CHAPTER 17: CATHODE RAYS AND THE CATHODE RAY OSCILLOSCOPE

What are cathode rays? How do old televisions work? How can we "see" electrical signals? The answers lie in the behavior of electrons in a vacuum. Cathode rays are streams of high-speed electrons. Understanding how they are produced and controlled led to the development of the Cathode Ray Oscilloscope (CRO)—a versatile instrument that can measure voltage, frequency, and display waveforms. This chapter will give you mastery over thermionic emission, the properties of cathode rays, and the construction and uses of the CRO.

17.1 CATHODE RAYS

17.1.1 Historical Background

Cathode rays were first observed in the 19th century during experiments with evacuated glass tubes (Crookes tubes). When a high voltage was applied across electrodes in the tube, a glow was observed at the anode end. Scientists discovered that rays were emitted from the cathode (negative electrode) and traveled to the anode (positive electrode). These were named cathode rays.

J.J. Thomson later proved that cathode rays were streams of negatively charged particles, which we now call electrons.

17.1.2 Production of Cathode Rays: Thermionic Emission

Cathode rays are produced by a process called thermionic emission. This occurs when a metal is heated to a high temperature, giving electrons enough energy to overcome the work function (the energy holding them in the metal) and escape from the surface.

The Electron Gun:

In a cathode ray tube, electrons are produced and controlled by an el