What is an agency relationship? How is an agency relationship formed? What are the duties of an agent to a principal? What duties does a principal owe to an agent?

What are the differences between a disclosed principal, an unidentified principal, and an undisclosed principal? What is the liability of both the agent and principal, respectively, on contracts by an agent of each of the three types of principals?

Consider the following short case study. Patricia asks Albert to buy $10,000 of widgets. Albert buys the widgets from Tom, but does so in his own name; Albert delivers the widgets to Patricia. Tom, however, learning that Patricia initiated the order, sends the invoice to her (instead of Albert). Patricia refuses to pay the invoice. Tom sues both Patricia and Albert. Who is liable? Why?