different frequencies to travel at different speeds and overtake each other like the trains in the book which the OP cited. \$\endgroup\$
    – Kostas
    Commented Dec 24, 2018 at 22:32
  • \$\begingroup\$ @Kostas Sure, they are just generally non-interacting. \$\endgroup\$
    – DK2AX
    Commented Dec 25, 2018 at 7:42
  • \$\begingroup\$ Your animation seems to have an extraneous last frame, which breaks smoothness of the animation on repetition (basically, two subsequent identical frames). \$\endgroup\$
    – Ruslan
    Commented Dec 26, 2018 at 11:46

There's your problem: telegraph signals are not electric currents. (We might as well say that telegraph signals are voltage instead.) Which is correct? Neither.

To solve this, give up on electronics and instead fall back to the physics behind it. Actually the telegraph signals (and even all electrical signals everywhere) are actually electrical energy; same thing as light and radio waves. Signals are changes, and a changing current involves voltage, same as changing voltages involving current. Signals are the watts, not just amperes and not just volts.

Signal-energy behaves differently than currents in circuits. While the energy zips across a circuit, instead the amps or charge-flow does not. Charges just rotate through the loop as a whole, or perhaps wiggle slightly back and forth, but current doesn't fly forwards at lightspeed. Something does fly along at lightspeed though. We measure it and discuss it in terms of watts or "wattage." The amps don't fly fast, the amps are different, amps are the slow motions of the "medium;" that charge-sea found inside every wire. Waves versus medium. Somewhat like sound waves versus wind. Electric current is like wind, while signals are like sound waves. (And of course sound waves are back-and-forth wind! The air wiggles, while the waves propagate forwards.)

How can two independent signals pass across an electric circuit? First ask yourself how two independent sound waves are able to pass through the same region of air. And on a pond, throw two pebbles and ask yourself how two bullseye ripple-patterns pass through each other without interacting. Why doesn't one laser beam block another one whenever they cross? It's just something that all waves can do, if the medium is linear. In a linear system, waves can add and then subtract again, so they cross each other without interacting. It works for light inside an optical fiber. It works for sound inside an organ-pipe. It works for coax cable with pulses going in opposite directions, and it works for telegraph signals propagating at lightspeed across a single pair, a single circuit.

The answer to your question involves the waves-chapter of your physics book. The answer to your particular circuits question opens up an entire fascinating field of electronics: cable reflections and standing waves on wires.

On the other hand, two direct currents cannot occupy the same circuit, since they lose their identity, combining to form a sum-current. (Don't forget that every circuit is a one-turn inductor. Similarly, two different voltages cannot occupy the same capacitor! In both cases they combine, and cannot be un-subtracted again.) Two direct currents can occupy a single wire, whenever that wire is a common section of two otherwise separate circuits. But they do this by adding up to form a third current within that common section. (For example, they might subtract to zero current in that section, if they happened to be equal and opposite. One electron can't actually flow in two directions simultaneously.)

Yet at the same time, two completely independent energy-waves (signals) can propagate across one single circuit. HOW? It involves E and M both, and that contains the secret: to understand it we must look at both wires of the long pair, and we must include voltage as well as current. Your question cannot be answered as long as we concentrate on single wires and currents alone, while ignoring the two wires and the voltage across them.

In a single circuit, current is a closed circle, like a flywheel. It doesn't start out in one place and flow to another (instead it just goes clockwise, CW, or perhaps CCW, much like a drive-belt.) A current in a circuit is like a spun flywheel, a closed loop. But something sure does go one-way, right? Whenever a battery lights a bulb, something must go from the battery to the bulb and not return back to the battery. That something is not current. Instead it's EM energy, where energy-flow is measured in terms of watts; of volts times amps. In a flashlight circuit, the wattage is a fast one-way flow from battery to bulb. But the current is a very slow circular flow. Again, the "signal" going from battery to bulb is made of EM energy, not amperes and not electrons.

