George Patrick Barnham was an inmate of Stangmoor Prison.
He was the latest inmate to be subjected to the Keller Machine, which extracted all the negative impulses from his brain. Due to this, Barnham's presence acted as a dampener on the machine, i.e. it would not function while he was present.
Working with the Third Doctor and Jo Grant, Barnham brought the machine to the site of the Thunderbolt rocket, which incapacitated the Master and allowed the Doctor to fix the missile's abort mechanism and stop its launch.