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14th Dec 2019 @ 10 min read
Sir Joseph John Thomson was a British physicist and Nobel Laureate. He was well-known for the discovery of the electron. In 1897, he showed that cathode rays were composed of very small negatively charged particles. These particles later were named electrons. The apparatus of his experiment is called the cathode-ray tube (CRT).
J. J. Thomson was not the only one working on cathode rays, but several other players like Julius Plücker, Johann Wilhelm Hittorf, William Crookes, Philipp Lenard had contributed or were busy studying it. However, Thomson's contributions remain more significant than the rest. His experimental results were further investigated by Rutherford and Bohr, which further provided important insights into the atomic world.
Before directly jumping Thomson's findings, let us understand some basic knowledge on cathode rays and the cathode-ray tube.
What are cathode rays? Cathode rays are streams of electrons emitted from the cathode (the electrode connected to the negative terminal of a battery). These rays travel in straight lines and can be deflected by electric and magnetic field.
The cathode-ray tube (CRT) is a hollow glass tube. The air in the tube is pumped out to create a vacuum.
The CRT consists of the following parts:
Back in those days, physicists were unclear whether cathode rays were immaterial like light or were material. Many diverse opinions were held on these rays. According to some, the rays are due to some process in the aether. The immaterial nature and the aetherial hypothesis of cathode rays were proved wrong by J. J. Thomson. He concluded that the rays were comprised of particles. His entire works can be divided into three different experiments. In the first, the magnetic effect on cathode rays was studied while in the second, the rays were deflected by an electric field. In the final experiment, he succeeded in measuring mass to charge ratio.
The experiment apparatus consisted of two metal cylinders. The cylinders were coaxial placed and insulated from each other. The outer cylinder was grounded while inner was attached to an electrometer to detect any electric current as shown in the figure below. Both cylinders had holes or slits. When a high potential difference was applied between the cathode (A in the diagram) and anode (B in the diagram), cathode rays, which were produced in the left tube, emitted from the cathode and entered into the main bell jar. The rays would not enter the cylinders unless deflected by a magnetic field.
He traced the path of the rays using the fluorescence on a squared screen in the jar. When the rays were bent by a magnetic field, they infiltrate the cylinders through the slits. And the presence of negatively charge was detected in the electrometer. If these rays were further bent, they overshot the slits and the electrometer failed to show any readings. “Thus this experiment shows that however we twist and deflect the cathode rays by magnetic forces, the negative electrification follows the same path as the rays and that this negative electrification is indissolubly connected with the cathode rays,” Thomson quoted.
Moreover, he repeated the experiment with different materials and gases and found the deflection of the rays was the same irrespective of materials and gases used.
He arrived at the two main points after this experiment.
The first experiment did demonstrate the behaviour of cathode rays as negatively charged particles under a magnetic field. This statement became deficient when cathode rays failed to deflect in an electric field. It was observed by Hertz well before Thomson. This resulted in a dilemma whether cathode rays are negatively charged particles or not. Thomson decided to investigate further through another experiment.
Thomson constructed a modified Crookes tube as depicted in the above figure. When a high potential difference was applied between the cathode and the anode, cathode rays were generated at the cathode (C in the diagram). As these rays passed through the anode (A in the diagram) and later through slit B, which was grounded, the rays were sharpened. This narrow beam propagated through aluminium plates (D and E) and finally struck the phosphorescent screen to produce a bright patch. The screen was scaled, so the deflection of the beam could be measured.
When Hertz had applied an electric field between the plates, he noticed no deflection of the beam. Hence, he concluded that cathode rays are not affected by an electric field.
After Hertz, when Thomson performed the same experiment, he also found the similar results. He repeated the same experiment under much lower pressure than the previous. This time the beam was deflected by an electric field. When the upper plate was attached to the positive terminal of the battery and the lower plate to the negative terminal, the beam deflected upwards. If the polarity of the plates was reversed, the beam would deflect downwards.
Finally, he succeeded in proving the beam are nothing but negatively charged particles.
As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter.
Note: One question, which may haunt the readers, is that why the beam deflected when the vacuum in the tube was increased. The high potential difference between the electrodes ionized the residual gas molecules into free electrons and ions, aka space charge. These free electrons and ions electrically screened the external electric field in the case of Hertz. Thus, it resulted in a damp electric field, and the beam remained unaffected by the electric field. But in the case of Thomson due to the higher vacuum, the density of the space charge was very less. And they did not significantly hinder the electric field.
After demonstrating the electrostatic properties of cathode rays, Thomson was still curious about these particles. He pondered whether what were these particles, were they atoms or molecules, or some unknown entities yet to discover. To find answers to such questions, he performed the third experiment. In this experiment, he measured the mass-to-charge ratio of particles.
The experimental apparatus for this experiment was the same as the previous one. Additionally, he applied a magnetic field by placing the poles of an electromagnet around the tube as shown in the above figure.
The magnetic field was applied such that it was perpendicular to both the electric field and cathode rays. This is depicted in the figure below.
Initially, he applied the only electric field, which deflected the beam to a particular direction. This electric deflection was measured by him. And then the magnetic field was varied until the beam returned to the original path i.e. it remained undeflected. At this condition, the magnetic force and the electric force had cancelled out each other. They were equal in magnitude but opposite in direction.
He calculated the mass-to-charge ratio (m⁄e) using the below expression.
Here, E and H are the electric field strength and the magnetic field strength, l is the length of the plates, and θ is the deflection when only the electric field is applied. All these parameters were known.
This notation is represented in the below figure.
When the electric force and the magnetic force cancel out each other, the rays are undeflected. Thus, the net force on the rays is zero.
We know FE = eE and FH = −evH. The negative sign shows the forces are in the opposite direction.
The displacement from the kinematic formulas is
In the x-direction, the initial velocity is v and the acceleration is zero.
Substituting the value of v in the above equation,
When t = T, x = l.
In the y-direction, the initial velocity is zero, but the beam accelerates as it advances in the electric field.
Acceleration is force divided mass.
Substituting the value of a,
When t = T, y = s.
Eliminating T,Thus, the mass-to-charge ratio is as follows:
For smaller values of θ, .
The value of the ratio reported by Thomson in his paper is (1.29 ± 0.17) × 10−7.
The reciprocal of m⁄e gives the charge-to-mass ratio (e⁄m). The value of e⁄m recommended by CODATA is 1.758 820 010 76(53) × 1011 C kg−1.
Thomson also noted that his calculated value of m⁄e was independent of the gas in the discharge tube and the metal used of the cathode. This also gave an inkling that particles were an integral part of atoms.
He also noted that the value of m⁄e was around 1000 times smaller than that the value of hydrogen ions. The value of m⁄e of hydrogen ions estimated at that time was around 10−4. It implied that the mass of the particles were much smaller than that of hydrogen ions or were heavily charged. Lenard had determined that the range, which is closely associated to the mean free path for collisions, of cathode rays; it was 0.5 cm. On the other hand, the mean free path of air molecules was 10−5 cm, which is very small in comparison with the range of cathode rays. Therefore, he argued the size of these particles must be much smaller than the molecules of air.
Thomson named these particles as corpuscles, later they were renamed as electrons. He concluded that the corpuscles were smaller than the size of the atoms and were an integral part of an atom.
Based on these experimental results Thomson also proposed his plum pudding model. He was honoured the Nobel Prize in Physics.
Thomson presented three hypotheses from his experiments.
The third hypothesis was proven wrong later when his own student Rutherford proposed the presence of the positively charged nucleus in an atom.
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