In a discharge tube experiment, cathode rays are composed of which fundamental particles?

Introduction / Context:
This question comes from the early history of atomic structure and experimental physics. J J Thomson and other scientists studied cathode rays in evacuated discharge tubes long before electrons were widely accepted as fundamental particles. By observing how these rays behave in electric and magnetic fields, they concluded that cathode rays are streams of tiny negatively charged particles, which we now call electrons. Recognising that cathode rays are made of electrons is essential for understanding how the electron was discovered.

Given Data / Assumptions:
- The setting is a gas discharge tube with a cathode and an anode at low pressure.
- Cathode rays are observed travelling from the cathode toward the anode.
- The options mention electrons, protons, neutrons, and positively charged ions as possible constituents.
- We assume standard school descriptions of cathode ray experiments.

Concept / Approach:
Cathode rays originate at the negatively charged cathode in a discharge tube when a high voltage is applied. The important clues about their nature come from experiments showing that they are deflected by electric and magnetic fields in a way that indicates a negative charge. They also produce fluorescence when they strike certain materials and can spin a small paddle wheel, showing that they have mass and momentum. These observations are consistent with a beam of negatively charged particles with a very small mass, which are electrons. Protons and positive ions are positively charged, so they would be deflected in the opposite direction. Neutrons are neutral and would not be deflected by electric fields at all.

Step-by-Step Solution:
Step 1: In a discharge tube, when voltage is applied, rays are seen to travel from the cathode (negative electrode) toward the anode (positive electrode).
Step 2: When an electric field is applied across the path of these rays, they are deflected toward the positive plate, which shows that they carry negative charge.
Step 3: When a magnetic field is applied, the rays are also deflected in a manner consistent with negatively charged particles moving through the field.
Step 4: The rays can cause mechanical effects, such as spinning a lightweight paddle wheel, indicating that the rays consist of particles with mass, not just energy like light.
Step 5: From the measured charge to mass ratio, scientists concluded that these particles are much lighter than atoms and are identical regardless of the gas used. They were named electrons.

Verification / Alternative check:
Standard textbooks describe how J J Thomson measured the charge to mass ratio of cathode ray particles and found the same value for all metals and gases tested. This universality pointed to a fundamental particle, the electron. Furthermore, the charge on the electron determined later by Millikan agreed with the values inferred from cathode ray experiments. No evidence suggested that cathode rays consisted of protons, neutrons, or positive ions, confirming that they are streams of electrons.

Why Other Options Are Wrong:
- Protons: These are positively charged particles found in atomic nuclei. A beam of protons would be deflected in the opposite direction under an electric field compared to cathode rays.

Why Other Options Are Wrong (continued):
- Neutrons: Neutrons carry no charge, so they would not be deflected by electric fields and would not match cathode ray behaviour.
- Positively charged ions: These are atoms that have lost electrons. A beam of such ions is called an anode ray or positive ray, not a cathode ray, and behaves differently in fields.

Common Pitfalls:
One common confusion is between cathode rays and anode rays. Anode rays are streams of positive ions that travel in the opposite direction and were studied by Goldstein. Another pitfall is thinking that any ray in a tube must be made of light, whereas cathode rays clearly show particle like properties with mass and charge. Remembering that cathode rays originate at the cathode and behave like negatively charged particles helps keep the concepts clear.

Final Answer:
Electrons are the particles that make up cathode rays in a discharge tube experiment.