So here's the start of your answer: in a single circuit, how can we know which direction the electrical energy is flowing? Simple: look at the value of wattage. Specifically: multiply the volts between the wires times the amps through them. If the result is positive, then energy is flowing in one direction, and if it's negative then it's flowing in the other. With a flashlight, hook up your voltmeter and ammeter so they give positive wattage when we multiply them together. Then, when you remove the bulb and install a battery charger instead, the current reverses, so we have energy flowing backwards, into the battery. (This idea is critical with AC, where if V and I waves are in synch, the energy flows continuously forward, but if V and I are at 180deg, the energy instead flows backward.)

So, on a long cable, with an electrical pulse which has positive wattage, the pulse is zooming along left, while if wattage was negative, the pulse is going right. If we suddenly connect and disconnect the flashlight battery, we're launching an energy-wave off along the two wires. It travels at lightspeed and gets absorbed by the flashlight bulb, which lights up. If we leave the battery continuously attached, then still an energy-wave flows to the bulb, even if there's no ripples at all. That's the first concept in basic waves-engineering: the propagation of electrical energy across circuits ...and the idea that "DC" is really just "AC" at very low frequency.

Back to the start again: how can two signal-pulses fly in opposite directions along the same wire-pair? (Note that it must be a wire pair with volts included. Not a single wire.) It can occur if one of the pulses has positive wattage and goes left, while the other pulse has negative wattage and is going right. One pulse might be composed of positive volts and positive amps, while the other pulse is made from negative volts and positive amps. Both pulses are EM waves.


Aha, I see another approach! (Ignore it if you wish, since sheesh this is long.) Suppose we have two separate circuits, two flashlights, but then we meld together one short wire-section from each? The two circuits have one piece of wire in common. Do they interact? Nope, because inside the common wire, the currents just add and subtract again. Each battery lights up its own bulb independently, because each circuit-loop has its own separate battery-voltage and its own separate loop-current. Yet in that common wire, it seems as if two different electric currents are flowing! They're not, not really, because one "circuit current" is the current in one entire loop, including one battery, bulb, and entire closed ring of conductors. In that combined wire, the two currents get added at one end of the wire, then subtracted again at the other. The two energy-waves in each circuit remain independent, even though the currents in their common wire can add and subtract.

This shows us that the answer to your original question cannot involve a single wire. It only can be answered by backing off and taking a wider view; by also including the voltage across two wires.

This also shows how "linear" versus "nonlinear" works. In the common wire, at one end the two currents have combined by adding together. But then they perfectly subtract apart again at the other end. This lets the two loops remain independent. But what if this didn't happen, and instead the currents in the single wire weren't a simple sum combination? Aha, that would be "NONLINEAR." In that case we couldn't cleanly separate them once combined. The "adding together" at one end of the wire wouldn't be perfectly equal to the "subtracting apart" at the other end, and in that case the two separate circuits would start interacting. One battery would begin lighting the other bulb slightly. The signals of the two circuits would truly mix together.


This sort of question has a long history, and a popular book about it is THE MAXWELLIANS, by BJ Hunt. The infamous Oliver Heaviside figured out that telegraph signals were actually EM waves, but then he was nearly suppressed by William Preece, head of the UK govt telegraphy office, who "knew" that dots and dashes were simply currents, period, end of story and don't ask questions or WH Preece will make you sorry! :) Heaviside used his new EM theory of cable-waves to solve a huge telegraphy problem: for any signals traveling along 100KM telegraph lines, the dots would either disappear or "ripple," and for telephone lines, long distance transmission was completely distorted and impossible. (The problem was found to be wave-dispersion or "chirp," where the low freqs travel faster than the high.) Heaviside's "telegrapher's equation" and his "loading coils" fixed this, letting telegraphy become broadband, even when over immense distances. He single-handedly created long-distance telephone. But Preece quickly halted this heresy by using his political power to start an anti-Heaviside bad-mouthing campaign in the press, and a whispering campaign among engineers. Then in the USA, Pupin of Columbia pretended to invent Heaviside's loading coils, patented them, and made millions via Bell Telephone, while Heaviside remained nearly penniless, not gaining fame until after he'd died. (Heh, a Tesla/Marconi story long before Tesla and Marconi. Pupin even played a big role in Tesla's downfall!) So now you see why I'm in love with the story of telegraph-is-EM-waves. Obsessed. Don't even get me started! Oops, too late. :)

