My Great-grandfather, John James, was a Warder at Dartmoor Prison, Princetown from 1870 to about 1890/5-ish.
See
http://www.flickr.com/photos/13225589@N02/1406998731/in/set-72157601970682222/ for a picture.
I've made extensive enquiries about his time with the Prison Service to little avail; the riot notwithstanding it seems that the service had, and has, a policy of destroying staff records on the 75th anniversary of their birth date. Ironically, you can find out far more if your ancestor was on the wrong side of the bars!
The curator at Dartmoor Prison Museum is Tina Pomerey.