anyone familiar with the radios of the previous generation will remember vacuum tubes: these glass tubes which functioned as amplifiers formed the heart of essentially all electronic gadgets from primitive computers to radios. The essential component of the vacuum tube was a cathode which was heated to give electrons. The vacuum tubes were bulky, unreliable and cumbersome to manufacture. With the advent of transistors, these tubes became obsolete except in certain specialised applications. But recently they have made a comeback, albeit in a modified form.

In the last few years, a lot of work has gone into the development of cold cathodes