  • \$\begingroup\$ Thanks for the answer. At the beginning of the same chapter, the author establishes the context as referring to electric telegraphy (Morse code, etc.), and says the following: “The space is the absence of an electric current, the dot is an electric current of short duration, and the dash is an electric current of longer duration.” So it seems that the author is saying that telegraph signals are electric currents? \$\endgroup\$ Commented Dec 22, 2018 at 15:47
  • \$\begingroup\$ This at least hints at the right idea, but has some problematic aspects. Also "Losing identity" isn't physics, but an engineering problem of application, that can in some cases be solved. \$\endgroup\$ Commented Dec 22, 2018 at 15:52
  • \$\begingroup\$ My problem with this answer is that it doesn't seem to answer my question at all. Instead, it just claims that the phenomenon is possible because of linearity, but it doesn't explain the physics of why it is possible, which was the point of my question. And the paragraph that begins with "To solve this, give up on electronics and instead fall back to the physics behind it. ..." doesn't seem to explain the physics of how it is possible at all; rather, it just goes on a tangent that avoids answering my question entirely. \$\endgroup\$ Commented Dec 23, 2018 at 6:36
  • \$\begingroup\$ @ThePointer The author has a misconception about telegraphy, or at least is teaching a "lie to children," an oversimplified description for beginners. To understand what's going on, try discarding the idea that telegraph signals are currents rather than waves. In fact, we can't pulse a telegraph sounder, or make noises in a telephone loudspeaker, flash light bulbs, without sending energy and performing work. That involves wattage, which is the measurement of lightspeed electromagnetic waves on wires. The dots/dashes are guided EM waves: pulses of voltage/current. \$\endgroup\$
    – wbeaty
    Commented Dec 23, 2018 at 21:54
  • \$\begingroup\$ @ThePointer >answer my question at all. True! There's no answer, so change the question."All waves act like that" doesn't apply to pure currents. But dots-dashes are wattage, volts AND amps, so include volts in the question. Or ask this: in one glass rod, how can two light signals pass in opposite directions without interacting? The answer will also apply to telegraph signals on wire-pairs. My answer "waves do that" is just the start of a whole chapter: cable reflections and standing waves, where volt-amp signals go in opposite directions on one long circuit. I'll add more above. \$\endgroup\$
    – wbeaty
    Commented Dec 23, 2018 at 22:49

Andreas H mentioned the circulator for waveguides. In analog phones this job is done by an imperfect hybrid circuit called the Anti sidetone induction coil (ASTIC). A perfect hybrid coil would transmit and receive speech concurrently and separately, ie the signal from your transmitter would travel through the wires to the receiver at the other end and the signal from the distant transmitter would travel to your receiver on the same pair of wires. It was realised early on that people needed to hear themselves speak so the ASTIC allows part of the signal from the local transmitter to pass across to the local receiver.

Within a local analog exchange area the circuit would be two wires all the way from one phone, via the relays in the exchange to the other phone. Once you start travelling between exchanges the signal would be split by a hybrid coil at the exchange and speech in one direction would travel on a different circuit to speech in the other direction (4-wire junction circuit). This allowed the speech to be amplified since amplifiers are unidirectional (one way only). At the remote exchange the two separate paths would be recombined by a hybrid coil and the last leg of the call would be on a pair of wires.

Speech on analog phones and exchanges was 300Hz to 3400Hz so this is low frequency EM waves.

However if you are transferring power, either AC or DC, then we don't have different currents going different ways in the same wire. For example, in a particular state the energy supply companies are required to supply a percentage of 'green' energy but they do not have sufficient 'green' generation resources so they buy the energy from out of state. At the same time they sell surplus non-green energy out of state. If they are buying and selling energy over the same interconnect (wires) then there isn't two competing power flows travelling in opposite directions on the same wire. If State A is buying 500MW of capacity from State B and State B is buying 400MW of capacity from State A then there is be a 100MW flow from State B to State A. The accounting might say 500MW and 400MW but the electrical reality is 100MW. State A will pay for 500MW of green energy but it will be 400MW local non-green plus 100MW out-of-state green.


