Replacing wires using earth in circuits

Question

I am reading the book "Code: The Hidden Language of Computer Hardware and Software", on page-36, the following replacement for a wire in a circuit for communicating in morse between people living in two nearby houses is described:

One can apparently throw out one wire in exchange for grounds

I have a concern regarding this set up, if two other people nearby wanted to talk in morse for whatever reason using circuit with ground, then would me and my friend using morse mess interfere with their circuit? (Provided both pairs communicate at same times) Would there be a way to avoid this using clever circuitry?

Answer 1

That's just a neat diagram to throw in a concept of common ground in a device and how to reduce the amount of wires drawn, by introducing a symbol which means that all things connected to this symbol are actually connected together.

You can replace all ground/common wires in a schematic with ground symbol, just like in a car wiring diagram you can omit the ground return wire by saying that both the battery and lamp have one terminal connected to the metal chassis.

For concept purposes, the planet earth used as ground return path instead of wire can be replaced by metallic car chassis, the reasoning is similar. There are currents flowing in resistance and it creates voltage differences.

As long as the currents are not too large to make too large potential differences, it will work, but there will be interference. Just like if you used a separate common wire shared with all neighbours instead of planet earth.

Please note that using a battery and lamp and using earth as conductor won't work in real life, the earth has too much resistance for that.

The claim that the book makes that the planet earth is virtually limitless source and sink for electrons is also just weird.

Answer 2

if two other people nearby wanted to talk in morse for whatever reason using circuit with ground, then would me and my friend using morse mess interfere with their circuit?

I wouldn't recommend that you try it for possible safety and maybe even legal reasons however, it could work; both Morse circuits are sharing the same common ground/earth return wire but, because that wire will be a low impedance, there will be virtually no cross-interference between the two circuits.

Remember also what you said here: -

a wire in a circuit for communicating in morse between people living in two nearby houses

Nearby houses will likely share a common ground/earth but, it isn't uncommon for this not to happen and, there may be some unforeseen problems if you try and implement it.

If you are going to string a wire between two houses (again a potentially illegal thing in some countries), you might as well use coax or twisted pair and forget about trying to hi-jack the electricity supply earth/ground wire.

Answer 3

When you press the switch and complete the circuit, you create a potential difference of maybe-up-to-1V (depending on the magnitude of the ground resistance as compared to the 100Ω of the bulb) between the two points in the physical ground that correspond to your two ground symbols. This potential spreads out throughout the ground (we can consider the ground as a very large grid of very small resistors, and in the simplest case we can consider it to be a uniform grid), which means that if you place probes at (almost any) two other points, there will also be a potential between those probes, with the ability to light up a bulb. This is called "ground lift".

And yet, in your scenario where multiple people have their own copies of exactly your second circuit, there is no interference as long as we stick to idealized models. Each circuit has a battery, a switch, and a lamp. If the switch is open, no current can flow, no matter what anyone else is doing. If the switch is closed, then current flows, inversely proportional to the total resistance around the loop. All the current entering one ground terminal has to leave the other. As long as the switch is an ideal switch, the voltage source is an ideal voltage source, and the ground is an ideal resistor, the ground lift caused by other people has no impact on the circuit.

However, if we let things be less-than-ideal then yes, there is interference. For example: a real switch has some capacitance between its terminals, which means that a tiny amount of current flows through your circuit every time someone presses or releases their switch, even when your switch is open. A real battery isn't an ideal voltage source either: it acts a little bit like a capacitor in series with a resistor. And the ground isn't a uniform grid of tiny resistors; it's a nonuniform aggregate of dirt, rocks, water (containing various minerals), buried metallic pipes, etc. Its resistance is probably not constant, but dependent on the voltage across it. So the brightness of your bulb while the switch is closed could vary depending on what other people are sending at the moment.

In reality, single wire earth return has been used successfully for telegraph, telephone, and even power transmission. When used for telephones there are some problems with crosstalk, which is one of the reasons it's largely been replaced since the 1930s, but it was still used for some pretty big installations without the interference being bad enough to make things unusable. I'll see if I can dig through my historical sources and add some numbers later on.