A duodynetic field core was part of a mnemonic memory circuit.
Stranded in 1930, Spock proposed creating one as part of a complex mechanism for retrieving information stored in his tricorder – information vital to determining how Leonard McCoy had altered history and erased the Federation. Unfortunately, the combined salaries of Kirk and Spock were the only resources available to them. At that time, platinum was a precious metal, and not common – a block of several pounds, necessary for such a core, was far beyond their means. (TOS: "The City on the Edge of Forever")