In Book II, Chapter 4 of a Tale of Two Cities, what is the golden thread?

In Chapter 4 we learn that in Book II ("The Golden Thread"), the golden thread most obviously refers to Lucie's hair. It also refers more abstractly to her curative power over her father. She commands the golden thread because she connects him to an earlier time that was not painful and to a present beyond his miserable period of imprisonment. Lucie is important not only for her golden nature, which has the power to redeem him, but also for the connective nature of her existence. She is the only person able to pull her father away from recollecting his time of misery.