Ancient history

How come some Mercury dimes have a large s mint mark and with small mark?

The large "S" mint mark on Mercury dimes was used by the San Francisco Mint from 1916 to 1942, while the small "S" mint mark was used from 1942 to 1954.

The change in mint mark size was due to a change in the minting process. In 1942, the San Francisco Mint switched from a steam-powered mint to an electric-powered mint. The new electric presses were more powerful than the old steam presses, and they could strike coins with a sharper impression. As a result, the mint mark on Mercury dimes minted after 1942 was smaller and more deeply impressed.