# What lead of a voltmeter is subtracted from the other to get the voltage?

As far as I understand, voltage is a difference of potential energy between two points, and that current flows from the negative to the positive pole of a battery in a circuit.

Thus, according to these premises, voltage would decrease from negative to positive as the potential energy is used to move the electrons.

In a multimeter, does the same logic apply, and is the energy at the red lead subtracted from the black? Or is it the other way around, and why?

• While it's technically correct to say "current flows from negative to positive", we never look at it like that. The whole world uses "conventional current" where we assume current flows from positive to negative. You should mentally operate the same way (unless you want to speak a different language from everybody else). Jan 22 at 7:21
• ”voltage is a difference of potential energy between 2 points” Almost. A stone on a hill has more potential energy than a stone in a valley, but your voltmeter can’t detect it. It can only detect electrical potential difference. Energy is a different physical quantity. Jan 22 at 8:42
• electrons flow from negative to positive. Current flows from positive to negative. Forget about electrons on this site until you are more proficient. Jan 22 at 11:48
• If the charge carriers in your circuit are electrons, then those will be moving from negative to positive, but a different part of the same circuit might have positively charged ions moving from positive to negative. But unless you're digging into the physics side of electronics, then current flows from positive to negative. Always. Jan 22 at 15:21
• @KyleB, no, it's not right to say "current flows from negative to positive". Even if you think of the physics of electrons as charge carriers, they're negative charges, so even if they flow from negative to positive, the electrical current itself flows from positive to negative. (Like Andy said, best forget about the electrons.) Jan 22 at 17:57

As far as I understand, voltage is a difference of potential energy

Just potential. Or voltage. You can express the voltage as 'potential energy per unit charge', but that's a bit of a mouthful.

between 2 points

Exactly, I'm glad you have that

and that current flows from negetive to positive poles of a battery in a circuit.

Conventional current flows from positive to negative, by convention. Current is made up of the movement of charge carriers. Those charge carriers can be anything mobile that carries charge, so positive and negative ions in an electrolyte, electrons and holes in a semiconductor, electrons in a wire, electrons and positive ions in a plasma. Positive carriers move positive to negative, negative carriers move negative to positive. As it happens, in metal wires, electrons are the only charge carrier, and they move negative to positive. But they are electrons, not 'current', which we always reserve for 'conventional current'.

Because they are negatively charged (by convention), the direction of conventional current is opposite to the flow of electrons that it consists of in wires. Be kind to the pioneers, they had to choose a direction for current ages before the electronic/nuclear theory of matter.

This may seem like an awful shame, as people first meet, and often only meet, electrons as charge carriers, but any time we have to deal with carriers of both signs, we need to consider their movements separately, so we would always have some that were negative.

Thus, according to these premises, voltage would decrease from negetive to positive as the potential energy is used to move the electrons.

No, the positive pole is defined to have a higher potential, a higher voltage than the negative. Whether it does, or the opposite, is down to convention. You're free to use whatever convention you like in your own work, but 100% of the scientific and engineering world has chosen the '+ve is higher convention' (for better or worse), and if you want to be able to communicate with them, you should too.

Now my question is, in a multimeter, does the same logic apply, and is the energy at the red lead subtracted from the black? Or is it the other way around and why?

A multimeter reads the voltage difference between the leads, and reads positive for the red lead being at a higher potential, a higher voltage, than the black. We usually express this as the voltage of the red lead 'with respect to the black'.

We often simplify this long-winded way of measuring voltages by choosing a terminal to be our '0 V' reference, connecting the black lead to that, and measuring all other voltages with the red lead. Which is fine, as long as we always remember that we are really dealing with voltage differences.

• Thanks for explaining the conventions lol, Seems like some confusion among the inventors of these terms has led to real confusion Jan 23 at 6:32
• @KrypticCoconut Yes, but you have to take into consideration that current flow was discovered (and the positive to negative convention established) by Benjamin Franklin in 1752. Whereas electrons were discovered by Joseph John Thomson in 1897 - roughly 150 years later. If the electron had somehow been discovered first, then Benjamin Franklin would likely have chosen the more physics-aligned direction for current flow. :) Jan 23 at 9:00
• @Frodyne one of these days I need to write a "introduction to electrical theory in the order it was discovered" web page. I think some learners might find it clarified a lot. Jan 23 at 9:48

• If words are not clear it is $RED-BLACK$ Jan 22 at 5:58