The Serial output on an Arduino is quite slow. By printing your output, you will slow down the readings so much that it will appear to be nonsense.
You should collect a series of readings into a C array of floats in RAM, stop recording, and then log that array of values.
Note that you don't have a lot of RAM to work with on most Arduinos, so you will only ...
To make that work you'd need to connect the HV connection to the Vin of the NodeMCU (as well as to the relay board) which should be the 5 V coming form the USB port. You'd also need to connect the GND on the HV side to the GND of the NodeMCU.
A signal is just a varying voltage. A wire is just a (very long) small resistor. The device reading the signal is just a current sink.
That's three terms there: voltage, resistance, and current. Three terms you should know are related by Ohm's Law.
Also, some simple rules:
The thinner the wire the bigger the resistance
The longer the wire the bigger the ...