In experiments with discharge tubes, cathode rays (streams emitted from the cathode) are known to possess which of the following properties?

Introduction / Context:
This question is about cathode rays, which were studied in early discharge tube experiments and were crucial in the discovery of the electron. Understanding whether cathode rays have mass, charge or both helps clarify the nature of electrons as fundamental particles of matter and charge, and explains why they are deflected by electric and magnetic fields.

Given Data / Assumptions:

• Cathode rays are produced in a discharge tube when a high voltage is applied between electrodes in a low pressure gas.
• They travel from the cathode towards the anode.
• We are asked whether these rays have mass, charge, both or neither.
• We assume standard school level understanding that cathode rays consist of electrons.

Concept / Approach:
Cathode rays are streams of electrons. An electron is a subatomic particle with a definite mass (although very small compared with atoms) and a definite negative electric charge. Because electrons carry charge, cathode rays are deflected by electric and magnetic fields. Because they have mass, they can impart momentum, rotate light paddle wheels in the tube and produce mechanical effects when they strike objects. Therefore, cathode rays clearly have both mass and charge, not one without the other.

Step-by-Step Solution:
1) Recall J. J. Thomson experiments which showed that cathode rays are deflected by electric and magnetic fields in a manner consistent with negatively charged particles.2) The direction of deflection indicates that these particles carry a negative charge.3) Thomson also measured the charge to mass ratio (e/m) of these particles, demonstrating that they have a finite mass as well as charge.4) In discharge tubes, cathode rays can turn a small paddle wheel placed in their path, showing that they transfer momentum and therefore have mass.5) At the same time, their deflection in electric and magnetic fields proves the presence of electric charge.6) These observations confirm that cathode rays consist of electrons, which possess both mass and electric charge.

Verification / Alternative check:
The modern model of the atom includes electrons as negatively charged particles with a rest mass of about 9.11 × 10^−31 kg and a charge of about −1.6 × 10^−19 coulomb. Cathode rays are nothing but streams of such electrons. If cathode rays had no mass, they would not convey mechanical effects to a paddle wheel; if they had no charge, they would not be deflected in fields. The fact that experiments show both deflection and mechanical effects supports the conclusion that they have both mass and charge.

Why Other Options Are Wrong:
Mass only but no electric charge: This would not explain the observed deflection in electric and magnetic fields.
Electric charge only but no mass: Pure charge without mass would not provide mechanical effects such as moving a paddle wheel and would not have a measurable e/m ratio.
Neither mass nor charge: This would correspond to neutral radiation like some forms of electromagnetic radiation, not to cathode rays.
Only magnetic moment without mass: This is not an accepted description of cathode rays; electrons indeed have mass and charge, and any magnetic moment is secondary to these properties.

Common Pitfalls:
Students sometimes confuse cathode rays with electromagnetic radiation such as X rays, which have energy but no rest mass and no net charge. It is important to remember that cathode rays are material particles (electrons), not light waves. Another confusion is mixing up cathode rays with anode rays (positive rays). Focusing on the key experimental facts of deflection and mechanical impact helps reinforce that cathode rays have both mass and electric charge.

Final Answer:
Cathode rays possess both mass and electric charge, because they are streams of electrons.