Measuring the speed of an electromagnetic wave using an oscilloscope and a couple of wires I've been trying to measure the speed of an electromagnetic wave with my oscilloscope.
I used :

*

*a 2 CH + SG function oscilloscope (Hantek DSO 2D15), SG set on square wave signal, amplitude 5V, frequency 10KHz

*2 probes (x10 mode), compensated, with both wires of the same length

*a couple of copper wires (2m each)

First of all, I plugged an aligator-clip ended probe into the signal generator, and plugged both CH1 and CH2 probes into the signal out, in order to mesure the delay (for instance, when tension starts to fall). With a time/div set to 2ns, I almost got zero delay. I inverted CH1 and CH2, same results.
Then I plugged a 2m length black wire into the black alligator probe coming from the SG, same thing with the red on with a 2m length red wire, and chose one of the 2 probes to connect it at the end of the wires. I started again the SG and...TADA !
A +/- 8 ns delay appeared. I tried to switch CH1 and CH2, delay still observed. As expected, the probe at the end of the wires is a bit late compared to the signal.
BUT : the light travels at about 30cm/ns (in vacuum). Then, to travel for 2+2=4 meters, it would have taken more than 13ns. And if we consider the material, the shape of the wires, the expected speed would have been no more than 80% * lightspeed.
In any case, the signal travels TOO FAST and I'm aware that lightspeed is a limit (as far as we know) so we can't "overspeed" it :)
So I suppose there is either a mistake in the way I reason or in my equipment.
If we consider the first possibility, for a reason I can't explain, the wave might travel for only 2 meters, what would give 6,67ns (for light in vacuum) and if we take into account the fact that the wave isn't light but electrons and travels 20% slower, then my results are pretty accurate.
So, what is wrong with my 2  2m cables ?
EDIT : have I somehow experienced a kind of reflexion in my setup ?
Is the quality of the signal generator affecting the results ("slow" rise time and fall time)
EDIT2 : here is a diagram of my very simple setup (made with Tinkercad)
 The delay I get with the channel measuring the circuit with the 2x 2m wires (in this diagram, CH1) is below the value of the delay I figured out
Thanks
 A: This is a transmission line problem.  You seem to be expecting your current to travel up your 2m red wire and then back down your 2m black wire, for a total of 4m signal propagation distance and a minimum of 12 nanoseconds propagation time.
But you have in fact built a two-meter cable.  Your voltage pulse travels along your cable as an electric field pointing from the negative wire to the positive wire.  The changing electric field in the gap between the wires induces a magnetic field in the gap as well.  And that changing magnetic field induces an electric field within the wires, pushing a positive current along the positive wire and pulling a negative current along the negative wire.  The light-speed propagation time along your two-meter cable would be about six nanoseconds, which is comfortably shorter than what you’ve observed.
A: Also please note that if the copper wires were covered in insulation, then the speed of an electromagnetic pulse in the wire will be slowed even more. This is called velocity factor and must be taken into proper account when shortwave radio antennas are manufactured, since the insulation alters the resonant length of an antenna. More info on this is available on the amateur radio stack exchange.
A: the speed of light in vacuum ist not 30cm/s nor is it 30cm/ms,  $c=3*10^8m/s$
for 4m in vacuum light would take $t=\frac{4}{3}*10^{-8}s$
A: I think you'll find this interesting.
This fellow did the experiment you're doing...
https://youtu.be/2Vrhk5OjBP8
A: It appeared that CH1 and CH2 shared the same ground and that is the reason why my setup behaved as if the EM wave traveled along a 2m wire ! Moreover, I did some research, and found that many oscilloscopes (actually most of them) have inputs that share a common ground. However, some oscilloscopes have differential inputs. Anyway, sometimes things are simpler than we think. Now I'll try to find out how do the geometry and the metal of the wire affect the propagation speed
