Dickens uses semantic field of time, with phrases such as "Five o'clock... ten o'clock... hour after hour... Seven hours left!" to show us that the experience of a prisoner sentenced to death is tough and nerve wracking while the time slowly comes to the end.
This affects the reader by making him imagine being in the prisoner's skin and how the time is slowly counting down until his death. Dickens talks to us about a 14 year old boy who is going to die for doing something silly making the reader unsteady and nervous as the time goes down.