The Cathode Ray Tube

Figure 9.1 shows schematically the essential features of the cathode ray tube in the oscilloscope.

   

Figure 9.1: The cathode ray tube (schematic)

Electrons leave the heated cathode by thermionic emission. They are accelerated through a fixed voltage and emerge as a narrow beam focussed through a hole in the accelerating plate. When the electron beam strikes the fluorescent screen on the face of the tube, it produces a small luminous spot. An external potential difference can be measured by applying it across a pair of parallel deflecting plates, through which the beam passes on its way to the screen. The beam is then deflected by the resultant transverse uniform electric field between the plates. There are two pairs of deflecting plates, one for vertical and the other for horizontal deflection.

If you double the externally applied potential across a pair of deflecting plates, you will also double

1. the electric field intensity between the plates.
2. the transverse force on the electrons.
3. the resultant transverse acceleration of the electrons, and thus,
4. the net transverse displacement and tangent of the angle at which the beam leaves the region between the plates.

The net displacement of the spot on the screen will therefore be proportional to the applied potential difference. It is this property which makes the oscilloscope useful as a voltmeter.