Thread: Discharge Tube

In a discharge tube how can electricity flow as there is no medium.

Air cannot conduct electricity.

And even if it conducts then from where does the cathode rays i.e. elctrons come from and when they Ionize the gas and forms ions there shoud be a limit.

Is it the ionized gas that glows or just the fluroscent effect of the glass.

In a discharge tube how can electricity flow as there is no medium.

Ummmm...i beg to differ. There is a medium in the discharge tube (I guess you are talking about fluorescent lights) allthough at rather low pressures. The main constituent is a noble gas (Krypton or Argon) as well as minute amounts of Mercury.

Air cannot conduct electricity.

Google "Jacobs Ladder"

And even if it conducts then from where does the cathode rays i.e. elctrons come from and when they Ionize the gas and forms ions there shoud be a limit.

This is basically the mechanism behind all this. The ionized gas i.e. plasma recombines and emits EM-radiation.

Is it the ionized gas that glows or just the fluroscent effect of the glass

Actually the primary effect is UV radiation of the plasma but you would not want this in your appartment. This UV-radiation is converted to visible by a coating on the inside of the tube, commonly called "phosphor". This has nothing to do with the element Phosphorous but it's rather a ceramic compound.

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Originally Posted by anand_kapadia

Air cannot conduct electricity.

There is no such thing as lightning?


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Cathode rays are electron beams that are produced at the cathode within a tube. The production of the electron beam and the conductivity of the gas (or vacuum) in the tube is a matter of the voltage used between the electrodes. If the voltage is high enough, electrons are released from the cathode (heating it increases the release) and are accelerated towards the anode. Depending on the velocity they gain, their kinetic energy can be high enough to ionise the gas inside the tube. When they hit the anode, they produce X rays.

There is a very nice historic related experiment: the Franck-Hertz experiment

Thanks Dishmaster but still there are traces of doubt.

U say that the cathode emits electrons. So is it the "Cold cathode emission or Field emission" which is responsible for the emission of the free electrons of the metal cathode. And the electrons emitted from the cathode strike the anode and then circulates through the external(outside the discharge tube) circuit and reaches cathode again.

My Hypothesis:-

As i went through different books i found that cathode rays are emitted just at 0.01 mm Hg presure(in a discharge tube 3cm diameter 30 cm long).
I wonder that it is just voltage which should be high. Even if the pressure is high electrons(cathode rays) are emitted but as there is no mean free path they donot reach pass the anode. But if the pressure is lowered to 0.01mm Hg the electrons reach the anode without collsion(wth ions, or atoms of gases) and produce fluroscene on the glass behind anode.

Is my hypothesis correct.

And also the wiki says that through sprutation cathode emits atoms rather than electrons.


Please Help

Originally Posted by anand_kapadia

U say that the cathode emits electrons. So is it the "Cold cathode emission or Field emission" which is responsible for the emission of the free electrons of the metal cathode.

You can have both, the hot and the cold cathodes. However, it seems that most common applications of discharge tubes use the cold cathode, i.e. the voltage is high enough to release the electrons from the cathode even at ambient temperature conditions.

Originally Posted by anand_kapadia

And the electrons emitted from the cathode strike the anode and then circulates through the external(outside the discharge tube) circuit and reaches cathode again.

Indeed. And depending on the velocity these electrons gain during their acceleration in the electric field, they cause secondary radiation, when they hit the anode. But this is not the main purpose of a discharge tube. In such a tube, it is the gas inside the tube that produces the primary radiation.

Originally Posted by anand_kapadia

As i went through different books i found that cathode rays are emitted just at 0.01 mm Hg pressure(in a discharge tube 3cm diameter 30 cm long).
I wonder that it is just voltage which should be high. Even if the pressure is high electrons(cathode rays) are emitted but as there is no mean free path they do not reach pass the anode. But if the pressure is lowered to 0.01mm Hg the electrons reach the anode without collision (with ions, or atoms of gases) and produce fluorescence on the glass behind anode.

I am not an expert in applications of discharge tubes, but it is easier for the emitted electrons to reach the anode, if the pressure within the tube is small. You correctly write that a low pressure means a long mean free path. But also the gas itself has an influence of the mean free path length (see Franck-Hertz experiment), because the interaction probability also depends on the energy levels of the gas atoms. Only if the kinetic energy of the electron matches the energy gap between two quantum levels of the gas atoms, the electron can cause ionisation or excitation and is absorbed. Here you see, that the voltage that is responsible for the acceleration of the electrons to a certain velocity (=> kinetic energy) also has an influence on the possibility how many electrons reach the anode.

The ionisation or excitation of the gas inside the tube is caused by collisions with the electrons which leads to the emission of photons that can be transformed into visible light by fluorescence. So, without collisions there is no light from the tube.

But you can also guide the electrons themselves onto a fluorescent (like in a TV or a computer display). In this way, the electrons produce the light by fluorescence on the screen and no reactive gas is needed. The advantage here is that electrons can be easily deflected by an electrical field. But this is not called a "discharge tube".

So it is clear that the emission of cathode rays depends upon the voltage.
And i am a bit confused that are 'cathode rays' and 'discharge' different things.

Because the book says that at very low pressure the no. of gaseous atoms becomes so less that ionisation does not occur and the discharge ceases leaving the tube dark. Then is it not that cathode rays should travel easily and reach anode easily.

Originally Posted by anand_kapadia

So it is clear that the emission of cathode rays depends upon the voltage.
And i am a bit confused that are 'cathode rays' and 'discharge' different things.

Because the book says that at very low pressure the no. of gaseous atoms becomes so less that ionisation does not occur and the discharge ceases leaving the tube dark. Then is it not that cathode rays should travel easily and reach anode easily.

"Discharge" means emission of photons from the gas that is excited or ionised by the electrons. Therefore, no gas = no discharge.

and so,
no gas=no cathode rays

Originally Posted by anand_kapadia

and so,
no gas=no cathode rays

No, the cathode rays are the electrons that are released from the cathode.

then even if the discharge ceases due to very low pressure cathode rays emit from cathode and pass to anode.

Thanks