What is the Tread Mill in A Christmas Carol?

The was like a wide water wheel in it appearance. The treads were separated across a room into cubicles. The poor would have to walk (in a similar manner to a hamster on a wheel) for up to 18 hours a day. They could not speak to anyone nor see anyone they faced the treadmill at all times. The mill turning did noting. It was simply there for people to be made to walk on. It was in effect a punishment for being poor.