Not all the currencies are of the same value. Some devaluate quickly, others get damaged easily, still others get lost. If a scientist is equipped with any kind of meter, he or she can instantly assess the usefulness of two different coins. It is enough to place a piece of blotting paper saturated with any kind of solution, e.g. saliva. 'Good coins' like Polish old aluminium grosz or Italian 10 lire coin act quite well when put together with brass new zloty or copper eurocents. It is enough to use just a few of them to light a small LED diode.
This was one of the greatest inventions - a cell, or a Voltaic pile. He placed together a several pairs of alternating zinc and silver coins separating each pair by cloth soaked in acid. Such a pile generated quite considerable sparks. What is more, the pile was easy to disassemble if you needed the money. (in our pile in the picture the electronegative aluminium coins digest quickly and become thinner).
Volta's pile (or Galvani's pile) is a device which is more a chemical one than physical one: the electric charge is transported by chemical reactions: reduction of ions (in the physical terminology "electron attachment) or oxidation ("ionization" or electron detachment).
For example, in the Zn/Cu pile zinc atoms move to the solution and copper ions deposit on the anode. A so-called electrochemical potential is the characteristic feature of a given metal, or more precisely, of a given ion in a given solution. For example, reduction of the single valence copper ion Cu+ + e -> Cu shows the electrochemical potential of +0,521 V and the bivalence ion Cu2+ + 2e -> Cu shows a potential of +0,342 V. The electrochemical potential of the same reaction but in the solution of Hg is slightly different, +0,345 V. This potential depends also on the crystallographic phase, for example the potential of forming the phosphine gas (poisonous, produced for example during the organic garbage fermentation) P + 3H+ + 3e -> PH3 is -0.11V for red phosphor and -0,06V for the white one.
To make Volta's pile you need coins made of two different metals and a piece of wet paper (better if they are submerged in a salt solution). Two pieces of different metal connected with a wire and inserted into a lemon, a vase with flowers, into a tomato or potato work equally well; see the IQ meter, lemon battery or Volta's tongue. So try your change, what is the real value of your money (at least as an electric pile).