They do interfere.

Electric signals travel down wires like waves on water. and when two waves meet you get inteferance.

But because the wires are linear, the interferance takes the form of addition, and so it is not destructive oth the info0rmation, and thus if you know what one of the signals is you can find the other signal by subtraction.

Telephone lines use (used?) a circuit called a hybrid that isolates the incoming and outgoing signals allowing a single copper circuit to carry voice signals in both directions.

The telegraph probably used something similar having the sender subtract its own signal from what it sees on the line, allowing it to determine what was arriving from the other end simultaneously with transmitting its own signal.

  • 1
    \$\begingroup\$ This is not correct. The waves propagating in opposite directions do not interfere at all.At any point in time and space both can be ideally recovered. The device that does this is a circulator. \$\endgroup\$
    – Andreas H.
    Commented Dec 23, 2018 at 9:11
  • \$\begingroup\$ perhaps you're using a different definition of interfere? a circulator is a microwave hybrid. \$\endgroup\$ Commented Dec 23, 2018 at 10:16
  • \$\begingroup\$ Perhaps: My definition of interference is that the amplitude of the wave(s) is attenuated (perhaps completely) at certain positions in space. This is not the case for forward and backward propagating waves. You are right about the hybrid. \$\endgroup\$
    – Andreas H.
    Commented Dec 23, 2018 at 10:53
  • \$\begingroup\$ I'm using the definition on the intererfance wikipedia page, the signals just add together with nothing lost. \$\endgroup\$ Commented Dec 23, 2018 at 10:57
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    \$\begingroup\$ This is the correct answer. Note that current will only flow in one direction at a time (where direction is determined by the voltage each sender puts on their end); and the book being referenced is about information theory and not electronics (and probably gets information theory right and electrical/electronics completely wrong). \$\endgroup\$
    – Brendan
    Commented Dec 23, 2018 at 23:56

You wrote:

How is it that two electric currents can travel in opposite directions on the same wire, at the same time, without interfering with each other?

but the original text says:

two telegraph signals can travel in opposite directions on the same wire at the same time without interfering with one another

Here's the contradiction: a telegraph signal and an electric current are not the same thing. The electric current is the linear superposition of waves set in motion on the line by the transducers at each end. The current at one instant at one point on the line can have only one value, but we can calculate that value by calculating the contribution of the waves from the signals imposed on each end of the line and adding them together.

As a simpler but directly observable system, consider a stereo playing music in a room. One speaker does not change how the pressure waves from the other speaker propagate. The net pressure gradient at any point in space and instant is the the result of adding of the pressure waves from each speaker.

Even though physical quantities like current or pressure can have only one value, if we know that those quantities are influenced by an additive combination of causes, the linear superposition principle allows the system to be broken into smaller parts which can be considered separately: in this case the telegraph station at each end of the line, and the waves it generates which propagate down the line.

  • \$\begingroup\$ This is the answer. It precisely addresses the OP's mistake, is short enough to read, and is correct. I especially like the loudspeaker/pressure analogy. \$\endgroup\$
    – Neil_UK
    Commented Dec 3, 2020 at 16:18

Signals are made up of waves. Waves pass each other, and after the passing are unchanged. Electromagnetic waves. Waves on the sea also pass each other (although they sometimes do have effects which I won't get into). "Interfere" was a bad choice of word by the author. Nobody can really tell you why. But you already instinctively know that waves can pass each other. Just think about light shining out of a window and in through a window at the same time. This does not seem to be perplexing, does it?

In your question you use the word "current." Currents are another matter. Current in a wire is basically defined as the flow of charge past a point. This would be net flow. So it does not make sense to talk about currents somehow passing each other.

I am trying to avoid talking about more advanced transmission line effects such as capacitance and inductance because I am afraid it will just muddy the waters even more. The bottom line is that signals can pass each other, and during the passing, at the location of the passing, they effect each other. But after the passing, they continue on as if the passing never happened. Just think of light passing both ways through a window.


It is not current, but SIGNAL that travels in any or all directions. This is why a telephone handset doesn't have to interrupt the received sound when you speak, and that is more familiar to us than telegraph protocols.

