system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly is measured to find emf?

I've been learning a lot about electricity as I'm in my first semester of HVACR school. I know voltage is a measurement of potential differences. Electrons flow to a positive source as they themselves are negatively charged. I've noticed when I measure across a live wire, the voltage is zero. I understand that there is no potential differences for electron energy flow between two points of the same wire, therefore the wire will always read zero volts no matter how hot it is. I guess my question is what is one probe reading and what is the other probe reading exactly to come up with the voltage?
Sorry if it's a dumb question, I just wanna further my understanding of what I'm doing and this seemed like an appropriate subreddit.
gpt
One probe on its own does nothing. Voltage always needs a reference 
- the other probe. The physical probes are just cables to make sure the voltage difference inside the voltmeter matches the voltage difference in the circuit.

How a voltmeter works internally depends on the device, but it always compares the two cables going into it. If both probes are connected to the same cable then you have a cable from one side of the voltmeter to the other, so the voltage will be zero.