COLD-CATHODE TUBES lack heaters or filaments and, therefore, do not use thermionic
emission. Instead, a voltage potential applied across the tube causes the internal gas to ionize. Once
ionization has occurred, the voltage drop across the tube remains constant, regardless of increased
The CRT is a special-purpose tube that has the unique ability to visually display electronic signals.
The CRT uses the principles of electrostatic attraction, repulsion, and fluorescence. Because of its unique
ability, the CRT makes up the heart of many types of test equipment that you will become familiar with
during your career in electronics.