Question: In an audio circuit, such as a guitar's electronics (the only thing I ever dabble with!), I have always thought (probably simplistically) of every point along a continuous earth path being essentially all at the same voltage (a 'zero volt' reference voltage).

However, I was wondering - if you extend an earth wire from some point along that path, is it possible for that in itself to introduce noise to the circuit by changing the '0V' of the earth reference? (Possibly I am wondering "What is the difference between a long earth wire and an antenna" - but I don't know enough about antennas to know if that's what I'm asking :)

Answer: 
0V is only 0V at the point of reference - it gradually degrades as you move away from that physical position. It may be nano volts difference up close but can rapidly become milli volts and, if sensitive input circuits have 0V connections that are not at the same point, the several milli volt difference can be an AC noise-signal and is quite often very annoying.

This is why PCBs use ground planes but, these are by no-means exempt from this problem. Other systems use star-point wiring to keep any 0V connections at the same physical point but these can suffer from magnetic pick up of AC.

It's a difficult nut to crack sometimes. Here's a nice picture of how a digital signal can create both ground bounce and power bounce: -

[![enter image description here](https://i.stack.imgur.com/LZoRm.jpg)](https://i.stack.imgur.com/LZoRm.jpg)

The fast rising edge on an output will inevitably charge up a PCB track's parasitic capacitance - this is seen as a small "bounce" on the power rail and associated distortion (side effect) on the outputted signal. When the edge falls there is a ground bounce. Now the signal is slightly corrupted by importantly, for this question, the ground plane and power plane have pulses of current injected into them and these can affect other circuits closeby.

Hears another example of where gaps in a ground plane can cause "bounce" because the return current of a signal has to "spread" out around the gap: -

[![enter image description here](https://i.stack.imgur.com/Q17zc.jpg)](https://i.stack.imgur.com/Q17zc.jpg)

Here's another idea of how mis-positioning functional parts can wreak havoc on sensitive analogue circuits: -

[![enter image description here](https://i.stack.imgur.com/CK4Of.png)](https://i.stack.imgur.com/CK4Of.png)

The clever thing to do here is avoid common 0V connections for things that can cause ground bounce to each other - this is a form of star-pointing i.e. separate power and ground connections from each individual circuit function only connect at a single "clean" pair of nodes (usually at the output of a voltage regulator or battery). 

[**This**](http://www.te.com/documentation/whitepapers/pdf/3jot_2.pdf) appears to be quite a useful document that explains the phenomena
The answer is YES if your application depends upon the the difference of voltage between the two ends of the wire to be close to zero. In other words if a functioning system is sensitive to signal levels at both ends of the wire at once then there can be reason to be aware that noise pickup can happen.

On the other hand if you connect two systems together using a cable such as a twisted pair (or a coax cable for high frequency) and make sure that the current traveling in one wire is equal and opposite to the current in the other wire and the target system is only sensitive to the signalling between the target ends of the two wires then any common mode pickup in the pair of wires will go unnoticed by the target equipment.