Cold cathode ray lamps
You have seen cathode ray lamps. You even using one in your house or you be in place where they are being used. Cathode ray lamps are the fluorescent lamps we use in our houses for lighting. They are a little dangerous and that is why people are discarding them in favor of LED lamps. Cathode ray lamps, despite their efficiency and desirable light, they use mercury vapor. Mercury is a dangerous element whether in vapor or in solid form. You should handle it while wearing protective gears. How cold cathode ray lamps work is similar to CRT since it involves emission of electrons.
We will discuss one of the two types of cathode ray lamps, the cold cathode. Does this mean there is a hot cathode ray lamp? Yes, there is.
What is cold cathode
We shall start by defining what Cathode rays. The term cathode refers to the negative terminal in a circuit. In short, they cathode rays are rays emitted by a negative terminal in circuit. These rays are just electrons. In our previous article on how CRT TV works, we saw that a special environment is needed to have the electrons (or the rays) emitted. It is also the same case with the cold cathode ray lamp.
A cold cathode is a type of cathode that does not have to heated by filament using electricity. The term cold was coined since this type of cathode is able to emit more electrons than what thermionic emission could emit. The term cold does not imply that it operates at very low temperatures. However, during operation, this type of cathode gets heated using other methods.
before we look into how cold cathode ray lamps work, Let us now looking into the history of cathode .
History of cathode
Michael faraday, is believed to have started the work with the cathode rays. However, it was not possible to attain a very good vacuum then so he used a partial vacuum during his experiments. When he connected the high voltage between two metal rods connected by a vacuum, he realized, there was an arc that appeared to move from negative terminal to the positive terminal. You will be fascinated to know that later, after a scientist managed to remove more air from the vacuum, the whole tube glowed. This is engineers later called Geissler tubes.
When the electrons moved in the partial vacuum, they collided with the air molecules creating ions. The positive ions were attracted to the cathode while the negative ions were attracted to the anode. When the ions hit the cathodes, more electrons were released. The ions generated allowed the current to flow freely in the glass tube. But the low speed of the electrons did not allow the development of cathode rays.
Crookes dark space
Michael faraday had notices there was a dark region near the cathode. The dark region is what we now call faraday dark space” or “Crookes dark space”. However, as you remove more airs from the glass tune, the faraday dark space” or “Crookes dark space” spreads. It can spread further even near the anode. However, the speed of electrons causes them to miss the anode and struct the wall behind.
This causes a glow when the electrons which were displaced to a higher energy returns back to their normal state. When the electrons hit the wall, they cause electrons on the wall to move to a higher energy level. This glow us what is referred to as fluorescence. However, the glow is either blue or green in color. A zinc sulfide paint made the grow to be more visible. This is what we use today to make gas discharge tubes such as the krytrons.
Figure 1: Crook’s tube.
This time, people had not known about the electrons. However, in the early 20th century, Thomas Edison received a Nobel peace prize for proving electrons existed.
Over time, the tubes would stop working and so the method was deemed unreliable. You know that matters are capable of absorbing other matters and hence gas would get absorbed by the wall. A better way of creating cathode rays was explored by Thomas Edison. The method as referred to as thermionic emission. This was achieved by heating a cathode while a separate current was being passed through it.
Vacuum tubes came to existence in early 20th century. A triode, was technique that revolutionized the world of electrical technology. Lee De Forest is the man who is credit to much of the triode technology. He realized that current could be controlled if a small voltage could be allowed. This is what is referred to as the accelerator voltage today in the CTR tubes. This technology got much use in the radar, talking movies, audio recording, and long-distance telephone service, radio as well as television.
By then, the cathode rays were referred to as the electron beams. The current design used in the CRT today was carefully developed in the 1897. This is what made televisions a possibility. However, they are no longer used in television. If you can remember our previous article on how LED tv display works, we talked of modern methods of producing the much-needed backlight. There are better uses of CRT such as the particle accelerators, electronic microscopes and electron beam lithography.
How does fluorescent lamp work?
the key to understanding how cold cathode ray lamps work is understanding how fluorescent lamps work. Now that we have look into the history of fluorescent and by extension cathode rays, lets see how the fluorescent lamps work. A simplified circuit diagram of fluorescent lamp is as shown in the figure below.
Fluorescent lamp circuit diagram.