Investigate, assess and model the experimental evidence supporting the existence and properties of the electron

Cathode Rays: Streams of electrons emitted within an evacuated tube from a cathode to an anode.
Cathode Ray Tube: Also known as a discharge tube, a highly evacuated sealed-glass tube with two electrodes contained within it, through which cathode rays can travel.
Cathode rays were discovered in 1855 by German physicists Heinrich Geissler and Julius Plucker.

In 1875, William Crookes designed a number of different discharge tubes to study cathode rays.
There were two main groups of scientists who supported two different theories about cathode rays:
- German physicists, such as Hertz and Lenard, who advocated an electromagnetic wave model.
- English physicists, such as Crookes and Thomson, who advocated a charged particle model
In 1897, J. J. Thomson demonstrated that cathode rays were actually deflected by electric fields, which led to the acceptance of the charged particle model.

Thomson’s charge-to-mass experiment

To find the charge/mass ratio of cathode rays (now known as electrons), J. J. Thomson performed two main steps:
- Varying perpendicular magnetic and electric fields acting on a beam of cathode rays until the beam was not deflected, and then equating electric and magnetic force equations, determining the velocity of the cathode rays.
- Applying the same strength magnetic field alone and determining the radius of the circle path travelled by the particles.

- Thus, the velocity of the cathode rays could be determined.
- Then, the beam was passed through the same magnetic field alone and the radius of the curvature of the beam was measured.

- Because the magnetic field exerts a centripetal force:
  - F_C = F_B
  - Hence,
- Thus, the charge/mass ratio could be determined.
Thomson confirmed the value found by passing the beam through the same electric field alone and measuring the radius of the curvature of the beam.
Thomson determined the charge/mass ratio to be 1.759 x 10¹¹ C kg^-1
As electrons produced by different cathodes had identical properties, Thomson concluded that electrons were a subatomic building block of all atoms.

Millikan’s oil drop experiment (ACSPH026)

In 1909 , Robert Millikan studied fine drops of oil within a uniform electric field to understand charge of a particle.

His setup is shown as above.
When the oil drops were sprayed , some of them drifted down to the region of uniform electric field.
As the oil drops entered the electric field, they were also momentarily exposed to X-rays so that the oil drops would get charged.
Once charged the electric field would act on the oil drops.
Two forces would be acting on the charged oil drop in this region:
- Gravitational force :
- Electric Force :
The electric field could be adjusted such that , and the oil drops move with constant velocity or are stationary.
- this implies :
- This motion of oil drop could be observed and verified with the help of a microscope.

To measure the mass of the oil drop, Millikan did the following:
- allowed the oil drop to fall without electric field.
- measured the terminal velocity
- using equations of fluid mechanics, calculated radius and hence volume of the oil drop.
- Using an oil of known density, and measured volume, calculated the mass of the drop.

Millikan observed that the charge appeared in quantised values. It was in multiple of a particular value.

He found the value to be 1.6 x 10^-19 C, now understood as the charge of the electron.

He received a Nobel prize for this discovery in 1923.