This is a bit of gimmickry, called a 'hybrid', that presents a signal to your ear that has mainly the signal from the distant phone, and creates signal (modulation of current) according to your voice applied to the microphone. What you HEAR isn't the 'current in the wire' which is equally modulated by two voices, it's 90% the distant voice that you hear, and only 10% of your own. A similar hybrid at the other end of the connection cancels the main part of his/her vocal input so that your voice is heard strongly in that phone receiver.

The hybrid is a signal-adding circuit, that has access to both your voice and the combination of two voices (on the line), and combines them to reinforce the message from a distance. Nothing in this scheme is unavailable to a telegraph office, which can also operate as a receive station even while it is transmitting.

It is NOT easily available to a wireless transmitter (the non-digital kind) which would usually have a push-to-talk interrupting switch. Our cellphones sending digital packets are doing a lot of interrupting, just fast enough that it rarely bothers us, because that hybrid function interacts badly with a receiver that overloads while a transmission is in progress.


Your analogy is broken. Don't think of a lane of cars, unless you think of the cars as bumper cars all put together.

The actual overall average speed of electrons moving through a wire is pretty slow. The drift velocity of electrons in a wire is typically several micrometers/second, not fast at all.

What propagates through the wire travels from electron to electron, from source to destination. That process happens very quickly, nearly the speed of light. In the highway analogy, it would be analogous to hitting the first car, and each car bumping the one in front of it. Despite each car moving slowly overall, a wave could propagate through the chain provided you can hit it hard enough.

Multiple sound waves can obviously travel through the air in multiple directions simultaneously. However, when you shout something out, a single molecule doesn't necessarily travel directly from your mouth to the listeners ear. Instead, the bouncing between molecules through the air is what transmits the sound. The same thing is basically true for electrical signals.

  • \$\begingroup\$ Thanks for the answer. But even using this corrected version of the analogy, it is not clear how/if the signal can propagate through the same wire, in opposite directions, at the same time? Using this analogy, it would seem that the electrons (bumper cars) would only allow signal propagation in a single direction at any one time? Otherwise, one would intuit that signals become “messy/corrupted/cancelled/whatever”? \$\endgroup\$ Commented Dec 23, 2018 at 4:36
  • \$\begingroup\$ @ThePointer not exactly bumper-cars, but more like long stretched Slinkys(tm). A long column of electrons inside a wire can behave like a long floppy spring. You can wiggle either end, and waves will zip along the spring. Waves going left will pass through any waves going right, but only if forces and motions in the spring can perfectly add and subtract. (Next, complete the analogy by using a Slinky as a closed-loop drive-belt passed around two separate pullies. Jerk one pulley and a "voltage-current wave" zooms along the "wire pair" to the other pulley. Not perfect analogy, but close. \$\endgroup\$
    – wbeaty
    Commented Dec 24, 2018 at 6:10
  • \$\begingroup\$ Actually bumper cars work also. Lets assume that logical one is when the line of cars moves one meter and zero when it doesn't. You can watch the cars move to receive and hit them to transmit. If both hit at the same time, the line doesn't move. If the line didn't move when you hitted it, you know that you have received a one. \$\endgroup\$
    – TemeV
    Commented Dec 24, 2018 at 20:23
  • \$\begingroup\$ Current in a wire is made up of electrons (current doesn't have to be electrons, but in a wire it is). But the signal is an electromagnetic wave. The wave travels at relativistic speeds. But the electrons move very slowly. Any electrons coming out of the wire at the far end are not the same ones that entered at the near end. \$\endgroup\$
    – user57037
    Commented Dec 25, 2018 at 20:22

Consider the following situation:

Lets assume we have a single wire pair with a controllable voltage source at one end and a controllable current sink at the other. Since both ends can measure the signal of the other end (at the voltage source we can measure current and at the current source we can measure voltage) we can transmit information in both directions. There is no frequency or time multiplexing involved. And there is no interference and we do not need to invoke wave theory.

More details are in my answer on Physics SE.

