What are these pipes?

[Jeremiah O'Neill]

This might not be the best place to ask, but here I go.  I am looking at a house that has old radiators (the cast iron type).  Can someone answer the following regarding the photo below:

1. Is the large white pipe the supply/return for the radiators?

2. Am I correct in assuming this is a hot water system not a steam system (maybe this picture is not enough to answer that question)

3. What is the knob looking thing that looks to be attached to an electrical junction box (right in front of the connection between the white and silver pipe)

Thanks.