Discharge Tube (Crookes Tube) | Definition, Working

Definition of Discharge Tube or Crookes Tube

discharge tube

An electron tube contains gas vapor at low pressure and through which conduction occurs when a high voltage is applied. A discharge tube (also Crookes–Hittorf tube) is an early experimental electrical discharge tube, with partial vacuum, invented by English physicist William Crookes and others around 1869-1875, cathode rays, streams of electrons, were discovered.

Discharge tube is also known as ‘CROOKES TUBE.’

History of Discharge Tube

Crookes tubes evolved from the earlier Geissler tubes invented by the German physicist and glassblower Heinrich Geissler in 1857, experimental tubes which are similar to modern neon tube lights.

Geissler tubes had only a low vacuum, around 10-3 atm (100 Pa). And the electrons in them could only travel a short distance before hitting a gas molecule. So the current of electrons moved in a slow diffusion process, constantly colliding with gas molecules, never gaining much energy.

These tubes did not create beams of cathode rays, only a colorful glow discharge. And that filled the tube as the electrons struck the gas molecules and excited them, producing light.

By the 1870s, Crookes (among other researchers) was able to evacuate his tubes to a lower pressure, 10−6 to 5×10-8 atm, using an improved Sprengel mercury vacuum pump invented by his coworker Charles A. Gimingham.

He found that as he pumped more air out of his tubes. And a dark area in the glowing gas formed next to the cathode. As the pressure got lower, the dark area, now called the Faraday dark space or Crookes dark space, spread down the tube until the inside of the tube was dark.

However, the glass envelope of the tube began to glow at the anode end.

Instrumentation of Discharge Tube

This tube is made up of a glass that has two metallic plates. One end is connected to the positive terminal and the other to the negative terminal of the high voltage power supply.

The plate connected to the positive terminal is the anode, and the plate attached to the negative terminal is the cathode. And the tube is filled with any gas.

Working of Discharge Tube

working of discharge tube

A tube used to study the electrical conduction through gases at low pressure is known as a discharge tube. The figure shows the experimental setup to study the discharge phenomenon.

It consists of a strong glass tube about 0.5m long and 0.04 diameter, closed at both ends and provided with two platinum electrodes A and C, called anode and cathode, respectively.

The two electrodes are connected to secondary terminates of a powerful induction coil. The discharge tube is combined with a vacuum pump to reduce the pressure inside the tube is decreased gradually using a pump.

The following phenomenon is observed as explained below:

1. At Pressure of about 10 mm of Hg

discharge tube at pressure of about 10mm of HG

When the pressure in the discharge rube is reduced to about 10 mm Hg, the discharge in the form of luminous streaks between the electrodes called blue streamers produced cracking noise, as shown in the figure.

2. At Pressure of about 5 mm of Hg

at pressure of about 5mm of Hg

When the pressure in the discharge tube is reduced to about 5 mm Hg, the blue streaks broaden out in a luminous column which bright and steady. The luminous column is called Geissler’s discharge. The Colour of the discharge depends on the nature of the gas.

3. At a Pressure of about 2 mm of Hg

at pressure of about 2mm of Hg

When the pressure in the discharge tube is reduced to about 2 mm Hg, a long luminous column appears from an anode to a cathode called the positive column, as shown in the figure. The Colour of the discharge depends upon the nature of the gas, e.g., it is red for and blue for hydrogen.

4. At a Pressure of about 1 mm of Hg

at pressure of about 1mm of Hg

When the pressure in the discharge tube is reduced to about 1 mm Hg, the positive column detaches itself from the cathode and moves towards the anode. A luminous blue glow appears at the cathode called a negative glow. A dark space appears between a positive column and negative glow called Faraday’s dark space, as shown in the figure.

5. At a Pressure of about 0.5 mm of Hg

At a Pressure of about 0.5 mm of Hg

When the pressure in the discharge tube is reduced to about 0.5 mm Hg, the positive column gets shortened, the Faraday dark space extending to a greater length.

The negative glow leaves the cathode, and another glow appears on the cathode called cathode glow. The negative glow moves towards the anode. The space between cathode glow and a negative glow appears dark and is called Crooke’s dark space, as shown in the figure.

6. At a Pressure of about 0.05 mm of Hg

At a Pressure of about 0.05 mm of Hg

In this pressure range, the positive column shortens and breaks into alternative bright and disc called striations, as shown in the figure.

7. At a Pressure of about 0.01 mm of Hg

At a Pressure of about 0.01 mm of Hg

At this pressure, first striations disappear, the negative and cathode glow vanishes, and the whole tube s filled with crook’s dark space.

And at this stage, luminous rays are seen to come out of the cathode. These rays are called cathode rays. Cathode rays are electrons traveling from the cathode to the anode.

Now the tube walls are seen o glow with a blue or green light depending on the nature of the glass. This is called fluorescence. If the pressure in the tube is reduced further, to maintain discharge, very high voltage is required, and finally, the tube stops conducting.

Role of Discharge Tube in Cathode Ray Experiment

Experiments on the discharge tube were first performed in 1878 by an English physicist, William Crookes. Later in the 19th century, J. J. Thomson studied the characteristics and the constituents of cathode rays. Certain properties of these cathode rays were concluded, so let us discuss them now;

  • Cathode rays originated from the cathode and traveled in straight lines carrying a negative charge.
  • Electrical and magnetic fields deflected the cathode rays. This behavior was similar to that of a negatively charged particle, suggesting that the rays contained electrons.
  • These rays consisted of material particles, electrons and it caused other materials to glow upon striking them. This experiment concluded that electrons were the basic constituent of the atoms, and matter contains atoms. Thus they concluded that matter consisted of electrons.

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