In a nutshell
Llewellyn-Jones was the top authority on the physics of electrified gases and on electrical contact phenomena - the type which cause blue flashes in a light switch.
As a child, Llewellyn-Jones used to build crystal wireless sets, model railways and make amazing machines out of meccano. Working in Oxford with great scifiles of the day, he began his lifetime study of how gases conduct electricity.
During the war he worked at the Royal Aircraft Establishment at Farnborough, applying his expertise to problems with spark-plug ignition and erosion in switches in aircraft engines. At the time these were problems which seriously limited flying time between essential servicing.
He was also a railway enthusiast, especially fond of the electric Mumbles Railway on the Swansea Bay foreshore.
It's common knowledge that electricity flows through metals but electricity also flows through gases, the most spectacular example being lightning. This is possible because atoms of gases can, under specific conditions, split into positive and negative ions which, in travelling in opposite directions to oppositely charged electrodes, form a current.
When you press a switch on or off you can see a spark - electricity flowing across a small gap. What you don't usually see is the heat damage to the metal contacts of the switch. At home this damage isn't significant, but in other specialist circuits the erosion can be critical and cause equipment to fail.