What is Bob Cratchit's job in A Christmas Carol?

Bob Cratchit is employed as Scrooge's clerk.

The door of Scrooge's counting-house was open that he might keep his eye upon his clerk, who in a dismal little cell beyond, a sort of tank, was copying letters. Scrooge had a very small fire, but the clerk's fire was so very much smaller that it looked like one coal.