Daniel invented the cell (Daniel cell) that had the advantage of NOT using acid solutions. Acid solutions produce toxic gases.
Instead, he used salt solutions. Different metals have different tendencies to liberate electrons when in contact. The Daniel cell uses Zinc in its ions Zn<sup>2+</sup> ( as ZnSO<sub>4</sub>) conected with Copper in its ions Cu<sup>2+</sup> ( as CuSO<sub>4</sub> ). Both solutions are separated by a porous medium, Externally, the metals are electrically bound. As the metal zinc has more tendency in loosing its electrons to copper, then Zinc will be corroded, like this :
Zn -> Zn<sup>2+</sup> + 2e<sup>-</sup>
These two electrons flow through the electric wire and reach the copper. There, the copper ions Cu<sup>2+</sup> will be attracted and deposit as metal, like this:
Cu<sup>2+</sup> + 2e<sup>-</sup> -> Cu
The voltage in this case will be 1.10 Volt theoretically. It depends on temperature, agitation, salt concentration, but it will be near 1 Volt. I have a practical written up, if you really need it, I guess I can type it up.