# Voltmeters / direct measurement of voltage in a circuit

I was wondering that many instruments that measure the ddp in a circuit works measuring instead current and then applying Ohm's law.

When I looked for how Ohm made his initial measurements for voltage, I found that he used a thermocouple, measuring the temperature.

But how was that thermocouple calibrated? Is there another instrument or something like this?

I think you are asking how he knew how much is 1 ampere or 1 volt. He didn't know. Almost every unit was arbitrarily defined. So, something like, for 1 degree temperature difference, current meter gives x degree deflection. Let's call it 1 ampere. Standard definitions of the units came much much later.

So, in short, Ohm arbitrarily decided the calibration.

Ohm initially use cells (voltaic piles) but found them to provide a stable voltage and have variable internal resistances so he then used a two thermocouples as the source of voltage. He assumed that the voltage provided by thermocouples was proportional to the difference in the temperature of the junctions. So there was no reference made to measuring the voltage in volts rather he assumed that doubling the temperature doubled the voltage.

So in essence he was investigation whether

$$\text{temperature difference} \,\,(\propto \rm voltage \propto current)\propto \text{gavanometer deflection}$$

• Wait, what? Didn't he use the definition of the potential that was known( for a point charge)? Where is the link between this assumed voltage and the potential I just mentioned? Nov 18 '21 at 12:57
• What I am trying to point out is that Ohm did not need to use any definition of the volt and the ampere to show the proportionality. The part of my equation outside the brackets is what he showed experimentally and the part inside the brackets is what he inferred from his observations. Nov 18 '21 at 13:16
• Ok, so i'm asking, who verified the seebeck effect? And using which instruments? My doubts are about the link between what i know about potential in situation where we have a distribution of fixed charge and the potential in a circuit Nov 18 '21 at 14:05
• I suggest that you start another post and ask about the Seebeck effect. Nov 18 '21 at 17:31