Conversation so far:
When did William Thomson, 1st Baron Kelvin attend Cambridge?
William's father was able to make a generous provision for his favourite son's education and, in 1841, installed him, with extensive letters of introduction and ample accommodation, at Peterhouse, Cambridge.
What did William Thomson, 1st Baron Kelvin study at Peterhouse, Cambridge?
William Thomson, 1st Baron Kelvin took a lively interest in the classics, music, and literature; but the real love of his intellectual life was the pursuit of science.
Did William Thomson, 1st Baron Kelvin belong to any clubs at Peterhouse, Cambridge?
Did William Thomson, 1st Baron Kelvin have any other interests besides classics, music, and literature?
The study of mathematics, physics, and in particular, of electricity, had captivated William Thomson's imagination.

Question: What did he do as a fellow?
Self-contained rewrite: What did William Thomson, 1st Baron Kelvin do as a fellow of St. Peter's?