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Ground return used for early telegraph?

  1. Nov 16, 2014 #1

    NTW

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    I have read that, in the early days of the telegraph, two wires were used, but Steinheil, of optics fame, discovered that the circuit could be completed by 'earth return', using the ground as return ...conductor?

    That's my question. It's hard to believe that currents strong enough to operate the telegraph could be produced by low-voltage batteries, with the high presumable resistance of the earth...
    In those telegraph systems, or in some single-wire, earth-return power supplies that exist in some countries, does the electricity really circulate through the ground? Or does the ground work as a 'capacitor', as I have seen mentioned somewhere? But, if it's a capacitor, then where are the plates...?
     
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  3. Nov 16, 2014 #2

    jedishrfu

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  4. Nov 16, 2014 #3

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    Thanks, but -in that web page- I can't find any mention to ground return...
     
  5. Nov 16, 2014 #4
    Currents really circulate through the ground
     
  6. Nov 16, 2014 #5

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    Well, I don't doubt that, in principle, but in the early telegraphs, where ground return was standard, the distances covered by the lines where often very large, and the ground wasn't always damp... How could the electromagnets of the receptors work with the little current produced by a few Daniels cells in series having to pass through many kilometers of desertic, high-resistance, very dry ground...?

    And the thousands of kilometers of the first transatlantic telegraph, employing an unipolar cable, also used 'ground return' to complete the circuit...
     
  7. Nov 16, 2014 #6
    When distances become so large, wire loses become more concern than ground return loses. Atenuation becomes so significant. IIRC, very long pulses with higher voltages are required for communication
     
  8. Nov 17, 2014 #7

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    For damp, or waterlogged ground, usual in wet countries, I find that quite plausible; but how is it possible that the 'ground return' may work in arid zones, where the ground is completely dry, and its electrical resistance extremely high...? And that for thousands of kilometers... Yes, there are aquifers even in deserts, but not everywhere, and they are often very deep...
     
  9. Nov 17, 2014 #8
    I find that plausible for a normal, average ground too. The point is that after leaving port, current flows deep underground and traverses large effective cross section of ground beneath. If the grounding instalation is decently enginered, a resistive part of zero sequence impedance per phase should be increased only by 0.1-0.2 Ω/km due to the resistance of ground. This is my data for 50/60 Hz system , but similar reasoning may be applied to single wire telegraphy . Certainly, you may increase wire cross section considerably and decrease it's resistance considerably to make ground loses highly dominant, but this is economically and practically bad decision.
     
  10. Nov 30, 2014 #9
    In my opinion telegraph transmission is not just the same that electric current, because in this case only electrical pulse are transmitted. Is not necessary to transport electrons through the wire but only changes of voltage. The receptor receive this changes in voltage and transform in an signal.
    It is anything like a pipe full of water. If we change the pressure in a extremity a barometer in the other move according with the changes, but no any water move through it.
     
    Last edited: Nov 30, 2014
  11. Nov 30, 2014 #10

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    Perhaps a 'static electricity' telegraph such as the one you propose might work. If you charge a cable with electricity, that charge may be detected, for example with an electroscope, at any point along the cable. But I doubt that the scheme might work in practice, save in very dry weather and for short distances. In the real world, telegraph lines need a physical return, that is usually a cable, but the ground does work too...
     
  12. Nov 30, 2014 #11
    Actually, laws of propagation are quite similar. Voltage (electrical) impulse propagation also carries electrical current propagation with it. The mayor difference is short telegraph pulse is way more atenuated than 60 Hz "pulse", and more energy is lost. That doesn't matter as long as we can detect (receive) pulse. Of course, this is unacceptable in electrical power transmission.
     
  13. Nov 30, 2014 #12

    Danger

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    A touch off-topic, but I believe appropriate here: I've always wondered why they didn't use the rails as a conductor rather than string dedicated wires.
     
  14. Nov 30, 2014 #13

    NTW

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    Steinheil discovered the 'earth return' when he tried to use the two rails as transmission lines for an early telegraph. The leakage that he detected induced him to use the earth as return...
     
  15. Nov 30, 2014 #14

    Danger

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    Oh, okay. Thanks. It still doesn't make a lot of sense to me, though. I never thought of using the two rails as separate paths, because with trains having metal wheels and chassis there would be a constant short across them. (In fact, that's how crossing signals, at least here in Canada, were triggered by an approaching train.) Using them in one direction, though, seems more efficient than the wires.
     
  16. Dec 1, 2014 #15

    NascentOxygen

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    That's an interesting alternative. Each rail would have to be electrically bonded to the next---so a lot of soldered or welded joints. Dissimilar metals in contact become sites of corrosion, and sources of galvanic potentials which may interfere with the telegraph signals.

    But I'm wondering whether slightly-magnitized steel trucks and bogies clanging and banging over every rail gap might induce interference/noise emf's in the rails either by themselves or by virtue of the rails being a conductor immersed in Earth's magnetic field?
     
  17. Dec 1, 2014 #16

    NTW

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    All electrified tracks have electrical continuity between one rail and the next. Besides, and in most present-day tracks, the rails themselves are often solidly welded together...
     
  18. Dec 1, 2014 #17

    NascentOxygen

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    Of course, I was referring to technical difficulties that must be surmounted in the days of the morse telegraph. They weren't electrified rails then, were they?? The rails then may have been higher resistance than todays, too.
     
  19. Dec 1, 2014 #18

    Danger

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    I'm not sure about in the beginning days, but here the non-welded rail segments are linked with steel brackets spiked into the ties. I don't know how it works with the new concrete ties, unless they're used only with welded rails.
     
  20. Dec 4, 2014 #19

    Baluncore

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    Rails are bolted together using “fish plates”. The thermal expansion and contraction of the rails keeps the surfaces sliding with the bolt and spring washer holding them together, through elliptical holes. http://en.wikipedia.org/wiki/Fishplate

    The problem with ground return telegraph was that the signal current was often only a small percentage of the ground induced current. Magnetic storms and electrochemical corrosion and weathering cause significant background currents.

    When using a ground return circuit, the natural interference effects were often reduced by cancellation. That problem disappeared when twisted pairs of wires were introduced.

    With ground return, the signal travelled along the wire above ground. That was in effect, a transmission line with self inductance and capacitance to ground. Charging and discharging the line capacitance was necessary to get the signal to propagate along the line so a current really did flow along the line, while an equal and opposite current flowed deeper in the ground.

    Ground resistance is rarely a problem if the ground connections at each end are installed and maintained correctly. There is much more resistance in the wire than the ground return.

    SWER (Single Wire Earth Return) lines are still used to provide HV AC power to remote sites. http://en.wikipedia.org/wiki/Single-wire_earth_return
     
  21. Dec 4, 2014 #20

    Danger

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    There's a side-note that I always found amusing. In the old days, iron particles in the coal smoke from a locomotive frequently got caught up in the magnetic field surrounding a telegraph line. There were strings of black "doughnuts" hovering all over the country. :D
     
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