Discovery of electron
Cathode rays and electrons
Electrical discharge through partially evacuated tubes produced radiation. This radiation originated from near the negative electrode, known as the cathode (thus, these rays were termed cathode rays).
- The “rays” traveled towards, or were attracted to the positive electrode (anode)
- Not directly visible but could be detected by their ability to cause other materials to glow, or fluoresce
- Traveled in straight line
- Their path could be “bent” by the influence of magnetic or electrical fields
- A metal plate in the path of the “cathode rays” aquired a negative charge
- The “cathode rays” produced by cathodes of different materials appeared to have the same properties
These observations indicated that the cathode ray were composed of negatively charged particles (now known as electrons).
J.J. Thompson (1897) measured the charge to mass ratio for a stream of electrons (using a cathode ray tube apparatus) at 1.76 x 108 coulombs/gram.
- Charged particle stream can be deflected by both an electric and by a magnetic field
- An electric field can be used to compensate for the magnetic deflection – the resulting beam thus behaves as if it were neutral
- The field needed to “neutralize” the magnetic field indicates the charge of the beam
Thompson determined the charge to mass ratio for the electron, but was not able to determine the mass of the electron.
However, from his data, if the charge of a single electron could be determined, then the mass of a single electron could also be determined.
Robert Millikan (1909) was able to successfully measure the charge on a single electron (the “Milliken oil drop experiment”). This value was determined to be 1.60 x 10-19 coulombs.
Thus, the mass of a single electron was determined to be:
(1 gram/1.76 x 108 coulombs)*(1.60 x 10-19 coulombs) = 9.10 x 10-28 grams
Note: the currently accepted value for the mass of the electron is 9.10939 x 10- 28 grams.