Why does the following "Battery" experiment work?

I did the following experiment:


**Experimental setup:**
=======================


**Two electrodes (one made out of Iron,the other made out of Copper)**
**are put into a (bluish) Copper(ii) sulfate lotion.**


[![enter image description here](https://i.stack.imgur.com/BQzyS.png)](https://i.stack.imgur.com/BQzyS.png)


**Then, one connect a voltmeter to them to measure the potential.**


[![enter image description here](https://i.stack.imgur.com/OX4fb.png)](https://i.stack.imgur.com/OX4fb.png)


**Finally, one disconnect the voltmeter and connect an Amperemeter to measure the electrical current.**


[![enter image description here](https://i.stack.imgur.com/DB3ea.png)](https://i.stack.imgur.com/DB3ea.png)


**Observations:**
=================


-Copper accumulates around the iron electrode,forming a thick copper coloured slick around it.


-there is a potential of about 0.5 V between the electrodes, which stays for several minutes.


-when one connect the amperemeter, one can measure a current which stays at least for a few seconds. (didn't measure further)


-there is no (visible) activity around the copper electrode.


-the solution doesn't change its colour notably.


-especially, one could not observe any evolution of gas. (although this does not mean that there is absolutely no gas evolution)


**How is the measured tension and current caused?**
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