The "ridges" that you are referring to are actually called "reeds". All dimes that are in circulation have 118 reeds. The number of reeds was established when the dime was commissioned by the Coinage Act of 1792. The first dime was minted in 1796 and continues to be produced today.
The US dime has 118 ridges around it's circumference. These ridges, also known as a reeded edge, were originally used as a counterfeit prevention tool. They are also useful to the seeing impaired for identifying the coin.
The modern Roosevelt dime which has been minted since 1946 is the smallest and lightest US coin. Despite its small size, the Roosevelt dime has 118 ridges along its edge. Dimes are currently made of the metals copper and nickle. Prior to 1965, dimes were made of silver and a small amount of copper.