Let’s take a minute to highlight the unique properties thyristors bring to a circuit by going further into the nature of the signal and the thyristor’s response. Electron (or Vacuum) Tubes An electron tube (also known as a 'Vacuum tube', or a 'Valve' ) is a glass or metal enclosure in which electrons move through the vacuum or gas from one metal electrode to another. The vacuum tube is often used to amplify weak currents or act as a one-way valve (rectifier) for electric current. http://www.weldh.com/electronic-tube-equipment/