Why Did a Glow at the Positive End Hint at Cathode Rays Having a Negative Charge in Thomson’s Experiment?

In Thomson’s Cathode Ray experiment, the observation of a glow at the positive end of the tube played a crucial role in inferring the nature of cathode rays. This article delves into the setup, observations, and conclusions drawn from this experiment, emphasizing why the glow at the positive end indicated the negative charge of cathode rays.

Setup of the Experiment

Thomson, a renowned physicist, conducted his experiment using a vacuum tube equipped with two electrodes: a cathode (negative electrode) and an anode (positive electrode). By applying a high voltage across these electrodes, Thomson was able to emit cathode rays, which are streams of electrons, from the cathode towards the anode.

Observation of the Glow

Crucially, when the cathode rays traveled through the tube and struck the anode, they caused a fluorescence that produced a visible glow. This glow was observed at the positive end of the tube, where the anode was located.

Direction of Cathode Rays

The cathode rays were observed to move from the cathode towards the anode. This behavior indicated that the rays carried a negative charge. If the rays had been positively charged, they would have been repelled by the anode instead of being attracted to it.

Conclusion on Charge

The observed movement of cathode rays towards the positively charged anode and the resulting glow there provided strong evidence that they were negatively charged particles, which we now know as electrons. This discovery was a pivotal moment in our understanding of atomic structure and the fundamental nature of electrical charge.

The Role of Benjamin Franklin’s Mistaken Theories

Benjamin Franklin, a pioneering figure in electricity, believed that electricity consisted of a single fluid that flowed from one terminal of a battery to the other. However, Franklin had mistakenly thought that two fluids existed in the circuit, which canceled each other out in the middle. This theory has left a lasting impact on our understanding of electricity, as people today still grapple with the consequences of his mislabeling.

Thomson, on the other hand, knew that his cathode ray tube produced a vacuum gap over which this mysterious fluid had to flow without the encumbrance of a wire. This led him to expect that the particles would be more energetic for the same reason. However, the direction of the mysterious current, as demonstrated by Thomson’s experiment, moved from the negative battery terminal to the positive battery terminal, the exact opposite of Franklin’s intended direction.

Thomson’s experiment not only confirmed the existence of electrons but also corrected a longstanding misconception. His findings provided a deeper understanding of electricity and enabled further advancements in the field.