EXPERIMENT | ANALYSIS
Sir John Joseph Thomsen began his investigation
of cathode rays by assuming that they were negatively charged particles. This
assumption allowed him to predict the path of the cathode rays when they passed through
electric and magnetic fields.
Thomsen knew that an electric field would force the cathode
rays in one direction and that a magnetic field would force them in an
opposite direction. When Thomsen performed his experiment, he used the
forces of each field to cancel each other out, allowing him to calculate the
charge-to-mass ratio (q/m) of the particle. Thomsen found that q/m = 1.76 x 1011
These experimental results were consistent with
Thomsens predictions, meaning that his assumptions were correct. Because
all cathode rays had the same properties, he concluded that all the particles were the
same. Thomsen named these particles electrons.
Since the q/m ratio of the electron was nearly
1800 times greater than the q/m ratio of a hydrogen ion, either:
The charge on the electron was much greater than the
the mass of the electron was much smaller than the hydrogen ion.
Thomsen determined that the amount of charge on the electron and the
hydrogen ion was the same. Therefore, the mass of the electron had to be very
Electrons have two important properties. They are:
- Emitted by
a wide range of materials
Much smaller in mass than a
In other words, electrons are small building blocks of atoms
and are a part of all matter.