EXPERIMENT  |  ANALYSIS   |  CONCLUSIONS

EXPERIMENT

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 coulombs/kg.

These experimental results were consistent with Thomsen’s 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.

ANALYSIS

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 hydrogen ion
-or-
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 small.

CONCLUSIONS

Electrons have two important properties. They are:

Emitted by a wide range of materials
Much smaller in mass than a hydrogen ion

In other words, electrons are small building blocks of atoms and are a part of all matter.

Cathode Ray Experiments

Thomsen's Plum Pudding Model