How does an electric field produce a potential difference across a wire?

Apr 2013
This is a conceptual question. I hope that's okay.

I've created a small circuit with an arduino that receives input from a copper wire connected to an analog pin, similar to what is shown in this tutorial:

The arduino reads the voltage across the sensor and interprets it as an integer from 0 (ground) to 1023 (5 volts). Left alone, the sensor tends to return a value of about 250-300. The value in this range oscillates due to, I believe, the power lines and other electromagnetic noise in the area. Is this correct?

My question is, what is going on at the atomic level between the electric field and the copper wire? I know that a magnetic field can induce a current through a coil of wire; how does that relate to electric field on a straight wire?