Aim
To study the behavior of gases at very low pressures when subjected to high electrical voltage, leading to the discovery of fundamental subatomic particles (electrons and protons).
Apparatus Required
- A heavy glass tube (discharge tube / Crookes tube)
- Two metallic electrodes (cathode and anode) sealed into the tube
- A high voltage induction coil (10,000 to 20,000 Volts)
- A vacuum pump connected to a side tube
- Fluorescent screen (e.g., zinc sulfide coating) inside the tube (optional)
Theory & Principle
Under normal conditions, gases are poor conductors of electricity. However, when the pressure inside a glass tube containing a gas is reduced significantly, and a very high potential difference is applied across the electrodes, the gas begins to conduct electricity. This results in the emission of invisible rays from the electrodes.
1. Cathode Rays (J.J. Thomson, 1897)
At extremely low pressures ($10^{-4}$ atm), a stream of particles originates from the negative electrode (cathode) and travels toward the positive electrode (anode). These are called Cathode Rays.
- They travel in straight lines.
- They consist of negatively charged particles (Electrons).
- They cast shadows of solid objects placed in their path.
- They are deflected by electric and magnetic fields toward the positive plate/pole.
- The charge-to-mass ratio ($e/m$) of these particles is constant and independent of the gas used in the tube or the material of the electrodes.
2. Anode Rays / Canal Rays (Eugen Goldstein, 1886)
If a perforated cathode is used, faint luminous rays are observed traveling in the opposite direction (from anode to cathode) and passing through the holes of the cathode. These are Anode Rays or Canal Rays.
- They consist of positively charged particles.
- Unlike cathode rays, the mass and charge-to-mass ratio ($e/m$) of anode rays depend on the nature of the gas present in the tube.
- When hydrogen gas is used, the particles obtained are the lightest and carry a unit positive charge. These particles were named Protons by Rutherford.
Procedure
- Connect the electrodes of the discharge tube to the high voltage induction coil.
- Connect the side tube to the vacuum pump.
- Observation at 1 atm: Apply the high voltage. No current flows, and nothing is observed, confirming gases are insulators at atmospheric pressure.
- Observation at $10^{-2}$ atm: Turn on the vacuum pump to reduce the pressure. The gas begins to conduct electricity, and the entire tube glows with a color dependent on the gas inside (e.g., pinkish for air).
- Observation at $10^{-4}$ atm: Reduce the pressure further. The glowing column disappears (Crookes dark space). Instead, the glass walls of the tube directly opposite the cathode begin to glow with a greenish fluorescent light. This indicates the emission of cathode rays.
- To observe anode rays, replace the standard cathode with a perforated cathode and observe the faint stream passing through the holes behind the cathode.
Result
The experiment proved that atoms are divisible and consist of fundamental charged particles. Cathode rays established the existence of the negatively charged electron, and anode rays led to the discovery of the positively charged proton.
Precautions
- Handle the induction coil with extreme care due to the dangerously high voltage.
- Ensure the discharge tube is securely connected to the vacuum pump to maintain low pressure.
- Avoid prolonged exposure to cathode rays, as striking certain targets at high speeds can produce X-rays.
Viva Questions & Answers
Q1: Why do gases conduct electricity only at very low pressures?
At atmospheric pressure, gas molecules are densely packed and collide frequently with any free electrons, preventing them from gaining enough kinetic energy from the electric field to ionize other atoms. At low pressure, the mean free path is longer, allowing electrons to accelerate and knock off electrons from gas molecules, causing an avalanche of ions that conduct electricity.
Q2: Why is the e/m ratio of cathode rays constant, but not for anode rays?
Cathode rays are simply electrons. Since all electrons are identical regardless of the source, their e/m ratio is constant. Anode rays, however, are gaseous positive ions left behind after electrons are knocked off. Since different gases have atoms of different masses, the positive ions have different masses, leading to varying e/m ratios.
Q3: How was it proven that cathode rays travel in straight lines?
When a solid object (like a metallic cross) was placed in the path of the cathode rays, a sharp shadow of the object was cast on the fluorescent screen behind it, proving they travel in straight lines.
