# Problem of measuring barrier potential by a voltmeter

I have read in my book that The barrier potential of a diode junction cannot be measured by a voltmeter only. It says that the thermal equilibrium is disrupted. Besides , the contact potential is also mentioned.

But I don't understand it. What does it mean actually?

One way to see this is that an ordinary voltmeter is a load and requires some current to show a value. But a diode cannot output energy to a load without input of energy. Other contact voltages will balance the built-in potential over the $pn$ junction.
• @UltraInstincts There will be contact potentials between the metal leads and the semiconductor on both the $p$- and the $n$-doped side, so that the built-in potential over the junction is cancelled. As for what the book means..., that is a bit of a mystery to me too. The wires should not get really warm. And maybe they are alluding to a Peltier effect, but that does not actually need a $pn$-junction. – Pieter Feb 24 '18 at 14:48