Dickens's text doesn't literally say that Bob Cratchit looked at a locked coal box, but he conveys that idea when his narrator states that

Scrooge had a very small fire, but the clerk's fire was so very much smaller that it looked like one coal. But he couldn't replenish it, for Scrooge kept the coal-box in his own room.

Bob is half frozen in his little "cell" off of Scrooge's office because Scrooge, though wealthy, is too miserly to heat his employee's office properly. Scrooge allows Bob so little coal that he is half frozen in the winter cold as he does his work. However, when Bob comes into Scrooge's office with the coal shovel to try to get some more coal, Scrooge threatens to fire him, so Bob suffers through. He tries to warm himself with his candle, but that doesn't work very well.

This small vignette at the beginning of the novel helps characterize Scrooge as an almost pathologically hard-hearted man who lacks any empathy. He underpays his clerk, begrudges him his one paid holiday a year—a day off for Christmas—and keeps him in miserable working conditions only so that he, Scrooge, can have a few more pennies he doesn't need and can't enjoy. It also shows the situation of the longsuffering Mr. Cratchit, who needs the job to support his family and, therefore, endures these petty tyrannies.

At the end of the story, Scrooge's transformation into a generous, caring person is expressed when he says to the no doubt astonished Bob Cratchit that he should

Make up the fires, and buy another coal-scuttle before you dot another i, Bob Cratchit!