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Metals Al and Au are connected in series. An ideal voltmeter, with both of its leads made of Cu, is connected across them. Then how come the voltmeter gives zero reading but why can't we measure contact potential between them if there is potential difference electrons flow right then why electron flow can't be measured If so how can we prove on paper that it is 0 V

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    \$\begingroup\$ Because it seems that you don't have a very sensitive voltmeter. \$\endgroup\$ Commented Sep 15, 2019 at 10:31
  • \$\begingroup\$ is that a school assignment question? \$\endgroup\$
    – jsotola
    Commented Sep 15, 2019 at 20:43

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Every junction between dissimilar metals has a potential associated. In your example, you have three such junctions: Cu-Al, Al-Au and Au-Cu. If they're all at the same temperature, these potentials will all cancel out. This is required by simple thermodynamics — if they didn't cancel out, you'd be able to extract arbitrary amounts of energy from the system.

But if they're not all at the same temperature, you will be able to measure a voltage, and you'll be able to extract electrical energy from the heat flow through the system. This is the basis of the Seebeck effect.

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