The Ferranti valve is a type of vacuum tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 developed by Messers. Ferranti Education ltd in measuring the charge to mass ratio of an electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

. It consists of an anode and two co-axial guard rings. A cathode, a thin wire of tungsten filament, runs through this arrangement. When heated to a particular temperature using an HT electricity supply, an electron is emitted.