• 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.

• More specifically:
  • He directed a beam of cathode rays through perpendicular electric and magnetic fields.
  • He adjusted the strength of the electric field until the beam passed through the perpendicular fields without being deflected.
  • For this to occur, the forces exerted by the two field would be equal, which allowed the formation of a mathematical expression:
    • F_b = F_E
    • qvB = qE
    • Hence ,

• • 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.

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)