The tube that Thomson built for his electrostatic deflection experiment (Figure 46) is the grandfather of the modern oscilloscope CRT.** As shown in Figure 48, the basic structures of Thomson’s tube are still recognizable in the modern CRT. The main difference is in the replacement of the “cold cathode,” where cathode-ray production relies on stripping electrons by ionizing traces of air between the cathode and anode. Instead, modern CRTs are sealed at a very high vacuum, and employ the more reliable thermionic emission method by which electrons are “boiled-off” the cathode when heated by a filament. In an improvement beyond Thomson’s tube, more than one anode electrode is used as part of the CRT’s “electron gun.” These anodes not only accelerate the electrons, but also act as lenses to focus the beam to a tight spot. The modern CRT also adds a set of deflector plates over Thomson’s tube, allowing the beam to be steered vertically and horizontally to reach any point on the fluorescent screen.
Figure 48 The modern electrostatic-deflection CRT used in CRT oscilloscopes is very similar to the one Thomson used in his electron-deflection experiment. The most dramatic change is the use of a thermionic cathode and an additional set of deflector plates that allow the beam to be steered in both axes (horizontal and vertical).
Now that microprocessor-controlled LCD screens have virtually replaced CRTs, small oscilloscope CRTs are plentiful and inexpensive in the surplus market, giving us the opportunity of using them to reproduce many of the classical experiments on cathode rays without having to pay careful attention to regulating the vacuum inside the tube.†† For our experiments, we built a rudimentary oscilloscope based on a design by J. B. Calvert of the University of Denver.12 We used the widely available 2AP1 CRT. However, the circuit should work equally well for other CRTs with 2-in.-diameter screen, such as the 2BP1 or 902. As shown in the schematic diagram of Figure 49, we use a dual-secondary transformer to power the CRT. The low-voltage secondary heats the filament, while the current from the high-voltage secondary is rectified and fed to a resistor divider that produces the various voltages needed to operate the CRT. Although the transformer is rated at only 500 V, the loading in our circuit is very low, so the voltage between +HV and -HV in our oscilloscope reaches well over 700 V. Be very careful when you build this circuit, since these voltages are dangerous!
Figure 49 We built this simple oscilloscope to experiment with the properties of cathode rays. The CRT is a surplus 2AP1 tube, but you can substitute one of the many other oscilloscope CRTs with 2-in.-diameter screens. We recommend that you use banana connectors between the CRT and your circuit so that you can reuse the CRT for other experiments.
Inside the CRT, electrons are produced by a thermionic cathode when sufficiently heated by the filament. Right after the cathode, electrons are squirted out of a small hole in a cup-shaped electrode called the “intensity grid.” The electrons are then accelerated and focused by two other anodes before reaching the region where they can be deflected by the deflection plates.
Potentiometers R11 and R12 place a voltage across their corresponding deflection plates. Simply turning the knobs on the horizontal or vertical centering electronics reproduces Thomson’s second experiment! Bring a magnet close to the CRT and see how the beam is deflected. Pay attention to the orientation of the magnet needed to cause the same deflection of the beam as an electrostatic field.‡‡ Another interesting experiment is to observe the deflection caused by turning the CRT around so that the electrons are affected by the earth’s magnetic field.
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