Experimental Discovery and Properties of the Electron

Experimental Setup and the Discovery of Cathode Rays by William Crooks

The fundamental research regarding the discovery of the electron began with the experiments conducted by William Crooks using a specialized apparatus known as a discharge tube, eventually referred to as a Crooks tube or a cathode ray tube. The experimental setup involved an airtight glass tube containing two metal electrodes: a cathode (the negative terminal) and an anode (the positive terminal). These electrodes were connected to a high-voltage generator capable of delivering a potential difference between $5000A$ and $11000A$. A vacuum pump was attached to the tube to control and lower the internal gas pressure.

Initially, when the high voltage was applied at regular atmospheric pressure, the ammeter did not show any reading, indicating that the gas inside the tube did not conduct electricity. However, as the vacuum pump was used to significantly reduce the pressure of the gas within the tube, the ammeter began to show a reading. This observation led to the conclusion that gases become conductive specifically under conditions of low pressure. During this state of low pressure, a distinct glow appeared starting from the side of the cathode and moving forward until it struck the glass wall, producing a characteristic bluish-green luminescence. William Crooks named these emissions "cathode rays."

The Physical Principles of Cathode Ray Production

The production of cathode rays is dependent on the specific physical environment within the discharge tube. The high potential difference (voltage) applied across the electrodes serves a critical function: it provides the necessary energy to eject electrons from the cathode material. Once ejected, these particles must travel across the tube to the anode. Under normal atmospheric pressure, the high density of gas particles creates constant obstructions, preventing the particles from moving freely. By reducing the internal pressure, the space between the remaining gas particles is increased, which minimizes the resistance or interference encountered by the moving electrons, allowing them to traverse the tube and manifest as cathode rays.

Observations on the Electric and Magnetic Properties

Careful observation and testing revealed several defining characteristics of cathode rays regarding their interaction with external fields. When subjected to an electric field, the rays were observed to bend or deflect toward the positive plate (the anode plate). This behavior provided definitive evidence that cathode rays carry a negative charge. Furthermore, when the rays were exposed to a magnetic field, they exhibited deflection in a direction perpendicular to the field. It was also noted that under neutral conditions, without the influence of external electric or magnetic plates, the rays do not deviate toward any specific side but maintain a direct path.

The Wave-Like and Rectilinear Nature of Cathode Rays

Cathode rays possess certain properties that resemble the behavior of light. To test this, a thick aluminum plate was placed in the path of the rays. This resulted in the formation of a sharp shadow on the glass wall directly opposite the cathode, demonstrating that cathode rays travel in straight lines. It was further discovered that these rays do not simply move toward the anode in an arbitrary fashion; rather, they are emitted perpendicularly from the surface of the cathode. This perpendicular emission was confirmed through experiments using a concave-shaped cathode, which focused the rays. However, when a very thin aluminum plate was substituted for the thick one, no shadow appeared on the glass wall. This implies that cathode rays consist of particles that are even smaller than atoms, allowing them to pass through the thin material.

Evidence for Particle Nature and Energy Manifestations

The particle nature of cathode rays was further reinforced by an experiment involving a small paddle wheel placed directly in the path of the rays. Upon being struck by the rays, the paddle wheel began to rotate. This rotation is a clear indication that the rays possess mass and velocity, which together generate momentum and kinetic energy. Because of their high velocity and particulate nature, cathode rays exhibit several energetic capabilities: they generate heat upon impact with surfaces, they have the power to ionize gases they pass through, and they possess the specific energy required to produce X-rays when they strike certain targets.

J.J. Thomson’s Determination of the Charge-to-Mass Ratio

J.J. Thomson conducted a landmark study to quantify the properties of these rays. By experimenting with different types of gases inside the discharge tube and using various materials for the electrodes, he calculated the ratio of the charge ($e$) to the mass ($m$) of the particles. He observed that the value of this ratio remained constant regardless of the gas used or the metal of the cathode. The specific charge-to-mass ratio was determined to be $1.759 \times 10^{11}\,C\,kg^{-1}$. This constancy led Thomson to the groundbreaking conclusion that cathode rays from all elements are composed of the same fundamental, universal particle.

The Naming and Fundamental Nature of the Electron

While J.J. Thomson is credited with the scientific discovery of the particle, the name "electron" was proposed by the scientist Stoney. It is now understood that electrons possess a dual nature, meaning they function as both particles and waves simultaneously at all times. This duality is central to their behavior in atomic structures and their role as the universal constituent of all matter identified during the discharge tube experiments.