what device separates Wemmick from the rest of the world?
There are two devices which seperate Wemmick from the rest of the world. The first is a physical construction. Wemmick's house is a castle, complete with cannon and drawbridge. He is able to seperate himself and his father off from the rest of the world by raising their drawbridge and receding into their private world together. Here we see him as a compassionate and jolly man, entertaining his father with the cannon and welcoming Pip into his family.
The second device is less concrete. Wemmick partitions off his life between work and home life. As clerk to Jaggers the solicitor he is abrupt, officious and distant. This image is projected to all including Pip, despite their closeness and trust for each other outside of their business dealings.