Volta’s pile — the first battery — was little more than silver and zinc discs separated by paper soaked in salt water. A classic classroom experiment is to build a pile from copper pennies, tin foil, and vinegar or lemon juice. [Omars2] has a different take on this old experiment. He creates a 9V battery using some zinc screws, copper wire, and salt water. There’s a video of the battery, below.
A syringe piston serves as a substrate for the cells, and each cell is just a screw with paper wrapped around it and then 35 turns of copper wire on top of that. The battery is soaked in salt water, although we suspect vinegar or lemon juice would work even better. Heating the electrolyte is also a good idea.
The theory is pretty simple: The salt dissolves into positively charged sodium ions and negatively charged chlorine atoms to form the electrolyte. The cathode loses electrons into the solution, leaving it positive. The other electrode — the anode — collects electrons, so it has a net negative charge. The difference between the charge on the two electrodes creates a potential difference. When you close the circuit, electrons flow from the anode back to the cathode, causing an electrical current.
If you’d rather make the penny version of this, it’s been easier in the US since 1982 when pennies became copper-covered zinc. Grinding the copper off one side of each penny allows them to act as both electrodes. Old pennies don’t have a zinc core, so to use those you’d need another metal like tin foil.