"There's no way this will work" - Roger that.
"a) Those currents switch in the MHz range so a multimeter will be totally unable to see them" - Yep, I should have understood/expected that.
"b) You'll probably cause errors so Ethernet will shut down and you'll see nothing" - In some cases it did shut down the Ehernet, others it didn't - I think it was teetering near the edge.
"Hint for getting good answers in forums: Tell us what you're trying to achieve, not what you're doing"- Well, I kind of did. I made a "breakout box" (more or less) and I measured to see what current was present. That's what I was trying to do, just measure, no other objective than to see if I could measure the current and how much was present. I expected the current to be relatively low (uA?) - and I'm still curious to know what the current might actually be; and I'd love to figure out what techniques and tools would enable a reasonably accurate measurement.
Among other things that went wrong, I didn't take into consideration the frequency but I think mostly what I don't have a grasp on is differential.
To be clear, I'm not interested in looking at PoE, just regular Ethernet data signal traffic. And I'm just measuring because that's how I learn - lame as that might be.
So, what I think I need to understand better is Ethernet and especially the principles of differential signaling.
"You can't just split a differential pair carrying 125 Mbit/s and add a massive long loop into just one half of it, then expect it to work." I sort of get this, but why? Here are some guesses: Because when you split the pair a) the differential is no longer at work? and b) even if it was, the DMM burden voltage is too high?, in addition to c) the frequency is way above what a regular DMM will handle? Feel free to correct me on any and all of this. Thx
"Ethernet is transformer isolated with 50 Ohm single ended / 100 Ohm differential impedance. If you want to know the current involved in the actual data signaling, measure the voltage differentially with a scope, and calculate it from there."- The transformer isolation takes place where? In the router or switch? What about in a PC port? Are the ports (in a router or switch) facing the user at 50 Ohm single ended? and 100 Ohm differential facing the interior of the Ethernet network, ie between routers or switches? That doesn't seem to make sense because the same Ethernet principle has to work all throughout a network (including PC to router/switch), so where is the 50 Ohm single ended / 100 Ohm differential impedance transformation taking place? And what drives the 50 Ohm / 100 Ohm relationship? (I'm guessing some use of Ohm's law like voltage dividers or something...? Maybe it's as simple as instead of one circuit at 50 Ohms it's 2 circuits x 50 Ohms = 100 Ohms?) As you can see, I'm kinda/very confused about the transformer isolation and where it takes place and why.
My objective is to figure this stuff out conceptually (and mathematically) and then be able to actually measure it. If anyone has a link to a primer or wants to post some explanations that would be great. Thanks!
PS, I've read this:
https://en.wikipedia.org/wiki/Differential_signaling - might just need to get it to sink in better and fill in some blank spots but any and all guidance welcome and appreciated