what experimental evidence led Thomson to conclude that cathode rays were negatively charged?
Look under "career" from this.
http://en.wikipedia.org/wiki/J._J._Thomson
To understand the experimental evidence that led J.J. Thomson to conclude that cathode rays were negatively charged, we need to examine his experiments and how he interpreted the results.
Thomson conducted a series of experiments involving cathode rays, which are streams of charged particles that travel from the negative electrode (cathode) to the positive electrode (anode) within a cathode ray tube.
One of Thomson's key experiments involved a cathode ray tube equipped with two metal plates: the cathode (negatively charged) and the anode (positively charged). Between these plates, he placed a pair of parallel metal plates called deflecting plates. Thomson applied an electric field between the deflecting plates, perpendicular to the path of the cathode rays.
Here's a step-by-step breakdown of Thomson's experimental setup and the observations that led to his conclusion:
1. Without any electric field: When no electric field was applied between the deflecting plates, Thomson observed that the cathode rays traveled in a straight line from the cathode to the anode, forming a straight beam. This indicated that the rays were not deflected by any external force or field.
2. With the electric field off-center: Thomson then turned on the electric field by applying a high voltage to the deflecting plates. He observed that the cathode rays were deflected from their original path towards the positively charged anode. This deflection indicated that the cathode rays were composed of charged particles.
3. With the electric field centered: Thomson observed that when he adjusted the strength of the electric field to accurately balance the deflection caused by the cathode rays, the rays could be made to follow a straight path again. This indicated that the deflection of the cathode rays was directly related to the strength of the electric field.
Based on these observations, Thomson drew the following conclusions:
a) Since the cathode rays were negatively charged (because they were attracted towards the positive anode), there must be negatively charged particles present within the cathode rays.
b) The magnitude of the deflection of the cathode rays was proportional to the strength of the electric field, suggesting that the particles composing the cathode rays had a fixed charge-to-mass ratio.
Overall, Thomson's experiments and observations led him to conclude that cathode rays were composed of negatively charged particles, which he later named electrons. This discovery laid the foundation for the understanding of atomic structure and the existence of subatomic particles.