Electron flow vs. Conventional current: Thompson's cathode ray experiment
228Today we will discuss the difference between these two concepts. The direction of the conventional current has been established in the beginning of the 18-th century, where scientists assumed that some positively charged atoms are flowing from the positive terminal of a power source towards the negative terminal. Since this direction was based on an old convention still used in physics and electronics, it is being called the conventional current direction.
In 1897 J.J. Thompson was performing experiments on cathode rays, which were causing a green fluorescence of the gas discharge tube next to the positively charged anode electrode. Thompson has realized, that the fluorescence is caused by negatively charged particles. These particles were ejected from the negatively charged cathode electrode, and they were traveling so fast, that some of them were not collected by the anode, but they have hit the glass wall of the tube instead. Since the particles were originating from the cathode, Thompson has proposed, that these negatively charged particles are responsible for carrying the electric current. Therefore, in contrast to the conventional current, the electrons are flowing from the negative terminal of a power source towards the positive terminal. However, by this time the conventional current was so well established on books and in scientific literature, that the conventional direction was kept for educational purposes.
In a series of further experiments by using external magnetic field and electric field applied as deflection coils or deflection plates Thompson was able to determine the charge to mass ratio of the particles. Surprisingly, the particles turned out to be rather light, since they had a weight almost 2000 times smaller than a single hydrogen atom. Because of this, Thompson has proposed, that these negatively charged particles have to be subatomic particles. This subatomic particle was named electron. The discovery of the electron has paved the way for modern chemistry, physics and electronics.