Question

4. What was the significance of Thomson’s experiment in which an off-center electrode was used to collect charge deflected by a magnetic field?

Short answer

This was the first conclusive demonstration that cathode rays really are negatively charged particles.

Step-by-step solution

Step 1. Given information

Thomson’s experiment in which an off-center electrode was used to collect charge deflected by a magnetic field.

Step 2. Explanation

Using an offset electrode, when no magnetic field was applied the cathode rays struck the center of the tube face and created a greenish spot on the glass. No current was measured under these circumstances. Thomson then placed the tube in a magnetic field to deflect the cathode rays to the side. He could determine their trajectory by the location of the green spot as it moved across the face of the tube. Just at the point when the field was strong enough to deflect the cathode rays onto the electrode, a current was detected! At an even stronger field, when the cathode rays were deflected completely to the other side of the electrode, the current ceased. From this he concluded that cathode rays are charged and the nature of charge, which is negatively charged, was concluded from the direction it was deflected.