How did the contact of two different metals affect a frog muscle in Alessandro Volta’s experiments?

In Alessandro Volta’s interpretation of the experiments conducted by Luigi Galvani, the contact of two different metals with the leg muscles of a skinned frog served as the source of an electric current that caused the muscle to twitch.1

Volta disagreed with Galvani’s theory of "animal electricity," which posited that the frog’s tissue contained an innate vital force. Instead, Volta argued that the frog’s leg acted merely as an indicating electroscope and that the true source of stimulation was the contact between the dissimilar metals, which he termed "metallic electricity". He observed that the agitation of the muscle increased based on the degree of dissimilarity between the metals used. This disagreement eventually led Volta to develop the voltaic pile, the first battery, which proved that a continuous electric current could be generated using metallic components without the need for animal tissue.12