  • \$\begingroup\$ Very good. A is modulating the voltage to transmit while B listens to voltage to receive. Meanwhile B modulates current to transmit and A listens to current to receive. \$\endgroup\$ Commented Dec 26, 2018 at 4:48

An aerial cable for a satellite dish carries currents in two directions - An 18 Volt DC signal is supplied by the tuner to power the LNB at the focal point of the dish, and at the same time the LNB sends a signal of 4-12 GHz back down to the tuner, over the same wire.

Both are electric currents, but one is DC and is flat, the other is Radio Frequency and varies.


Its because any wave can pass through each other. Interference will occur, but this does not stop waves.

Its like asking why two waves in a pond can pass each other. If the waves are completely opposite, they will annihilate, otherwise they will weaken each other and continue on.

  • \$\begingroup\$ "Because telegraph wires are linear, which is just to say because telegraph wires are such that electrical signals on them behave independently without interacting with one another, two telegraph signals can travel in opposite directions on the same wire at the same time without interfering with one another. " \$\endgroup\$ Commented Dec 23, 2018 at 8:28
  • \$\begingroup\$ Is the author wrong? \$\endgroup\$ Commented Dec 23, 2018 at 8:28
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    \$\begingroup\$ @ThePointer You can literally take two waves and add together the currents and voltages from each wave, to find out what the combined wave looks like, if it confuses you. What counts as interference? Each end will see what the other end sent, but if you look in the middle you'll get a jumble. \$\endgroup\$ Commented Dec 24, 2018 at 3:14

It has been observed, by many engineers and reasearchers (including myself) that metal conductors have linear behaviour with regards to electric currents and voltages. However, as with most materials, linear behaviour only exists over a certain range. High levels of current will result in non linear behaviour. With good conductors, like copper, silver and gold, the range of linear behaviour is quite large. These metals have low (but not zero) resistance. (If you assume metals have zero resistance, then you will end up with strange predictions that do not match reality)

At low current density, there are lots of free spaces in the metal for electons to move into, and they don't bump into each other or get stuck very often. so not much energy is absorbed by the metal and behaviour appears linear ( the bumper cars are far appart)

When the current density in the metal becomes high enough, then the current transfers significant energey to the metal, which changes it s resistance, and the behvaiour has become non linear. As simple example is hooking a thin wire (like 28 gauge) across the terminals of a large 12V car battery. The metal becomes hot, eventually it melts and breaks the circuit. This is VERY nonlinear behaviour. That wire is probably carrying 50 Amps or so. (DO NOT try this yourself - you can get bits of molten metal flying around, can cause fires and severe damage to your eyes) On the other hand if I put two signals on this same wire (before I melt it) with 0.001 amps each, behaviour will be quite linear.


This guy is armwaving to make a more existential point. It works in principle, but not like he says. And with signal, not current.

Heck, even in radio, two transmitters can and do block concurent usage. Listen to this at 1:25. That "Booooop" is both airplanes acknowledging "their" takeoff clearance, but stepping on each other so at least one is not heard.

If you are using a DC telegraph system, same problem. If either telegraph key pressed down, it will cause both sounders to activate. It really isn't possible to send DC signals in opposite directions in the DC domain, (except via a CSMA-CD style courtesy of waiting until the other person is finished, and being wary of two people starting up at once).

However, imagine if telegraph station 1 transmits DC, and telegrph station 2 has its sounder connected through an AC-blocking choke. Station 2 transmits by switching 1000Hz AC on and off, which only station 1 can hear because its sounder has a right-sized capacitor, which passes 1000 Hz AC but blocks DC.

You can expand this to multiple AC frequencies by using "bandpass" filters which only allow through a certain frequency. Consider the bah-boo-BEEP tone that hails the fire station on the TV series Chicago Fire. That show is a huge tribute and shout-out to a 1970s show called Emergency, the origin of the tones. Emergency portrays a 1960s-era fire dispatching system where multiple frequencies were used in just that fashion.

Two stations transmitting at once simply create a chord on the wire. The frequencies have to be chosen smartly so the chords do not interfere with each other.

All the stations hear all the signals. They just disregard "their own dogfood", i.e. the signal they are transmitting.

It can get more complex, with carrier waves being modulated. At this point we are talking about radio spectrum, but on a wire.


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