A Helmholtz coil is one of the equipment that produces cathode rays. Cathode rays and cathode-ray tubes have many applications in the modern world. Let’s dig in and find out more.
By the end of this topic, you are expected to;
Other terms that can be used to refer to cathode rays are e-beam or electron beam. Cathode rays refer to streams of electrons that are observed in vacuum tubes. When an evacuated glass tube is equipped with two electrodes and voltage is applied, the glass that is behind the positive electrode is seen to glow. This is as a result of electrons that are emitted from the cathode (the electrode that is connected to the voltage supply’s negative terminal). Cathode ray tubes (CRTs) make use of a focused beam of electrons that is deflected by magnetic or electric fields in order to render an image on a screen.
Cathode rays are named so because their emission is from the negative electrode, which is known as the cathode. In order for electrons to be released into the tube, they must be detached from the cathode’s atoms first. In the Crookes tubes (early cold cathode vacuum tubes), this was done using the high electrical potential of thousands of volts between the cathode and the anode to ionize the residual gas atoms found in the tube. The electric field accelerates the positive ions toward the cathode, on collision with it, electrons are knocked out of its surface. These are the cathode rays. Thermionic emission is used by most modern vacuum tubes, in this, the cathode is composed of a thin wire filament that is heated by a separate electric current passing through it. The increase in the random heat motion of the filament is responsible for knocking electrons out of the filament’s surface into the evacuated space of the tube.
Because electrons have a negative charge, the negative cathode repels them and the positive anode attracts them. They travel through the empty tube in straight lines. Cathode rays are not visible, but they were first detected in the early vacuum tubes when they struck the glass wall of the tube. This excited the glass atoms resulting in them emitting light, a glow known as fluorescence.
There was an ongoing debate about whether cathode rays are a wave or a particle. This was because of their conflicting properties.
Later, J.J.Thomson used an electric field to deflect the rays. This demonstrated that the beams were composed of particles because scientists knew it was impossible to deflect electromagnetic waves with an electric field. These can also create mechanical effects, fluorescence, etc.
The cathode-ray tube (CRT) refers to a vacuum tube containing one or more electron guns and a phosphorescent screen and it is used for image display. It is responsible for modulating, accelerating and deflecting electron beams onto screens in order to create images. These images may represent pictures (computer monitor, television), radar targets, electrical waveforms (oscilloscope), or other phenomena.
A color CRT is made up of:
