Cathode Ray Experiment

Cathode Ray Experiment - J J Thomson

What are Cathode rays?

Cathode rays are a beam of negatively charged electrons traveling from the negative end of an electrode to the positive end within a vacuum, across a potential difference between the electrodes.

How do the Cathode Rays work?

The cathode is a negative electrode, Anode is the positive electrode. Since electrons are repelled by the negative electrode, the cathode is the source of cathode rays inside a vacuum environment. When a potential difference is applied, the electrons jump to an excited state and travel at high speeds to jump back-and-forth inside the vacuum glass chamber and when some cathode rays certain molecules of the cathode screen, they emit light energy. A wire is connected from anode to cathode to complete the electrical circuit.

Construction of a Cathode Ray Tube

Its basic components are: -

# Electron Gun Assembly: - It is the source of the electron beams. The electron gun has a heater, cathode, pre-accelerating anode, focusing anode and accelerating anode.
# Deflecting Plates: - They produce a uniform electrostatic field only in one direction, and accelerate particles in only one direction.
# Screen: - The inner layer of the screen is coated with phosphorus, and produces fluorescence when cathode ray hit the screen by a process of phosphorus excitation.
# Aquadag: - It is an aqueous solution of graphite used to collect the secondary emitted electrons which are required to keep the cathode ray in electrical equilibrium.

What is the Cathode Ray Tube Experiment?

In 1897, great physician J.J. Thompson, conducted his first cathode ray tube experiment to prove that rays emitted from an electron gun are inseparable from the latent charge. He built his cathode ray tube with a metal cylinder on the other end. The metal had two small diversions(slits), leading to electrometer that could measure small electric charge. From the first experiment, he discovered that the electrometers stopped measuring electric charge. From this, he deduced that the electric charge and the cathode rays must be combined together and are the same entity.

Then he conducted a Second experiment, to prove the charge carried by the cathode rays was negative or positive. Now, he put a negatively charged metal plate on one side of the cathode rays to go past the anode, and a positively charged metal plate on the other side. Instead of an electrometer at one end of the Cathode Ray Tube, he used a fluorescent coated tube that would glow where the cathode ray hit it. When the charged metal plates were introduced he found that the cathode rays bent away from the negative plate and towards the positive plate. This proved that the cathode rays were negatively charged.

Then he performed the third experiment, to know the nature of the particles and reduce the mass of the particle as they had too small of a mass to be calculated directly. For the experiment, he used the cathode ray tube and with a high applied potential difference between the two electrodes, with the negatively charged cathode producing the cathode rays. He had already deduced that the particles were negatively charged. Firstly, he applied an electric field in the path between anode and cathode and measured the deflections from the straight path. Now he applied a magnetic field across the cathode ray tube by using an external magnetic field. The cathode ray is deflected by the magnetic field. Now he changed the direction of the external magnetic field and found that the beam of electrons is deflected in the opposite direction. From this experiment he concluded, that the electrostatic deflection is the same as the electromagnetic deflection for the cathode rays and he was able to calculate the charge to mass ratio of the electron.

After these three experiments, he deduced that inside the atom there consist a subatomic particle, originally named ‘corpuscle’, then changed to ‘electron’ which is 1800 times lighter than the mass of hydrogen atom (Lightest atom).

Formula used

The derivation of the formula used to calculate the charge to mass ratio:
For Electric Field the force on a particle is
   Force(F)=Charge(Q)*Electric field(E) ---<1>

For Magnetic Field the force on a particle moving with velocity is:
             F=q*velocity(v)*Magnetic Field(B) ---<2>

From 1 and 2 we get,
                                         V=E/B ----<3>

From the definition of Force,
             Acceleration(a)= Force(f)/mass(m) ----<4>

Combining 1 and 4
                                      a=q*E/m ----<5>

From Newton’s law Of motion vertical displacement is:
                                      Y= (1/2)*a*t*t ----<6>

From 5 and 6

Postulates of J.J. Thomson’s Atomic Model

After the Cathode ray tube experiment, Thomson gave one of the first atomic models including the newly discovered particle. 
His model stated: -

1) An atom resembles a sphere of positive charge with negative charge present inside the sphere.
2) The positive charge and the negative charge were equal in magnitude and thus the atom had no charge as a whole and is electrically neutral.
3) His model resembles a plum pudding or watermelon. It assumed that positive and negative charge inside an atom is randomly spread across the whole sphere like the red part of watermelon (positive charge) and the black seeds (negative charge).

Practical uses of Cathode ray tube experiment

In ancient times, the cathode ray tubes were used in the beam where the electron was considered with no inertia but have higher frequencies and can be made visible for a short time.

There are many scientists who were trying to get the secrets of cathode rays, while others were in search of the practical uses or applications of cathode ray tube experiments. And the first search was ended and released in 1897 which was introduced as the Karl Ferdinand Braun’s oscilloscope. It was used for producing luminescence on a chemical affected screen in which cathode rays were allowed to pass through the narrow aperture by focusing into the beams that looks like a dot. This dot was passed for scanning across the screen which was represented visually by the electrical pulse generator.

Then during the first two to three decades of the twentieth century, inventors continued to search the uses of cathode ray tube technology. Then inspired by Braun's oscilloscope, A. A. Campbell advised that cathode ray tube would be used for projecting video image on the screen. But, this technology of the time did not get matched with the vision of Campbell-Swinton. It was only till 1922, when the Philo T. Farnsworth developed a magnet to get focused on the stream of electrons on the screen, for producing the image. Thus, the first kind of it, Farnsworth was quickly backed up by Zworykin’s kinescope, known as the ancestor of modern TV sets.

Nowadays, most of the image viewer devices are made with the help of cathode ray tube technology including the guns of electrons which are used in huge areas of science as well as medical applications. One such use for cathode-ray tube research is the microscope invented by Ernst Ruska in 1928. The microscope based on electrons uses the stream of electrons to magnify the image as the electron have a small wavelength which is used for magnifying the objects which are very small to get resolved by visible light. Just like plucker and Crookes work, Ernst Ruska used a strong field of magnetic lines for getting it focused on the stream of electrons into an image.